code:
=========================================
Kernel CAPI Interface to Hardware Drivers
=========================================

1. Overview
===========

From the CAPI 2.0 specification:
COMMON-ISDN-API (CAPI) is an application programming interface standard used
to access ISDN equipment connected to basic rate interfaces (BRI) and primary
rate interfaces (PRI).

Kernel CAPI operates as a dispatching layer between CAPI applications and CAPI
hardware drivers. Hardware drivers register ISDN devices (controllers, in CAPI
lingo) with Kernel CAPI to indicate their readiness to provide their service
to CAPI applications. CAPI applications also register with Kernel CAPI,
requesting association with a CAPI device. Kernel CAPI then dispatches the
application registration to an available device, forwarding it to the
corresponding hardware driver. Kernel CAPI then forwards CAPI messages in both
directions between the application and the hardware driver.

Format and semantics of CAPI messages are specified in the CAPI 2.0 standard.
This standard is freely available from https://www.capi.org.

2. Driver and Device Registration
=================================

CAPI drivers must register each of the ISDN devices they control with Kernel
CAPI by calling the Kernel CAPI function attach_capi_ctr() with a pointer to a
struct capi_ctr before they can be used. This structure must be filled with
the names of the driver and controller, and a number of callback function
pointers which are subsequently used by Kernel CAPI for communicating with the
driver. The registration can be revoked by calling the function
detach_capi_ctr() with a pointer to the same struct capi_ctr.

Before the device can be actually used, the driver must fill in the device
information fields 'manu', 'version', 'profile' and 'serial' in the capi_ctr
structure of the device, and signal its readiness by calling capi_ctr_ready().
From then on, Kernel CAPI may call the registered callback functions for the
device.

If the device becomes unusable for any reason (shutdown, disconnect ...), the
driver has to call capi_ctr_down(). This will prevent further calls to the
callback functions by Kernel CAPI.

3. Application Registration and Communication
=============================================

Kernel CAPI forwards registration requests from applications (calls to CAPI
operation CAPI_REGISTER) to an appropriate hardware driver by calling its
register_appl() callback function. A unique Application ID (ApplID, u16) is
allocated by Kernel CAPI and passed to register_appl() along with the
parameter structure provided by the application. This is analogous to the
open() operation on regular files or character devices.

After a successful return from register_appl(), CAPI messages from the
application may be passed to the driver for the device via calls to the
send_message() callback function. Conversely, the driver may call Kernel
CAPI's capi_ctr_handle_message() function to pass a received CAPI message to
Kernel CAPI for forwarding to an application, specifying its ApplID.

Deregistration requests (CAPI operation CAPI_RELEASE) from applications are
forwarded as calls to the release_appl() callback function, passing the same
ApplID as with register_appl(). After return from release_appl(), no CAPI
messages for that application may be passed to or from the device anymore.

4. Data Structures
==================

4.1 struct capi_driver
----------------------

This structure describes a Kernel CAPI driver itself. It is used in the
register_capi_driver() and unregister_capi_driver() functions, and contains
the following non-private fields, all to be set by the driver before calling
register_capi_driver():

``char name[32]``
	the name of the driver, as a zero-terminated ASCII string
``char revision[32]``
	the revision number of the driver, as a zero-terminated ASCII string

4.2 struct capi_ctr
-------------------

This structure describes an ISDN device (controller) handled by a Kernel CAPI
driver. After registration via the attach_capi_ctr() function it is passed to
all controller specific lower layer interface and callback functions to
identify the controller to operate on.

It contains the following non-private fields:

to be set by the driver before calling attach_capi_ctr():
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

``struct module *owner``
	pointer to the driver module owning the device

``void *driverdata``
	an opaque pointer to driver specific data, not touched by Kernel CAPI

``char name[32]``
	the name of the controller, as a zero-terminated ASCII string

``char *driver_name``
	the name of the driver, as a zero-terminated ASCII string

``int (*load_firmware)(struct capi_ctr *ctrlr, capiloaddata *ldata)``
	(optional) pointer to a callback function for sending firmware and
	configuration data to the device

	The function may return before the operation has completed.

	Completion must be signalled by a call to capi_ctr_ready().

	Return value: 0 on success, error code on error
	Called in process context.

``void (*reset_ctr)(struct capi_ctr *ctrlr)``
	(optional) pointer to a callback function for stopping the device,
	releasing all registered applications

	The function may return before the operation has completed.

	Completion must be signalled by a call to capi_ctr_down().

	Called in process context.

``void (*register_appl)(struct capi_ctr *ctrlr, u16 applid, capi_register_params *rparam)``
	pointers to callback function for registration of
	applications with the device

	Calls to these functions are serialized by Kernel CAPI so that only
	one call to any of them is active at any time.

``void (*release_appl)(struct capi_ctr *ctrlr, u16 applid)``
	pointers to callback functions deregistration of
	applications with the device

	Calls to these functions are serialized by Kernel CAPI so that only
	one call to any of them is active at any time.

``u16  (*send_message)(struct capi_ctr *ctrlr, struct sk_buff *skb)``
	pointer to a callback function for sending a CAPI message to the
	device

	Return value: CAPI error code

	If the method returns 0 (CAPI_NOERROR) the driver has taken ownership
	of the skb and the caller may no longer access it. If it returns a
	non-zero (error) value then ownership of the skb returns to the caller
	who may reuse or free it.

	The return value should only be used to signal problems with respect
	to accepting or queueing the message. Errors occurring during the
	actual processing of the message should be signaled with an
	appropriate reply message.

	May be called in process or interrupt context.

	Calls to this function are not serialized by Kernel CAPI, ie. it must
	be prepared to be re-entered.

``char *(*procinfo)(struct capi_ctr *ctrlr)``
	pointer to a callback function returning the entry for the device in
	the CAPI controller info table, /proc/capi/controller

Note:
  Callback functions except send_message() are never called in interrupt
  context.

to be filled in before calling capi_ctr_ready():
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

``u8 manu[CAPI_MANUFACTURER_LEN]``
	value to return for CAPI_GET_MANUFACTURER

``capi_version version``
	value to return for CAPI_GET_VERSION

``capi_profile profile``
	value to return for CAPI_GET_PROFILE

``u8 serial[CAPI_SERIAL_LEN]``
	value to return for CAPI_GET_SERIAL

4.3 SKBs
--------

CAPI messages are passed between Kernel CAPI and the driver via send_message()
and capi_ctr_handle_message(), stored in the data portion of a socket buffer
(skb).  Each skb contains a single CAPI message coded according to the CAPI 2.0
standard.

For the data transfer messages, DATA_B3_REQ and DATA_B3_IND, the actual
payload data immediately follows the CAPI message itself within the same skb.
The Data and Data64 parameters are not used for processing. The Data64
parameter may be omitted by setting the length field of the CAPI message to 22
instead of 30.

4.4 The _cmsg Structure
-----------------------

(declared in <linux/isdn/capiutil.h>)

The _cmsg structure stores the contents of a CAPI 2.0 message in an easily
accessible form. It contains members for all possible CAPI 2.0 parameters,
including subparameters of the Additional Info and B Protocol structured
parameters, with the following exceptions:

* second Calling party number (CONNECT_IND)

* Data64 (DATA_B3_REQ and DATA_B3_IND)

* Sending complete (subparameter of Additional Info, CONNECT_REQ and INFO_REQ)

* Global Configuration (subparameter of B Protocol, CONNECT_REQ, CONNECT_RESP
  and SELECT_B_PROTOCOL_REQ)

Only those parameters appearing in the message type currently being processed
are actually used. Unused members should be set to zero.

Members are named after the CAPI 2.0 standard names of the parameters they
represent. See <linux/isdn/capiutil.h> for the exact spelling. Member data
types are:

=========== =================================================================
u8          for CAPI parameters of type 'byte'

u16         for CAPI parameters of type 'word'

u32         for CAPI parameters of type 'dword'

_cstruct    for CAPI parameters of type 'struct'
	    The member is a pointer to a buffer containing the parameter in
	    CAPI encoding (length + content). It may also be NULL, which will
	    be taken to represent an empty (zero length) parameter.
	    Subparameters are stored in encoded form within the content part.

_cmstruct   alternative representation for CAPI parameters of type 'struct'
	    (used only for the 'Additional Info' and 'B Protocol' parameters)
	    The representation is a single byte containing one of the values:
	    CAPI_DEFAULT: The parameter is empty/absent.
	    CAPI_COMPOSE: The parameter is present.
	    Subparameter values are stored individually in the corresponding
	    _cmsg structure members.
=========== =================================================================

5. Lower Layer Interface Functions
==================================

::

  int attach_capi_ctr(struct capi_ctr *ctrlr)
  int detach_capi_ctr(struct capi_ctr *ctrlr)

register/unregister a device (controller) with Kernel CAPI

::

  void capi_ctr_ready(struct capi_ctr *ctrlr)
  void capi_ctr_down(struct capi_ctr *ctrlr)

signal controller ready/not ready

::

  void capi_ctr_handle_message(struct capi_ctr * ctrlr, u16 applid,
			       struct sk_buff *skb)

pass a received CAPI message to Kernel CAPI
for forwarding to the specified application

6. Helper Functions and Macros
==============================

Macros to extract/set element values from/in a CAPI message header
(from <linux/isdn/capiutil.h>):

======================  =============================   ====================
Get Macro		Set Macro			Element (Type)
======================  =============================   ====================
CAPIMSG_LEN(m)		CAPIMSG_SETLEN(m, len)		Total Length (u16)
CAPIMSG_APPID(m)	CAPIMSG_SETAPPID(m, applid)	ApplID (u16)
CAPIMSG_COMMAND(m)	CAPIMSG_SETCOMMAND(m,cmd)	Command (u8)
CAPIMSG_SUBCOMMAND(m)	CAPIMSG_SETSUBCOMMAND(m, cmd)	Subcommand (u8)
CAPIMSG_CMD(m)		-				Command*256
							+ Subcommand (u16)
CAPIMSG_MSGID(m)	CAPIMSG_SETMSGID(m, msgid)	Message Number (u16)

CAPIMSG_CONTROL(m)	CAPIMSG_SETCONTROL(m, contr)	Controller/PLCI/NCCI
							(u32)
CAPIMSG_DATALEN(m)	CAPIMSG_SETDATALEN(m, len)	Data Length (u16)
======================  =============================   ====================

Library functions for working with _cmsg structures
(from <linux/isdn/capiutil.h>):

``char *capi_cmd2str(u8 Command, u8 Subcommand)``
	Returns the CAPI 2.0 message name corresponding to the given command
	and subcommand values, as a static ASCII string. The return value may
	be NULL if the command/subcommand is not one of those defined in the
	CAPI 2.0 standard.

7. Debugging
============

The module kernelcapi has a module parameter showcapimsgs controlling some
debugging output produced by the module. It can only be set when the module is
loaded, via a parameter "showcapimsgs=<n>" to the modprobe command, either on
the command line or in the configuration file.

If the lowest bit of showcapimsgs is set, kernelcapi logs controller and
application up and down events.

In addition, every registered CAPI controller has an associated traceflag
parameter controlling how CAPI messages sent from and to tha controller are
logged. The traceflag parameter is initialized with the value of the
showcapimsgs parameter when the controller is registered, but can later be
changed via the MANUFACTURER_REQ command KCAPI_CMD_TRACE.

If the value of traceflag is non-zero, CAPI messages are logged.
DATA_B3 messages are only logged if the value of traceflag is > 2.

If the lowest bit of traceflag is set, only the command/subcommand and message
length are logged. Otherwise, kernelcapi logs a readable representation of
the entire message.


code:
============
mISDN Driver
============

mISDN is a new modular ISDN driver, in the long term it should replace
the old I4L driver architecture for passiv ISDN cards.
It was designed to allow a broad range of applications and interfaces
but only have the basic function in kernel, the interface to the user
space is based on sockets with a own address family AF_ISDN.

repo_name: angery/linux


code:
# Simple Kconfig recursive issue
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#
# Test with:
#
# make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig
#
# This Kconfig file has a simple recursive dependency issue. In order to
# understand why this recursive dependency issue occurs lets consider what
# Kconfig needs to address. We iterate over what Kconfig needs to address
# by stepping through the questions it needs to address sequentially.
#
#  * What values are possible for CORE?
#
# CORE_BELL_A_ADVANCED selects CORE, which means that it influences the values
# that are possible for CORE. So for example if CORE_BELL_A_ADVANCED is 'y',
# CORE must be 'y' too.
#
#  * What influences CORE_BELL_A_ADVANCED ?
#
# As the name implies CORE_BELL_A_ADVANCED is an advanced feature of
# CORE_BELL_A so naturally it depends on CORE_BELL_A. So if CORE_BELL_A is 'y'
# we know CORE_BELL_A_ADVANCED can be 'y' too.
#
#   * What influences CORE_BELL_A ?
#
# CORE_BELL_A depends on CORE, so CORE influences CORE_BELL_A.
#
# But that is a problem, because this means that in order to determine
# what values are possible for CORE we ended up needing to address questions
# regarding possible values of CORE itself again. Answering the original
# question of what are the possible values of CORE would make the kconfig
# tools run in a loop. When this happens Kconfig exits and complains about
# the "recursive dependency detected" error.
#
# Reading the Documentation/kbuild/Kconfig.recursion-issue-01 file it may be
# obvious that an easy to solution to this problem should just be the removal
# of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already
# since CORE_BELL_A depends on CORE. Recursive dependency issues are not always
# so trivial to resolve, we provide another example below of practical
# implications of this recursive issue where the solution is perhaps not so
# easy to understand. Note that matching semantics on the dependency on
# CORE also consist of a solution to this recursive problem.

mainmenu "Simple example to demo kconfig recursive dependency issue"

config CORE
	tristate

config CORE_BELL_A
	tristate
	depends on CORE

config CORE_BELL_A_ADVANCED
	tristate
	depends on CORE_BELL_A
	select CORE


code:
# Cumulative Kconfig recursive issue
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#
# Test with:
#
# make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig
#
# The recursive limitations with Kconfig has some non intuitive implications on
# kconfig semantics which are documented here. One known practical implication
# of the recursive limitation is that drivers cannot negate features from other
# drivers if they share a common core requirement and use disjoint semantics to
# annotate those requirements, ie, some drivers use "depends on" while others
# use "select". For instance it means if a driver A and driver B share the same
# core requirement, and one uses "select" while the other uses "depends on" to
# annotate this, all features that driver A selects cannot now be negated by
# driver B.
#
# A perhaps not so obvious implication of this is that, if semantics on these
# core requirements are not carefully synced, as drivers evolve features
# they select or depend on end up becoming shared requirements which cannot be
# negated by other drivers.
#
# The example provided in Documentation/kbuild/Kconfig.recursion-issue-02
# describes a simple driver core layout of example features a kernel might
# have. Let's assume we have some CORE functionality, then the kernel has a
# series of bells and whistles it desires to implement, its not so advanced so
# it only supports bells at this time: CORE_BELL_A and CORE_BELL_B. If
# CORE_BELL_A has some advanced feature CORE_BELL_A_ADVANCED which selects
# CORE_BELL_A then CORE_BELL_A ends up becoming a common BELL feature which
# other bells in the system cannot negate. The reason for this issue is
# due to the disjoint use of semantics on expressing each bell's relationship
# with CORE, one uses "depends on" while the other uses "select". Another
# more important reason is that kconfig does not check for dependencies listed
# under 'select' for a symbol, when such symbols are selected kconfig them
# as mandatory required symbols. For more details on the heavy handed nature
# of select refer to Documentation/kbuild/Kconfig.select-break
#
# To fix this the "depends on CORE" must be changed to "select CORE", or the
# "select CORE" must be changed to "depends on CORE".
#
# For an example real world scenario issue refer to the attempt to remove
# "select FW_LOADER" [0], in the end the simple alternative solution to this
# problem consisted on matching semantics with newly introduced features.
#
# [0] https://lore.kernel.org/r/1432241149-8762-1-git-send-email-mcgrof@do-not-panic.com

mainmenu "Simple example to demo cumulative kconfig recursive dependency implication"

config CORE
	tristate

config CORE_BELL_A
	tristate
	depends on CORE

config CORE_BELL_A_ADVANCED
	tristate
	select CORE_BELL_A

config CORE_BELL_B
	tristate
	depends on !CORE_BELL_A
	select CORE


code:
# Select broken dependency issue
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#
# Test with:
#
# make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.select-break menuconfig
#
# kconfig will not complain and enable this layout for configuration. This is
# currently a feature of kconfig, given select was designed to be heavy handed.
# Kconfig currently does not check the list of symbols listed on a symbol's
# "select" list, this is done on purpose to help load a set of known required
# symbols. Because of this use of select should be used with caution. An
# example of this issue is below.
#
# The option B and C are clearly contradicting with respect to A.
# However, when A is set, C can be set as well because Kconfig does not
# visit the dependencies of the select target (in this case B).  And since
# Kconfig does not visit the dependencies, it breaks the dependencies of B
# (!A).

mainmenu "Simple example to demo kconfig select broken dependency issue"

config A
	bool "CONFIG A"

config B
	bool "CONFIG B"
	depends on !A

config C
	bool "CONFIG C"
	depends on A
	select B


code:
=========================
GCC plugin infrastructure
=========================

Introduction
============

GCC plugins are loadable modules that provide extra features to the
compiler [1]_. They are useful for runtime instrumentation and static analysis.
We can analyse, change and add further code during compilation via
callbacks [2]_, GIMPLE [3]_, IPA [4]_ and RTL passes [5]_.

The GCC plugin infrastructure of the kernel supports building out-of-tree
modules, cross-compilation and building in a separate directory.
Plugin source files have to be compilable by a C++ compiler.

Currently the GCC plugin infrastructure supports only some architectures.
Grep "select HAVE_GCC_PLUGINS" to find out which architectures support
GCC plugins.

This infrastructure was ported from grsecurity [6]_ and PaX [7]_.

--

.. [1] https://gcc.gnu.org/onlinedocs/gccint/Plugins.html
.. [2] https://gcc.gnu.org/onlinedocs/gccint/Plugin-API.html#Plugin-API
.. [3] https://gcc.gnu.org/onlinedocs/gccint/GIMPLE.html
.. [4] https://gcc.gnu.org/onlinedocs/gccint/IPA.html
.. [5] https://gcc.gnu.org/onlinedocs/gccint/RTL.html
.. [6] https://grsecurity.net/
.. [7] https://pax.grsecurity.net/

Purpose
=======

GCC plugins are designed to provide a place to experiment with potential
compiler features that are neither in GCC nor Clang upstream. Once
their utility is proven, the goal is to upstream the feature into GCC
(and Clang), and then to finally remove them from the kernel once the
feature is available in all supported versions of GCC.

Specifically, new plugins should implement only features that have no
upstream compiler support (in either GCC or Clang).

When a feature exists in Clang but not GCC, effort should be made to
bring the feature to upstream GCC (rather than just as a kernel-specific
GCC plugin), so the entire ecosystem can benefit from it.

Similarly, even if a feature provided by a GCC plugin does *not* exist
in Clang, but the feature is proven to be useful, effort should be spent
to upstream the feature to GCC (and Clang).

After a feature is available in upstream GCC, the plugin will be made
unbuildable for the corresponding GCC version (and later). Once all
kernel-supported versions of GCC provide the feature, the plugin will
be removed from the kernel.

Files
=====

**$(src)/scripts/gcc-plugins**

	This is the directory of the GCC plugins.

**$(src)/scripts/gcc-plugins/gcc-common.h**

	This is a compatibility header for GCC plugins.
	It should be always included instead of individual gcc headers.

**$(src)/scripts/gcc-plugins/gcc-generate-gimple-pass.h,
$(src)/scripts/gcc-plugins/gcc-generate-ipa-pass.h,
$(src)/scripts/gcc-plugins/gcc-generate-simple_ipa-pass.h,
$(src)/scripts/gcc-plugins/gcc-generate-rtl-pass.h**

	These headers automatically generate the registration structures for
	GIMPLE, SIMPLE_IPA, IPA and RTL passes.
	They should be preferred to creating the structures by hand.

Usage
=====

You must install the gcc plugin headers for your gcc version,
e.g., on Ubuntu for gcc-10::

	apt-get install gcc-10-plugin-dev

Or on Fedora::

	dnf install gcc-plugin-devel libmpc-devel

Or on Fedora when using cross-compilers that include plugins::

	dnf install libmpc-devel

Enable the GCC plugin infrastructure and some plugin(s) you want to use
in the kernel config::

	CONFIG_GCC_PLUGINS=y
	CONFIG_GCC_PLUGIN_LATENT_ENTROPY=y
	...

Run gcc (native or cross-compiler) to ensure plugin headers are detected::

	gcc -print-file-name=plugin
	CROSS_COMPILE=arm-linux-gnu- ${CROSS_COMPILE}gcc -print-file-name=plugin

The word "plugin" means they are not detected::

	plugin

A full path means they are detected::

       /usr/lib/gcc/x86_64-redhat-linux/12/plugin

To compile the minimum tool set including the plugin(s)::

	make scripts

or just run the kernel make and compile the whole kernel with
the cyclomatic complexity GCC plugin.

4. How to add a new GCC plugin
==============================

The GCC plugins are in scripts/gcc-plugins/. You need to put plugin source files
right under scripts/gcc-plugins/. Creating subdirectories is not supported.
It must be added to scripts/gcc-plugins/Makefile, scripts/Makefile.gcc-plugins
and a relevant Kconfig file.


code:
=============================================
Exporting kernel headers for use by userspace
=============================================

The "make headers_install" command exports the kernel's header files in a
form suitable for use by userspace programs.

The linux kernel's exported header files describe the API for user space
programs attempting to use kernel services.  These kernel header files are
used by the system's C library (such as glibc or uClibc) to define available
system calls, as well as constants and structures to be used with these
system calls.  The C library's header files include the kernel header files
from the "linux" subdirectory.  The system's libc headers are usually
installed at the default location /usr/include and the kernel headers in
subdirectories under that (most notably /usr/include/linux and
/usr/include/asm).

Kernel headers are backwards compatible, but not forwards compatible.  This
means that a program built against a C library using older kernel headers
should run on a newer kernel (although it may not have access to new
features), but a program built against newer kernel headers may not work on an
older kernel.

The "make headers_install" command can be run in the top level directory of the
kernel source code (or using a standard out-of-tree build).  It takes two
optional arguments::

  make headers_install ARCH=i386 INSTALL_HDR_PATH=/usr

ARCH indicates which architecture to produce headers for, and defaults to the
current architecture.  The linux/asm directory of the exported kernel headers
is platform-specific, to see a complete list of supported architectures use
the command::

  ls -d include/asm-* | sed 's/.*-//'

INSTALL_HDR_PATH indicates where to install the headers. It defaults to
"./usr".

An 'include' directory is automatically created inside INSTALL_HDR_PATH and
headers are installed in 'INSTALL_HDR_PATH/include'.

The kernel header export infrastructure is maintained by David Woodhouse
<dwmw2@infradead.org>.


code:
.. SPDX-License-Identifier: GPL-2.0

===================
Kernel Build System
===================

.. toctree::
    :maxdepth: 1

    kconfig-language
    kconfig-macro-language

    kbuild
    kconfig
    makefiles
    modules

    headers_install

    issues
    reproducible-builds
    gcc-plugins
    llvm

.. only::  subproject and html

   Indices
   =======

   * :ref:`genindex`


code:
================
Recursion issues
================

issue #1
--------

.. literalinclude:: Kconfig.recursion-issue-01
   :language: kconfig

issue #2
--------

.. literalinclude:: Kconfig.recursion-issue-02
   :language: kconfig


code:
======
Kbuild
======

Output files
============

modules.order
-------------
This file records the order in which modules appear in Makefiles. This
is used by modprobe to deterministically resolve aliases that match
multiple modules.

modules.builtin
---------------
This file lists all modules that are built into the kernel. This is used
by modprobe to not fail when trying to load something builtin.

modules.builtin.modinfo
-----------------------
This file contains modinfo from all modules that are built into the kernel.
Unlike modinfo of a separate module, all fields are prefixed with module name.

Environment variables
=====================

KCPPFLAGS
---------
Additional options to pass when preprocessing. The preprocessing options
will be used in all cases where kbuild does preprocessing including
building C files and assembler files.

KAFLAGS
-------
Additional options to the assembler (for built-in and modules).

AFLAGS_MODULE
-------------
Additional assembler options for modules.

AFLAGS_KERNEL
-------------
Additional assembler options for built-in.

KCFLAGS
-------
Additional options to the C compiler (for built-in and modules).

KRUSTFLAGS
----------
Additional options to the Rust compiler (for built-in and modules).

CFLAGS_KERNEL
-------------
Additional options for $(CC) when used to compile
code that is compiled as built-in.

CFLAGS_MODULE
-------------
Additional module specific options to use for $(CC).

RUSTFLAGS_KERNEL
----------------
Additional options for $(RUSTC) when used to compile
code that is compiled as built-in.

RUSTFLAGS_MODULE
----------------
Additional module specific options to use for $(RUSTC).

LDFLAGS_MODULE
--------------
Additional options used for $(LD) when linking modules.

HOSTCFLAGS
----------
Additional flags to be passed to $(HOSTCC) when building host programs.

HOSTCXXFLAGS
------------
Additional flags to be passed to $(HOSTCXX) when building host programs.

HOSTRUSTFLAGS
-------------
Additional flags to be passed to $(HOSTRUSTC) when building host programs.

HOSTLDFLAGS
-----------
Additional flags to be passed when linking host programs.

HOSTLDLIBS
----------
Additional libraries to link against when building host programs.

.. _userkbuildflags:

USERCFLAGS
----------
Additional options used for $(CC) when compiling userprogs.

USERLDFLAGS
-----------
Additional options used for $(LD) when linking userprogs. userprogs are linked
with CC, so $(USERLDFLAGS) should include "-Wl," prefix as applicable.

KBUILD_KCONFIG
--------------
Set the top-level Kconfig file to the value of this environment
variable.  The default name is "Kconfig".

KBUILD_VERBOSE
--------------
Set the kbuild verbosity. Can be assigned same values as "V=...".

See make help for the full list.

Setting "V=..." takes precedence over KBUILD_VERBOSE.

KBUILD_EXTMOD
-------------
Set the directory to look for the kernel source when building external
modules.

Setting "M=..." takes precedence over KBUILD_EXTMOD.

KBUILD_OUTPUT
-------------
Specify the output directory when building the kernel.

The output directory can also be specified using "O=...".

Setting "O=..." takes precedence over KBUILD_OUTPUT.

KBUILD_EXTRA_WARN
-----------------
Specify the extra build checks. The same value can be assigned by passing
W=... from the command line.

See `make help` for the list of the supported values.

Setting "W=..." takes precedence over KBUILD_EXTRA_WARN.

KBUILD_DEBARCH
--------------
For the deb-pkg target, allows overriding the normal heuristics deployed by
deb-pkg. Normally deb-pkg attempts to guess the right architecture based on
the UTS_MACHINE variable, and on some architectures also the kernel config.
The value of KBUILD_DEBARCH is assumed (not checked) to be a valid Debian
architecture.

ARCH
----
Set ARCH to the architecture to be built.

In most cases the name of the architecture is the same as the
directory name found in the arch/ directory.

But some architectures such as x86 and sparc have aliases.

- x86: i386 for 32 bit, x86_64 for 64 bit
- sh: sh for 32 bit, sh64 for 64 bit
- sparc: sparc32 for 32 bit, sparc64 for 64 bit

CROSS_COMPILE
-------------
Specify an optional fixed part of the binutils filename.
CROSS_COMPILE can be a part of the filename or the full path.

CROSS_COMPILE is also used for ccache in some setups.

CF
--
Additional options for sparse.

CF is often used on the command-line like this::

    make CF=-Wbitwise C=2

INSTALL_PATH
------------
INSTALL_PATH specifies where to place the updated kernel and system map
images. Default is /boot, but you can set it to other values.

INSTALLKERNEL
-------------
Install script called when using "make install".
The default name is "installkernel".

The script will be called with the following arguments:

   - $1 - kernel version
   - $2 - kernel image file
   - $3 - kernel map file
   - $4 - default install path (use root directory if blank)

The implementation of "make install" is architecture specific
and it may differ from the above.

INSTALLKERNEL is provided to enable the possibility to
specify a custom installer when cross compiling a kernel.

MODLIB
------
Specify where to install modules.
The default value is::

     $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE)

The value can be overridden in which case the default value is ignored.

INSTALL_MOD_PATH
----------------
INSTALL_MOD_PATH specifies a prefix to MODLIB for module directory
relocations required by build roots.  This is not defined in the
makefile but the argument can be passed to make if needed.

INSTALL_MOD_STRIP
-----------------
INSTALL_MOD_STRIP, if defined, will cause modules to be
stripped after they are installed.  If INSTALL_MOD_STRIP is '1', then
the default option --strip-debug will be used.  Otherwise,
INSTALL_MOD_STRIP value will be used as the options to the strip command.

INSTALL_HDR_PATH
----------------
INSTALL_HDR_PATH specifies where to install user space headers when
executing "make headers_*".

The default value is::

    $(objtree)/usr

$(objtree) is the directory where output files are saved.
The output directory is often set using "O=..." on the commandline.

The value can be overridden in which case the default value is ignored.

KBUILD_ABS_SRCTREE
--------------------------------------------------
Kbuild uses a relative path to point to the tree when possible. For instance,
when building in the source tree, the source tree path is '.'

Setting this flag requests Kbuild to use absolute path to the source tree.
There are some useful cases to do so, like when generating tag files with
absolute path entries etc.

KBUILD_SIGN_PIN
---------------
This variable allows a passphrase or PIN to be passed to the sign-file
utility when signing kernel modules, if the private key requires such.

KBUILD_MODPOST_WARN
-------------------
KBUILD_MODPOST_WARN can be set to avoid errors in case of undefined
symbols in the final module linking stage. It changes such errors
into warnings.

KBUILD_MODPOST_NOFINAL
----------------------
KBUILD_MODPOST_NOFINAL can be set to skip the final link of modules.
This is solely useful to speed up test compiles.

KBUILD_EXTRA_SYMBOLS
--------------------
For modules that use symbols from other modules.
See more details in modules.rst.

ALLSOURCE_ARCHS
---------------
For tags/TAGS/cscope targets, you can specify more than one arch
to be included in the databases, separated by blank space. E.g.::

    $ make ALLSOURCE_ARCHS="x86 mips arm" tags

To get all available archs you can also specify all. E.g.::

    $ make ALLSOURCE_ARCHS=all tags

KBUILD_BUILD_TIMESTAMP
----------------------
Setting this to a date string overrides the timestamp used in the
UTS_VERSION definition (uname -v in the running kernel). The value has to
be a string that can be passed to date -d. The default value
is the output of the date command at one point during build.

KBUILD_BUILD_USER, KBUILD_BUILD_HOST
------------------------------------
These two variables allow to override the user@host string displayed during
boot and in /proc/version. The default value is the output of the commands
whoami and host, respectively.

LLVM
----
If this variable is set to 1, Kbuild will use Clang and LLVM utilities instead
of GCC and GNU binutils to build the kernel.


code:
================
Kconfig Language
================

Introduction
------------

The configuration database is a collection of configuration options
organized in a tree structure::

	+- Code maturity level options
	|  +- Prompt for development and/or incomplete code/drivers
	+- General setup
	|  +- Networking support
	|  +- System V IPC
	|  +- BSD Process Accounting
	|  +- Sysctl support
	+- Loadable module support
	|  +- Enable loadable module support
	|     +- Set version information on all module symbols
	|     +- Kernel module loader
	+- ...

Every entry has its own dependencies. These dependencies are used
to determine the visibility of an entry. Any child entry is only
visible if its parent entry is also visible.

Menu entries
------------

Most entries define a config option; all other entries help to organize
them. A single configuration option is defined like this::

  config MODVERSIONS
	bool "Set version information on all module symbols"
	depends on MODULES
	help
	  Usually, modules have to be recompiled whenever you switch to a new
	  kernel.  ...

Every line starts with a key word and can be followed by multiple
arguments.  "config" starts a new config entry. The following lines
define attributes for this config option. Attributes can be the type of
the config option, input prompt, dependencies, help text and default
values. A config option can be defined multiple times with the same
name, but every definition can have only a single input prompt and the
type must not conflict.

Menu attributes
---------------

A menu entry can have a number of attributes. Not all of them are
applicable everywhere (see syntax).

- type definition: "bool"/"tristate"/"string"/"hex"/"int"

  Every config option must have a type. There are only two basic types:
  tristate and string; the other types are based on these two. The type
  definition optionally accepts an input prompt, so these two examples
  are equivalent::

	bool "Networking support"

  and::

	bool
	prompt "Networking support"

- input prompt: "prompt" <prompt> ["if" <expr>]

  Every menu entry can have at most one prompt, which is used to display
  to the user. Optionally dependencies only for this prompt can be added
  with "if".

- default value: "default" <expr> ["if" <expr>]

  A config option can have any number of default values. If multiple
  default values are visible, only the first defined one is active.
  Default values are not limited to the menu entry where they are
  defined. This means the default can be defined somewhere else or be
  overridden by an earlier definition.
  The default value is only assigned to the config symbol if no other
  value was set by the user (via the input prompt above). If an input
  prompt is visible the default value is presented to the user and can
  be overridden by him.
  Optionally, dependencies only for this default value can be added with
  "if".

 The default value deliberately defaults to 'n' in order to avoid bloating the
 build. With few exceptions, new config options should not change this. The
 intent is for "make oldconfig" to add as little as possible to the config from
 release to release.

 Note:
	Things that merit "default y/m" include:

	a) A new Kconfig option for something that used to always be built
	   should be "default y".

	b) A new gatekeeping Kconfig option that hides/shows other Kconfig
	   options (but does not generate any code of its own), should be
	   "default y" so people will see those other options.

	c) Sub-driver behavior or similar options for a driver that is
	   "default n". This allows you to provide sane defaults.

	d) Hardware or infrastructure that everybody expects, such as CONFIG_NET
	   or CONFIG_BLOCK. These are rare exceptions.

- type definition + default value::

	"def_bool"/"def_tristate" <expr> ["if" <expr>]

  This is a shorthand notation for a type definition plus a value.
  Optionally dependencies for this default value can be added with "if".

- dependencies: "depends on" <expr>

  This defines a dependency for this menu entry. If multiple
  dependencies are defined, they are connected with '&&'. Dependencies
  are applied to all other options within this menu entry (which also
  accept an "if" expression), so these two examples are equivalent::

	bool "foo" if BAR
	default y if BAR

  and::

	depends on BAR
	bool "foo"
	default y

- reverse dependencies: "select" <symbol> ["if" <expr>]

  While normal dependencies reduce the upper limit of a symbol (see
  below), reverse dependencies can be used to force a lower limit of
  another symbol. The value of the current menu symbol is used as the
  minimal value <symbol> can be set to. If <symbol> is selected multiple
  times, the limit is set to the largest selection.
  Reverse dependencies can only be used with boolean or tristate
  symbols.

  Note:
	select should be used with care. select will force
	a symbol to a value without visiting the dependencies.
	By abusing select you are able to select a symbol FOO even
	if FOO depends on BAR that is not set.
	In general use select only for non-visible symbols
	(no prompts anywhere) and for symbols with no dependencies.
	That will limit the usefulness but on the other hand avoid
	the illegal configurations all over.

- weak reverse dependencies: "imply" <symbol> ["if" <expr>]

  This is similar to "select" as it enforces a lower limit on another
  symbol except that the "implied" symbol's value may still be set to n
  from a direct dependency or with a visible prompt.

  Given the following example::

    config FOO
	tristate "foo"
	imply BAZ

    config BAZ
	tristate "baz"
	depends on BAR

  The following values are possible:

	===		===		=============	==============
	FOO		BAR		BAZ's default	choice for BAZ
	===		===		=============	==============
	n		y		n		N/m/y
	m		y		m		M/y/n
	y		y		y		Y/m/n
	n		m		n		N/m
	m		m		m		M/n
	y		m		m		M/n
	y		n		*		N
	===		===		=============	==============

  This is useful e.g. with multiple drivers that want to indicate their
  ability to hook into a secondary subsystem while allowing the user to
  configure that subsystem out without also having to unset these drivers.

  Note: If the combination of FOO=y and BAR=m causes a link error,
  you can guard the function call with IS_REACHABLE()::

	foo_init()
	{
		if (IS_REACHABLE(CONFIG_BAZ))
			baz_register(&foo);
		...
	}

  Note: If the feature provided by BAZ is highly desirable for FOO,
  FOO should imply not only BAZ, but also its dependency BAR::

    config FOO
	tristate "foo"
	imply BAR
	imply BAZ

- limiting menu display: "visible if" <expr>

  This attribute is only applicable to menu blocks, if the condition is
  false, the menu block is not displayed to the user (the symbols
  contained there can still be selected by other symbols, though). It is
  similar to a conditional "prompt" attribute for individual menu
  entries. Default value of "visible" is true.

- numerical ranges: "range" <symbol> <symbol> ["if" <expr>]

  This allows to limit the range of possible input values for int
  and hex symbols. The user can only input a value which is larger than
  or equal to the first symbol and smaller than or equal to the second
  symbol.

- help text: "help"

  This defines a help text. The end of the help text is determined by
  the indentation level, this means it ends at the first line which has
  a smaller indentation than the first line of the help text.

- module attribute: "modules"
  This declares the symbol to be used as the MODULES symbol, which
  enables the third modular state for all config symbols.
  At most one symbol may have the "modules" option set.

Menu dependencies
-----------------

Dependencies define the visibility of a menu entry and can also reduce
the input range of tristate symbols. The tristate logic used in the
expressions uses one more state than normal boolean logic to express the
module state. Dependency expressions have the following syntax::

  <expr> ::= <symbol>                           (1)
           <symbol> '=' <symbol>                (2)
           <symbol> '!=' <symbol>               (3)
           <symbol1> '<' <symbol2>              (4)
           <symbol1> '>' <symbol2>              (4)
           <symbol1> '<=' <symbol2>             (4)
           <symbol1> '>=' <symbol2>             (4)
           '(' <expr> ')'                       (5)
           '!' <expr>                           (6)
           <expr> '&&' <expr>                   (7)
           <expr> '||' <expr>                   (8)

Expressions are listed in decreasing order of precedence.

(1) Convert the symbol into an expression. Boolean and tristate symbols
    are simply converted into the respective expression values. All
    other symbol types result in 'n'.
(2) If the values of both symbols are equal, it returns 'y',
    otherwise 'n'.
(3) If the values of both symbols are equal, it returns 'n',
    otherwise 'y'.
(4) If value of <symbol1> is respectively lower, greater, lower-or-equal,
    or greater-or-equal than value of <symbol2>, it returns 'y',
    otherwise 'n'.
(5) Returns the value of the expression. Used to override precedence.
(6) Returns the result of (2-/expr/).
(7) Returns the result of min(/expr/, /expr/).
(8) Returns the result of max(/expr/, /expr/).

An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
respectively for calculations). A menu entry becomes visible when its
expression evaluates to 'm' or 'y'.

There are two types of symbols: constant and non-constant symbols.
Non-constant symbols are the most common ones and are defined with the
'config' statement. Non-constant symbols consist entirely of alphanumeric
characters or underscores.
Constant symbols are only part of expressions. Constant symbols are
always surrounded by single or double quotes. Within the quote, any
other character is allowed and the quotes can be escaped using '\'.

Menu structure
--------------

The position of a menu entry in the tree is determined in two ways. First
it can be specified explicitly::

  menu "Network device support"
	depends on NET

  config NETDEVICES
	...

  endmenu

All entries within the "menu" ... "endmenu" block become a submenu of
"Network device support". All subentries inherit the dependencies from
the menu entry, e.g. this means the dependency "NET" is added to the
dependency list of the config option NETDEVICES.

The other way to generate the menu structure is done by analyzing the
dependencies. If a menu entry somehow depends on the previous entry, it
can be made a submenu of it. First, the previous (parent) symbol must
be part of the dependency list and then one of these two conditions
must be true:

- the child entry must become invisible, if the parent is set to 'n'
- the child entry must only be visible, if the parent is visible::

    config MODULES
	bool "Enable loadable module support"

    config MODVERSIONS
	bool "Set version information on all module symbols"
	depends on MODULES

    comment "module support disabled"
	depends on !MODULES

MODVERSIONS directly depends on MODULES, this means it's only visible if
MODULES is different from 'n'. The comment on the other hand is only
visible when MODULES is set to 'n'.

Kconfig syntax
--------------

The configuration file describes a series of menu entries, where every
line starts with a keyword (except help texts). The following keywords
end a menu entry:

- config
- menuconfig
- choice/endchoice
- comment
- menu/endmenu
- if/endif
- source

The first five also start the definition of a menu entry.

config::

	"config" <symbol>
	<config options>

This defines a config symbol <symbol> and accepts any of above
attributes as options.

menuconfig::

	"menuconfig" <symbol>
	<config options>

This is similar to the simple config entry above, but it also gives a
hint to front ends, that all suboptions should be displayed as a
separate list of options. To make sure all the suboptions will really
show up under the menuconfig entry and not outside of it, every item
from the <config options> list must depend on the menuconfig symbol.
In practice, this is achieved by using one of the next two constructs::

  (1):
  menuconfig M
  if M
      config C1
      config C2
  endif

  (2):
  menuconfig M
  config C1
      depends on M
  config C2
      depends on M

In the following examples (3) and (4), C1 and C2 still have the M
dependency, but will not appear under menuconfig M anymore, because
of C0, which doesn't depend on M::

  (3):
  menuconfig M
      config C0
  if M
      config C1
      config C2
  endif

  (4):
  menuconfig M
  config C0
  config C1
      depends on M
  config C2
      depends on M

choices::

	"choice" [symbol]
	<choice options>
	<choice block>
	"endchoice"

This defines a choice group and accepts any of the above attributes as
options. A choice can only be of type bool or tristate.  If no type is
specified for a choice, its type will be determined by the type of
the first choice element in the group or remain unknown if none of the
choice elements have a type specified, as well.

While a boolean choice only allows a single config entry to be
selected, a tristate choice also allows any number of config entries
to be set to 'm'. This can be used if multiple drivers for a single
hardware exists and only a single driver can be compiled/loaded into
the kernel, but all drivers can be compiled as modules.

A choice accepts another option "optional", which allows to set the
choice to 'n' and no entry needs to be selected.
If no [symbol] is associated with a choice, then you can not have multiple
definitions of that choice. If a [symbol] is associated to the choice,
then you may define the same choice (i.e. with the same entries) in another
place.

comment::

	"comment" <prompt>
	<comment options>

This defines a comment which is displayed to the user during the
configuration process and is also echoed to the output files. The only
possible options are dependencies.

menu::

	"menu" <prompt>
	<menu options>
	<menu block>
	"endmenu"

This defines a menu block, see "Menu structure" above for more
information. The only possible options are dependencies and "visible"
attributes.

if::

	"if" <expr>
	<if block>
	"endif"

This defines an if block. The dependency expression <expr> is appended
to all enclosed menu entries.

source::

	"source" <prompt>

This reads the specified configuration file. This file is always parsed.

mainmenu::

	"mainmenu" <prompt>

This sets the config program's title bar if the config program chooses
to use it. It should be placed at the top of the configuration, before any
other statement.

'#' Kconfig source file comment:

An unquoted '#' character anywhere in a source file line indicates
the beginning of a source file comment.  The remainder of that line
is a comment.

Kconfig hints
-------------
This is a collection of Kconfig tips, most of which aren't obvious at
first glance and most of which have become idioms in several Kconfig
files.

Adding common features and make the usage configurable
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
It is a common idiom to implement a feature/functionality that are
relevant for some architectures but not all.
The recommended way to do so is to use a config variable named HAVE_*
that is defined in a common Kconfig file and selected by the relevant
architectures.
An example is the generic IOMAP functionality.

We would in lib/Kconfig see::

  # Generic IOMAP is used to ...
  config HAVE_GENERIC_IOMAP

  config GENERIC_IOMAP
	depends on HAVE_GENERIC_IOMAP && FOO

And in lib/Makefile we would see::

	obj-$(CONFIG_GENERIC_IOMAP) += iomap.o

For each architecture using the generic IOMAP functionality we would see::

  config X86
	select ...
	select HAVE_GENERIC_IOMAP
	select ...

Note: we use the existing config option and avoid creating a new
config variable to select HAVE_GENERIC_IOMAP.

Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is
introduced to overcome the limitation of select which will force a
config option to 'y' no matter the dependencies.
The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
situation where select forces a symbol equals to 'y'.

Adding features that need compiler support
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

There are several features that need compiler support. The recommended way
to describe the dependency on the compiler feature is to use "depends on"
followed by a test macro::

  config STACKPROTECTOR
	bool "Stack Protector buffer overflow detection"
	depends on $(cc-option,-fstack-protector)
	...

If you need to expose a compiler capability to makefiles and/or C source files,
`CC_HAS_` is the recommended prefix for the config option::

  config CC_HAS_FOO
	def_bool $(success,$(srctree)/scripts/cc-check-foo.sh $(CC))

Build as module only
~~~~~~~~~~~~~~~~~~~~
To restrict a component build to module-only, qualify its config symbol
with "depends on m".  E.g.::

  config FOO
	depends on BAR && m

limits FOO to module (=m) or disabled (=n).

Compile-testing
~~~~~~~~~~~~~~~
If a config symbol has a dependency, but the code controlled by the config
symbol can still be compiled if the dependency is not met, it is encouraged to
increase build coverage by adding an "|| COMPILE_TEST" clause to the
dependency. This is especially useful for drivers for more exotic hardware, as
it allows continuous-integration systems to compile-test the code on a more
common system, and detect bugs that way.
Note that compile-tested code should avoid crashing when run on a system where
the dependency is not met.

Architecture and platform dependencies
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Due to the presence of stubs, most drivers can now be compiled on most
architectures. However, this does not mean it makes sense to have all drivers
available everywhere, as the actual hardware may only exist on specific
architectures and platforms. This is especially true for on-SoC IP cores,
which may be limited to a specific vendor or SoC family.

To prevent asking the user about drivers that cannot be used on the system(s)
the user is compiling a kernel for, and if it makes sense, config symbols
controlling the compilation of a driver should contain proper dependencies,
limiting the visibility of the symbol to (a superset of) the platform(s) the
driver can be used on. The dependency can be an architecture (e.g. ARM) or
platform (e.g. ARCH_OMAP4) dependency. This makes life simpler not only for
distro config owners, but also for every single developer or user who
configures a kernel.

Such a dependency can be relaxed by combining it with the compile-testing rule
above, leading to:

  config FOO
	bool "Support for foo hardware"
	depends on ARCH_FOO_VENDOR || COMPILE_TEST

Kconfig recursive dependency limitations
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If you've hit the Kconfig error: "recursive dependency detected" you've run
into a recursive dependency issue with Kconfig, a recursive dependency can be
summarized as a circular dependency. The kconfig tools need to ensure that
Kconfig files comply with specified configuration requirements. In order to do
that kconfig must determine the values that are possible for all Kconfig
symbols, this is currently not possible if there is a circular relation
between two or more Kconfig symbols. For more details refer to the "Simple
Kconfig recursive issue" subsection below. Kconfig does not do recursive
dependency resolution; this has a few implications for Kconfig file writers.
We'll first explain why this issues exists and then provide an example
technical limitation which this brings upon Kconfig developers. Eager
developers wishing to try to address this limitation should read the next
subsections.

Simple Kconfig recursive issue
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Read: Documentation/kbuild/Kconfig.recursion-issue-01

Test with::

  make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig

Cumulative Kconfig recursive issue
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Read: Documentation/kbuild/Kconfig.recursion-issue-02

Test with::

  make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig

Practical solutions to kconfig recursive issue
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Developers who run into the recursive Kconfig issue have two options
at their disposal. We document them below and also provide a list of
historical issues resolved through these different solutions.

  a) Remove any superfluous "select FOO" or "depends on FOO"
  b) Match dependency semantics:

	b1) Swap all "select FOO" to "depends on FOO" or,

	b2) Swap all "depends on FOO" to "select FOO"

The resolution to a) can be tested with the sample Kconfig file
Documentation/kbuild/Kconfig.recursion-issue-01 through the removal
of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already
since CORE_BELL_A depends on CORE. At times it may not be possible to remove
some dependency criteria, for such cases you can work with solution b).

The two different resolutions for b) can be tested in the sample Kconfig file
Documentation/kbuild/Kconfig.recursion-issue-02.

Below is a list of examples of prior fixes for these types of recursive issues;
all errors appear to involve one or more "select" statements and one or more
"depends on".

============    ===================================
commit          fix
============    ===================================
06b718c01208    select A -> depends on A
c22eacfe82f9    depends on A -> depends on B
6a91e854442c    select A -> depends on A
118c565a8f2e    select A -> select B
f004e5594705    select A -> depends on A
c7861f37b4c6    depends on A -> (null)
80c69915e5fb    select A -> (null)              (1)
c2218e26c0d0    select A -> depends on A        (1)
d6ae99d04e1c    select A -> depends on A
95ca19cf8cbf    select A -> depends on A
8f057d7bca54    depends on A -> (null)
8f057d7bca54    depends on A -> select A
a0701f04846e    select A -> depends on A
0c8b92f7f259    depends on A -> (null)
e4e9e0540928    select A -> depends on A        (2)
7453ea886e87    depends on A > (null)           (1)
7b1fff7e4fdf    select A -> depends on A
86c747d2a4f0    select A -> depends on A
d9f9ab51e55e    select A -> depends on A
0c51a4d8abd6    depends on A -> select A        (3)
e98062ed6dc4    select A -> depends on A        (3)
91e5d284a7f1    select A -> (null)
============    ===================================

(1) Partial (or no) quote of error.
(2) That seems to be the gist of that fix.
(3) Same error.

Future kconfig work
~~~~~~~~~~~~~~~~~~~

Work on kconfig is welcomed on both areas of clarifying semantics and on
evaluating the use of a full SAT solver for it. A full SAT solver can be
desirable to enable more complex dependency mappings and / or queries,
for instance one possible use case for a SAT solver could be that of handling
the current known recursive dependency issues. It is not known if this would
address such issues but such evaluation is desirable. If support for a full SAT
solver proves too complex or that it cannot address recursive dependency issues
Kconfig should have at least clear and well defined semantics which also
addresses and documents limitations or requirements such as the ones dealing
with recursive dependencies.

Further work on both of these areas is welcomed on Kconfig. We elaborate
on both of these in the next two subsections.

Semantics of Kconfig
~~~~~~~~~~~~~~~~~~~~

The use of Kconfig is broad, Linux is now only one of Kconfig's users:
one study has completed a broad analysis of Kconfig use in 12 projects [0]_.
Despite its widespread use, and although this document does a reasonable job
in documenting basic Kconfig syntax a more precise definition of Kconfig
semantics is welcomed. One project deduced Kconfig semantics through
the use of the xconfig configurator [1]_. Work should be done to confirm if
the deduced semantics matches our intended Kconfig design goals.
Another project formalized a denotational semantics of a core subset of
the Kconfig language [10]_.

Having well defined semantics can be useful for tools for practical
evaluation of dependencies, for instance one such case was work to
express in boolean abstraction of the inferred semantics of Kconfig to
translate Kconfig logic into boolean formulas and run a SAT solver on this to
find dead code / features (always inactive), 114 dead features were found in
Linux using this methodology [1]_ (Section 8: Threats to validity).
The kismet tool, based on the semantics in [10]_, finds abuses of reverse
dependencies and has led to dozens of committed fixes to Linux Kconfig files [11]_.

Confirming this could prove useful as Kconfig stands as one of the leading
industrial variability modeling languages [1]_ [2]_. Its study would help
evaluate practical uses of such languages, their use was only theoretical
and real world requirements were not well understood. As it stands though
only reverse engineering techniques have been used to deduce semantics from
variability modeling languages such as Kconfig [3]_.

.. [0] https://www.eng.uwaterloo.ca/~shshe/kconfig_semantics.pdf
.. [1] https://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf
.. [2] https://gsd.uwaterloo.ca/sites/default/files/ase241-berger_0.pdf
.. [3] https://gsd.uwaterloo.ca/sites/default/files/icse2011.pdf

Full SAT solver for Kconfig
~~~~~~~~~~~~~~~~~~~~~~~~~~~

Although SAT solvers [4]_ haven't yet been used by Kconfig directly, as noted
in the previous subsection, work has been done however to express in boolean
abstraction the inferred semantics of Kconfig to translate Kconfig logic into
boolean formulas and run a SAT solver on it [5]_. Another known related project
is CADOS [6]_ (former VAMOS [7]_) and the tools, mainly undertaker [8]_, which
has been introduced first with [9]_.  The basic concept of undertaker is to
extract variability models from Kconfig and put them together with a
propositional formula extracted from CPP #ifdefs and build-rules into a SAT
solver in order to find dead code, dead files, and dead symbols. If using a SAT
solver is desirable on Kconfig one approach would be to evaluate repurposing
such efforts somehow on Kconfig. There is enough interest from mentors of
existing projects to not only help advise how to integrate this work upstream
but also help maintain it long term. Interested developers should visit:

https://kernelnewbies.org/KernelProjects/kconfig-sat

.. [4] https://www.cs.cornell.edu/~sabhar/chapters/SATSolvers-KR-Handbook.pdf
.. [5] https://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf
.. [6] https://cados.cs.fau.de
.. [7] https://vamos.cs.fau.de
.. [8] https://undertaker.cs.fau.de
.. [9] https://www4.cs.fau.de/Publications/2011/tartler_11_eurosys.pdf
.. [10] https://paulgazzillo.com/papers/esecfse21.pdf
.. [11] https://github.com/paulgazz/kmax


code:
======================
Kconfig macro language
======================

Concept
-------

The basic idea was inspired by Make. When we look at Make, we notice sort of
two languages in one. One language describes dependency graphs consisting of
targets and prerequisites. The other is a macro language for performing textual
substitution.

There is clear distinction between the two language stages. For example, you
can write a makefile like follows::

    APP := foo
    SRC := foo.c
    CC := gcc

    $(APP): $(SRC)
            $(CC) -o $(APP) $(SRC)

The macro language replaces the variable references with their expanded form,
and handles as if the source file were input like follows::

    foo: foo.c
            gcc -o foo foo.c

Then, Make analyzes the dependency graph and determines the targets to be
updated.

The idea is quite similar in Kconfig - it is possible to describe a Kconfig
file like this::

    CC := gcc

    config CC_HAS_FOO
            def_bool $(shell, $(srctree)/scripts/gcc-check-foo.sh $(CC))

The macro language in Kconfig processes the source file into the following
intermediate::

    config CC_HAS_FOO
            def_bool y

Then, Kconfig moves onto the evaluation stage to resolve inter-symbol
dependency as explained in kconfig-language.rst.

Variables
---------

Like in Make, a variable in Kconfig works as a macro variable.  A macro
variable is expanded "in place" to yield a text string that may then be
expanded further. To get the value of a variable, enclose the variable name in
$( ). The parentheses are required even for single-letter variable names; $X is
a syntax error. The curly brace form as in ${CC} is not supported either.

There are two types of variables: simply expanded variables and recursively
expanded variables.

A simply expanded variable is defined using the := assignment operator. Its
righthand side is expanded immediately upon reading the line from the Kconfig
file.

A recursively expanded variable is defined using the = assignment operator.
Its righthand side is simply stored as the value of the variable without
expanding it in any way. Instead, the expansion is performed when the variable
is used.

There is another type of assignment operator; += is used to append text to a
variable. The righthand side of += is expanded immediately if the lefthand
side was originally defined as a simple variable. Otherwise, its evaluation is
deferred.

The variable reference can take parameters, in the following form::

  $(name,arg1,arg2,arg3)

You can consider the parameterized reference as a function. (more precisely,
"user-defined function" in contrast to "built-in function" listed below).

Useful functions must be expanded when they are used since the same function is
expanded differently if different parameters are passed. Hence, a user-defined
function is defined using the = assignment operator. The parameters are
referenced within the body definition with $(1), $(2), etc.

In fact, recursively expanded variables and user-defined functions are the same
internally. (In other words, "variable" is "function with zero argument".)
When we say "variable" in a broad sense, it includes "user-defined function".

Built-in functions
------------------

Like Make, Kconfig provides several built-in functions. Every function takes a
particular number of arguments.

In Make, every built-in function takes at least one argument. Kconfig allows
zero argument for built-in functions, such as $(filename), $(lineno). You could
consider those as "built-in variable", but it is just a matter of how we call
it after all. Let's say "built-in function" here to refer to natively supported
functionality.

Kconfig currently supports the following built-in functions.

 - $(shell,command)

  The "shell" function accepts a single argument that is expanded and passed
  to a subshell for execution. The standard output of the command is then read
  and returned as the value of the function. Every newline in the output is
  replaced with a space. Any trailing newlines are deleted. The standard error
  is not returned, nor is any program exit status.

 - $(info,text)

  The "info" function takes a single argument and prints it to stdout.
  It evaluates to an empty string.

 - $(warning-if,condition,text)

  The "warning-if" function takes two arguments. If the condition part is "y",
  the text part is sent to stderr. The text is prefixed with the name of the
  current Kconfig file and the current line number.

 - $(error-if,condition,text)

  The "error-if" function is similar to "warning-if", but it terminates the
  parsing immediately if the condition part is "y".

 - $(filename)

  The 'filename' takes no argument, and $(filename) is expanded to the file
  name being parsed.

 - $(lineno)

  The 'lineno' takes no argument, and $(lineno) is expanded to the line number
  being parsed.

Make vs Kconfig
---------------

Kconfig adopts Make-like macro language, but the function call syntax is
slightly different.

A function call in Make looks like this::

  $(func-name arg1,arg2,arg3)

The function name and the first argument are separated by at least one
whitespace. Then, leading whitespaces are trimmed from the first argument,
while whitespaces in the other arguments are kept. You need to use a kind of
trick to start the first parameter with spaces. For example, if you want
to make "info" function print "  hello", you can write like follows::

  empty :=
  space := $(empty) $(empty)
  $(info $(space)$(space)hello)

Kconfig uses only commas for delimiters, and keeps all whitespaces in the
function call. Some people prefer putting a space after each comma delimiter::

  $(func-name, arg1, arg2, arg3)

In this case, "func-name" will receive " arg1", " arg2", " arg3". The presence
of leading spaces may matter depending on the function. The same applies to
Make - for example, $(subst .c, .o, $(sources)) is a typical mistake; it
replaces ".c" with " .o".

In Make, a user-defined function is referenced by using a built-in function,
'call', like this::

    $(call my-func,arg1,arg2,arg3)

Kconfig invokes user-defined functions and built-in functions in the same way.
The omission of 'call' makes the syntax shorter.

In Make, some functions treat commas verbatim instead of argument separators.
For example, $(shell echo hello, world) runs the command "echo hello, world".
Likewise, $(info hello, world) prints "hello, world" to stdout. You could say
this is _useful_ inconsistency.

In Kconfig, for simpler implementation and grammatical consistency, commas that
appear in the $( ) context are always delimiters. It means::

  $(shell, echo hello, world)

is an error because it is passing two parameters where the 'shell' function
accepts only one. To pass commas in arguments, you can use the following trick::

  comma := ,
  $(shell, echo hello$(comma) world)

Caveats
-------

A variable (or function) cannot be expanded across tokens. So, you cannot use
a variable as a shorthand for an expression that consists of multiple tokens.
The following works::

    RANGE_MIN := 1
    RANGE_MAX := 3

    config FOO
            int "foo"
            range $(RANGE_MIN) $(RANGE_MAX)

But, the following does not work::

    RANGES := 1 3

    config FOO
            int "foo"
            range $(RANGES)

A variable cannot be expanded to any keyword in Kconfig.  The following does
not work::

    MY_TYPE := tristate

    config FOO
            $(MY_TYPE) "foo"
            default y

Obviously from the design, $(shell command) is expanded in the textual
substitution phase. You cannot pass symbols to the 'shell' function.

The following does not work as expected::

    config ENDIAN_FLAG
            string
            default "-mbig-endian" if CPU_BIG_ENDIAN
            default "-mlittle-endian" if CPU_LITTLE_ENDIAN

    config CC_HAS_ENDIAN_FLAG
            def_bool $(shell $(srctree)/scripts/gcc-check-flag ENDIAN_FLAG)

Instead, you can do like follows so that any function call is statically
expanded::

    config CC_HAS_ENDIAN_FLAG
            bool
            default $(shell $(srctree)/scripts/gcc-check-flag -mbig-endian) if CPU_BIG_ENDIAN
            default $(shell $(srctree)/scripts/gcc-check-flag -mlittle-endian) if CPU_LITTLE_ENDIAN


code:
===================
Kconfig make config
===================

This file contains some assistance for using `make *config`.

Use "make help" to list all of the possible configuration targets.

The xconfig ('qconf'), menuconfig ('mconf'), and nconfig ('nconf')
programs also have embedded help text.  Be sure to check that for
navigation, search, and other general help text.

General
-------

New kernel releases often introduce new config symbols.  Often more
important, new kernel releases may rename config symbols.  When
this happens, using a previously working .config file and running
"make oldconfig" won't necessarily produce a working new kernel
for you, so you may find that you need to see what NEW kernel
symbols have been introduced.

To see a list of new config symbols, use::

	cp user/some/old.config .config
	make listnewconfig

and the config program will list any new symbols, one per line.

Alternatively, you can use the brute force method::

	make oldconfig
	scripts/diffconfig .config.old .config | less

----------------------------------------------------------------------

Environment variables for `*config`

KCONFIG_CONFIG
--------------
This environment variable can be used to specify a default kernel config
file name to override the default name of ".config".

KCONFIG_DEFCONFIG_LIST
----------------------

This environment variable specifies a list of config files which can be used
as a base configuration in case the .config does not exist yet. Entries in
the list are separated with whitespaces to each other, and the first one
that exists is used.

KCONFIG_OVERWRITECONFIG
-----------------------
If you set KCONFIG_OVERWRITECONFIG in the environment, Kconfig will not
break symlinks when .config is a symlink to somewhere else.

`CONFIG_`
---------
If you set `CONFIG_` in the environment, Kconfig will prefix all symbols
with its value when saving the configuration, instead of using the default,
`CONFIG_`.

----------------------------------------------------------------------

Environment variables for '{allyes/allmod/allno/rand}config'

KCONFIG_ALLCONFIG
-----------------
(partially based on lkml email from/by Rob Landley, re: miniconfig)

--------------------------------------------------

The allyesconfig/allmodconfig/allnoconfig/randconfig variants can also
use the environment variable KCONFIG_ALLCONFIG as a flag or a filename
that contains config symbols that the user requires to be set to a
specific value.  If KCONFIG_ALLCONFIG is used without a filename where
KCONFIG_ALLCONFIG == "" or KCONFIG_ALLCONFIG == "1", `make *config`
checks for a file named "all{yes/mod/no/def/random}.config"
(corresponding to the `*config` command that was used) for symbol values
that are to be forced.  If this file is not found, it checks for a
file named "all.config" to contain forced values.

This enables you to create "miniature" config (miniconfig) or custom
config files containing just the config symbols that you are interested
in.  Then the kernel config system generates the full .config file,
including symbols of your miniconfig file.

This 'KCONFIG_ALLCONFIG' file is a config file which contains
(usually a subset of all) preset config symbols.  These variable
settings are still subject to normal dependency checks.

Examples::

	KCONFIG_ALLCONFIG=custom-notebook.config make allnoconfig

or::

	KCONFIG_ALLCONFIG=mini.config make allnoconfig

or::

	make KCONFIG_ALLCONFIG=mini.config allnoconfig

These examples will disable most options (allnoconfig) but enable or
disable the options that are explicitly listed in the specified
mini-config files.

----------------------------------------------------------------------

Environment variables for 'randconfig'

KCONFIG_SEED
------------
You can set this to the integer value used to seed the RNG, if you want
to somehow debug the behaviour of the kconfig parser/frontends.
If not set, the current time will be used.

KCONFIG_PROBABILITY
-------------------
This variable can be used to skew the probabilities. This variable can
be unset or empty, or set to three different formats:

    =======================     ==================  =====================
	KCONFIG_PROBABILITY     y:n split           y:m:n split
    =======================     ==================  =====================
	unset or empty          50  : 50            33  : 33  : 34
	N                        N  : 100-N         N/2 : N/2 : 100-N
    [1] N:M                     N+M : 100-(N+M)      N  :  M  : 100-(N+M)
    [2] N:M:L                    N  : 100-N          M  :  L  : 100-(M+L)
    =======================     ==================  =====================

where N, M and L are integers (in base 10) in the range [0,100], and so
that:

    [1] N+M is in the range [0,100]

    [2] M+L is in the range [0,100]

Examples::

	KCONFIG_PROBABILITY=10
		10% of booleans will be set to 'y', 90% to 'n'
		5% of tristates will be set to 'y', 5% to 'm', 90% to 'n'
	KCONFIG_PROBABILITY=15:25
		40% of booleans will be set to 'y', 60% to 'n'
		15% of tristates will be set to 'y', 25% to 'm', 60% to 'n'
	KCONFIG_PROBABILITY=10:15:15
		10% of booleans will be set to 'y', 90% to 'n'
		15% of tristates will be set to 'y', 15% to 'm', 70% to 'n'

----------------------------------------------------------------------

Environment variables for 'syncconfig'

KCONFIG_NOSILENTUPDATE
----------------------
If this variable has a non-blank value, it prevents silent kernel
config updates (requires explicit updates).

KCONFIG_AUTOCONFIG
------------------
This environment variable can be set to specify the path & name of the
"auto.conf" file.  Its default value is "include/config/auto.conf".

KCONFIG_AUTOHEADER
------------------
This environment variable can be set to specify the path & name of the
"autoconf.h" (header) file.
Its default value is "include/generated/autoconf.h".

----------------------------------------------------------------------

menuconfig
----------

SEARCHING for CONFIG symbols

Searching in menuconfig:

	The Search function searches for kernel configuration symbol
	names, so you have to know something close to what you are
	looking for.

	Example::

		/hotplug
		This lists all config symbols that contain "hotplug",
		e.g., HOTPLUG_CPU, MEMORY_HOTPLUG.

	For search help, enter / followed by TAB-TAB (to highlight
	<Help>) and Enter.  This will tell you that you can also use
	regular expressions (regexes) in the search string, so if you
	are not interested in MEMORY_HOTPLUG, you could try::

		/^hotplug

	When searching, symbols are sorted thus:

	  - first, exact matches, sorted alphabetically (an exact match
	    is when the search matches the complete symbol name);
	  - then, other matches, sorted alphabetically.

	For example: ^ATH.K matches:

	    ATH5K ATH9K ATH5K_AHB ATH5K_DEBUG [...] ATH6KL ATH6KL_DEBUG
	    [...] ATH9K_AHB ATH9K_BTCOEX_SUPPORT ATH9K_COMMON [...]

	of which only ATH5K and ATH9K match exactly and so are sorted
	first (and in alphabetical order), then come all other symbols,
	sorted in alphabetical order.

----------------------------------------------------------------------

User interface options for 'menuconfig'

MENUCONFIG_COLOR
----------------
It is possible to select different color themes using the variable
MENUCONFIG_COLOR.  To select a theme use::

	make MENUCONFIG_COLOR=<theme> menuconfig

Available themes are::

  - mono       => selects colors suitable for monochrome displays
  - blackbg    => selects a color scheme with black background
  - classic    => theme with blue background. The classic look
  - bluetitle  => a LCD friendly version of classic. (default)

MENUCONFIG_MODE
---------------
This mode shows all sub-menus in one large tree.

Example::

	make MENUCONFIG_MODE=single_menu menuconfig

----------------------------------------------------------------------

nconfig
-------

nconfig is an alternate text-based configurator.  It lists function
keys across the bottom of the terminal (window) that execute commands.
You can also just use the corresponding numeric key to execute the
commands unless you are in a data entry window.  E.g., instead of F6
for Save, you can just press 6.

Use F1 for Global help or F3 for the Short help menu.

Searching in nconfig:

	You can search either in the menu entry "prompt" strings
	or in the configuration symbols.

	Use / to begin a search through the menu entries.  This does
	not support regular expressions.  Use <Down> or <Up> for
	Next hit and Previous hit, respectively.  Use <Esc> to
	terminate the search mode.

	F8 (SymSearch) searches the configuration symbols for the
	given string or regular expression (regex).

NCONFIG_MODE
------------
This mode shows all sub-menus in one large tree.

Example::

	make NCONFIG_MODE=single_menu nconfig

----------------------------------------------------------------------

xconfig
-------

Searching in xconfig:

	The Search function searches for kernel configuration symbol
	names, so you have to know something close to what you are
	looking for.

	Example::

		Ctrl-F hotplug

	or::

		Menu: File, Search, hotplug

	lists all config symbol entries that contain "hotplug" in
	the symbol name.  In this Search dialog, you may change the
	config setting for any of the entries that are not grayed out.
	You can also enter a different search string without having
	to return to the main menu.

----------------------------------------------------------------------

gconfig
-------

Searching in gconfig:

	There is no search command in gconfig.  However, gconfig does
	have several different viewing choices, modes, and options.


code:
.. _kbuild_llvm:

==============================
Building Linux with Clang/LLVM
==============================

This document covers how to build the Linux kernel with Clang and LLVM
utilities.

About
-----

The Linux kernel has always traditionally been compiled with GNU toolchains
such as GCC and binutils. Ongoing work has allowed for `Clang
<https://clang.llvm.org/>`_ and `LLVM <https://llvm.org/>`_ utilities to be
used as viable substitutes. Distributions such as `Android
<https://www.android.com/>`_, `ChromeOS
<https://www.chromium.org/chromium-os>`_, and `OpenMandriva
<https://www.openmandriva.org/>`_ use Clang built kernels.  `LLVM is a
collection of toolchain components implemented in terms of C++ objects
<https://www.aosabook.org/en/llvm.html>`_. Clang is a front-end to LLVM that
supports C and the GNU C extensions required by the kernel, and is pronounced
"klang," not "see-lang."

Clang
-----

The compiler used can be swapped out via ``CC=`` command line argument to ``make``.
``CC=`` should be set when selecting a config and during a build. ::

	make CC=clang defconfig

	make CC=clang

Cross Compiling
---------------

A single Clang compiler binary will typically contain all supported backends,
which can help simplify cross compiling. ::

	make ARCH=arm64 CC=clang CROSS_COMPILE=aarch64-linux-gnu-

``CROSS_COMPILE`` is not used to prefix the Clang compiler binary, instead
``CROSS_COMPILE`` is used to set a command line flag: ``--target=<triple>``. For
example: ::

	clang --target=aarch64-linux-gnu foo.c

LLVM Utilities
--------------

LLVM has substitutes for GNU binutils utilities. They can be enabled individually.
The full list of supported make variables::

	make CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \
	  OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump READELF=llvm-readelf \
	  HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar HOSTLD=ld.lld

To simplify the above command, Kbuild supports the ``LLVM`` variable::

	make LLVM=1

If your LLVM tools are not available in your PATH, you can supply their
location using the LLVM variable with a trailing slash::

	make LLVM=/path/to/llvm/

which will use ``/path/to/llvm/clang``, ``/path/to/llvm/ld.lld``, etc.

If your LLVM tools have a version suffix and you want to test with that
explicit version rather than the unsuffixed executables like ``LLVM=1``, you
can pass the suffix using the ``LLVM`` variable::

	make LLVM=-14

which will use ``clang-14``, ``ld.lld-14``, etc.

``LLVM=0`` is not the same as omitting ``LLVM`` altogether, it will behave like
``LLVM=1``. If you only wish to use certain LLVM utilities, use their respective
make variables.

The integrated assembler is enabled by default. You can pass ``LLVM_IAS=0`` to
disable it.

Omitting CROSS_COMPILE
----------------------

As explained above, ``CROSS_COMPILE`` is used to set ``--target=<triple>``.

If ``CROSS_COMPILE`` is not specified, the ``--target=<triple>`` is inferred
from ``ARCH``.

That means if you use only LLVM tools, ``CROSS_COMPILE`` becomes unnecessary.

For example, to cross-compile the arm64 kernel::

	make ARCH=arm64 LLVM=1

If ``LLVM_IAS=0`` is specified, ``CROSS_COMPILE`` is also used to derive
``--prefix=<path>`` to search for the GNU assembler and linker. ::

	make ARCH=arm64 LLVM=1 LLVM_IAS=0 CROSS_COMPILE=aarch64-linux-gnu-

Supported Architectures
-----------------------

LLVM does not target all of the architectures that Linux supports and
just because a target is supported in LLVM does not mean that the kernel
will build or work without any issues. Below is a general summary of
architectures that currently work with ``CC=clang`` or ``LLVM=1``. Level
of support corresponds to "S" values in the MAINTAINERS files. If an
architecture is not present, it either means that LLVM does not target
it or there are known issues. Using the latest stable version of LLVM or
even the development tree will generally yield the best results.
An architecture's ``defconfig`` is generally expected to work well,
certain configurations may have problems that have not been uncovered
yet. Bug reports are always welcome at the issue tracker below!

.. list-table::
   :widths: 10 10 10
   :header-rows: 1

   * - Architecture
     - Level of support
     - ``make`` command
   * - arm
     - Supported
     - ``LLVM=1``
   * - arm64
     - Supported
     - ``LLVM=1``
   * - hexagon
     - Maintained
     - ``LLVM=1``
   * - mips
     - Maintained
     - ``LLVM=1``
   * - powerpc
     - Maintained
     - ``CC=clang``
   * - riscv
     - Maintained
     - ``LLVM=1``
   * - s390
     - Maintained
     - ``CC=clang``
   * - um (User Mode)
     - Maintained
     - ``LLVM=1``
   * - x86
     - Supported
     - ``LLVM=1``

Getting Help
------------

- `Website <https://clangbuiltlinux.github.io/>`_
- `Mailing List <https://lore.kernel.org/llvm/>`_: <llvm@lists.linux.dev>
- `Old Mailing List Archives <https://groups.google.com/g/clang-built-linux>`_
- `Issue Tracker <https://github.com/ClangBuiltLinux/linux/issues>`_
- IRC: #clangbuiltlinux on irc.libera.chat
- `Telegram <https://t.me/ClangBuiltLinux>`_: @ClangBuiltLinux
- `Wiki <https://github.com/ClangBuiltLinux/linux/wiki>`_
- `Beginner Bugs <https://github.com/ClangBuiltLinux/linux/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22>`_

.. _getting_llvm:

Getting LLVM
-------------

- https://releases.llvm.org/download.html
- https://github.com/llvm/llvm-project
- https://llvm.org/docs/GettingStarted.html
- https://llvm.org/docs/CMake.html
- https://apt.llvm.org/
- https://www.archlinux.org/packages/extra/x86_64/llvm/
- https://github.com/ClangBuiltLinux/tc-build
- https://github.com/ClangBuiltLinux/linux/wiki/Building-Clang-from-source
- https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/