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linux/Documentation/kbuild/kbuild.rst
Masahiro Yamada 2eb5d7f242 kbuild: doc: describe the -C option precisely for external module builds 
Building external modules is typically done using this command:

  $ make -C <KERNEL_DIR> M=<EXTMOD_DIR>

Here, <KERNEL_DIR> refers to the output directory where the kernel was
built, not the kernel source directory.

When the kernel is built in the source tree, there is no ambiguity, as
the output directory and the source directory are the same.

If the kernel was built in a separate build directory, <KERNEL_DIR>
should be the kernel output directory. Otherwise, Kbuild cannot locate
necessary build artifacts. This has been the method for building
external modules against a pre-built kernel in a separate directory
for over 20 years. [1]

If you pass the kernel source directory to the -C option, you must also
specify the kernel build directory using the O= option. This approach
works as well, though it results in a slightly longer command:

  $ make -C <KERNEL_SOURCE_DIR> O=<KERNEL_BUILD_DIR> M=<EXTMOD_DIR>

Some people mistakenly believe that O= should specify a build directory
for external modules when used together with M=. This commit adds more
clarification to Documentation/kbuild/kbuild.rst.

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/history/history.git/commit/?id=e321b2ec2eb2993b3d0116e5163c78ad923e3c54

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>
2024-09-24 03:07:21 +09:00

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======
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.
modules.builtin.ranges
----------------------
This file contains address offset ranges (per ELF section) for all modules
that are built into the kernel. Together with System.map, it can be used
to associate module names with symbols.
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.
This variable can also be used to point to the kernel output directory when
building external modules against a pre-built kernel in a separate build
directory. Please note that this does NOT specify the output directory for the
external modules themselves.
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.
KDOCFLAGS
---------
Specify extra (warning/error) flags for kernel-doc checks during the build,
see scripts/kernel-doc for which flags are supported. Note that this doesn't
(currently) apply to documentation builds.
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
- parisc: parisc64 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.
INSTALL_DTBS_PATH
-----------------
INSTALL_DTBS_PATH specifies where to install device tree blobs for
relocations required by build roots. This is not defined in the
makefile but the argument can be passed to make if needed.
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
IGNORE_DIRS
-----------
For tags/TAGS/cscope targets, you can choose which directories won't
be included in the databases, separated by blank space. E.g.::
$ make IGNORE_DIRS="drivers/gpu/drm/radeon tools" cscope
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.
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