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Message-ID: <20241004-little-rigorous-ammonite-6a8ce2@lindesnes>
Date: Fri, 4 Oct 2024 22:05:20 +0200
From: Nicolas Schier <nicolas@...sle.eu>
To: Masahiro Yamada <masahiroy@...nel.org>
Cc: linux-kbuild@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
Nathan Chancellor <nathan@...nel.org>
Subject: Re: [PATCH 23/23] kbuild: allow to start building external module in
any directory
On Tue, Sep 17, 2024 at 11:16:51PM +0900, Masahiro Yamada wrote:
> Unless an explicit O= option is provided, external module builds must
> start from the kernel directory.
>
> This can be achieved by using the -C option:
>
> $ make -C /path/to/kernel M=/path/to/external/module
>
> This commit allows starting external module builds from any directory,
> so you can also do the following:
>
> $ make -f /path/to/kernel/Makefile M=/path/to/external/module
>
> The key difference is that the -C option changes the working directory
> and parses the Makefile located there, while the -f option only
> specifies the Makefile to use.
>
> As shown in the examples in Documentation/kbuild/modules.rst, external
> modules usually have a wrapper Makefile that allows you to build them
> without specifying any make arguments. The Makefile typically contains
> a rule as follows:
>
> KDIR ?= /path/to/kernel
> default:
> $(MAKE) -C $(KDIR) M=$(CURDIR) $(MAKECMDGOALS)
>
> The log will appear as follows:
>
> $ make
> make -C /path/to/kernel M=/path/to/external/module
> make[1]: Entering directory '/path/to/kernel'
> make[2]: Entering directory '/path/to/exernal/module'
> CC [M] helloworld.o
> MODPOST Module.symvers
> CC [M] helloworld.mod.o
> CC [M] .module-common.o
> LD [M] helloworld.ko
> make[2]: Leaving directory '/path/to/exernal/module'
s/exernal/external/, also above.
> make[1]: Leaving directory '/path/to/kernel'
>
> This changes the working directory twice because the -C option first
> switches to the kernel directory, and then Kbuild internally recurses
> back to the external module directory.
>
> With this commit, the wrapper Makefile can directly include the kernel
> Makefile:
>
> KDIR ?= /path/to/kernel
> export KBUILD_EXTMOD = $(CURDIR)
> include $(KDIR)/Makefile
>
> This avoids unnecessary sub-make invocations:
>
> $ make
> CC [M] helloworld.o
> MODPOST Module.symvers
> CC [M] helloworld.mod.o
> CC [M] .module-common.o
> LD [M] helloworld.ko
>
> Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> ---
>
> Documentation/kbuild/modules.rst | 21 +++++++++++++++++++++
> Makefile | 8 ++++++--
> 2 files changed, 27 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/kbuild/modules.rst b/Documentation/kbuild/modules.rst
> index 03347e13eeb5..5610cfa426bb 100644
> --- a/Documentation/kbuild/modules.rst
> +++ b/Documentation/kbuild/modules.rst
> @@ -88,6 +88,12 @@ executed to make module versioning work.
>
> $ make -C /lib/modules/`uname -r`/build M=$PWD modules_install
>
> + Starting from Linux 6.13, you can use the -f option instead of -C. This
> + will avoid unnecessary change of the working directory. The external
> + module will be output to the directory where you invoke make.
> +
> + $ make -f /lib/modules/`uname -r`/build/Makefile M=$PWD
> +
> 2.2 Options
> ===========
>
> @@ -246,6 +252,21 @@ module 8123.ko, which is built from the following files::
> consisting of several hundred lines, and here it really pays
> off to separate the kbuild part from the rest.
>
> + Linux 6.13 and later support another way. The external module Makefile
> + can include the kernel Makefile directly, rather than invoking sub Make.
> +
> + Example 3::
> +
> + --> filename: Kbuild
> + obj-m := 8123.o
> + 8123-y := 8123_if.o 8123_pci.o
> +
> + --> filename: Makefile
> + KDIR ?= /lib/modules/`uname -r`/build
KDIR ?= /lib/modules/$(shell uname -r)/build
This patch is another very good idea, thanks!
Reviewed-by: Nicolas Schier <nicolas@...sle.eu>
> + export KBUILD_EXTMOD = $(CURDIR)
> + include $(KDIR)/Makefile
> +
> +
> 3.4 Building Multiple Modules
> =============================
>
> diff --git a/Makefile b/Makefile
> index 81603a50c757..88c160ac7b3c 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -189,9 +189,13 @@ ifdef KBUILD_EXTMOD
> objtree := $(realpath $(KBUILD_OUTPUT))
> $(if $(objtree),,$(error specified kernel directory "$(KBUILD_OUTPUT)" does not exist))
> else
> - objtree := $(CURDIR)
> + objtree := $(abs_srctree)
> endif
> - output := $(or $(KBUILD_EXTMOD_OUTPUT),$(KBUILD_EXTMOD))
> + # If Make is invoked from the kernel directory (either kernel
> + # source directory or kernel build directory), external modules
> + # are built in $(KBUILD_EXTMOD) for backward compatibility,
> + # otherwise, built in the current directory.
> + output := $(or $(KBUILD_EXTMOD_OUTPUT),$(if $(filter $(CURDIR),$(objtree) $(abs_srctree)),$(KBUILD_EXTMOD)))
> # KBUILD_EXTMOD might be a relative path. Remember its absolute path before
> # Make changes the working directory.
> export abs_extmodtree := $(realpath $(KBUILD_EXTMOD))
> --
> 2.43.0
>
--
Nicolas
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