Re: [PATCH 23/23] kbuild: allow to start building external module in any directory

[Nicolas Schier <nicolas@fjasle.eu>]

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@kernel.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@fjasle.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