[PATCH v3 14/15] kbuild: make built-in.a rule robust against too long argument error

[Nicolas Schier]

On Thu, May 05, 2022 at 04:22:43PM +0900 Masahiro Yamada wrote:
> Kbuild runs at the top of objtree instead of changing the working
> directory to subdirectories. I think this design is nice overall but
> some commands have a scapability issue.
> 
> The build command of built-in.a is one of them whose length scales with:
> 
>     O(D * N)
> 
> Here, D is the length of the directory path (i.e. $(obj)/ prefix),
> N is the number of objects in the Makefile, O() is the big O notation.
> 
> The deeper directory the Makefile directory is located, the more easily
> it will hit the too long argument error.
> 
> We can make it better. Trim the $(obj)/ by Make's builtin function, and
> restore it by a shell command (sed).
> 
> With this, the command length scales with:
> 
>     O(D + N)
> 
> In-tree modules still have some room to the limit (ARG_MAX=2097152),
> but this is more future-proof for big modules in a deep directory.
> 
> For example, you can build i915 as builtin (CONFIG_DRM_I915=y) and
> compare drivers/gpu/drm/i915/.built-in.a.cmd with/without this commit.
> 
> Signed-off-by: Masahiro Yamada <masahiroy at kernel.org>
> ---
> 
> (no changes since v2)
> 
> Changes in v2:
>   - New patch
> 
>  scripts/Makefile.build | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> index 0436ff94800e..cea48762299c 100644
> --- a/scripts/Makefile.build
> +++ b/scripts/Makefile.build
> @@ -370,7 +370,10 @@ $(subdir-modorder): $(obj)/%/modules.order: $(obj)/% ;
>  #
>  
>  quiet_cmd_ar_builtin = AR      $@
> -      cmd_ar_builtin = rm -f $@; $(AR) cDPrST $@ $(real-prereqs)
> +      cmd_ar_builtin = rm -f $@; \
> +		echo $(patsubst $(obj)/%,%,$(real-prereqs)) | \
> +		sed -E 's:([^ ]+):$(obj)/\1:g' | \
> +		xargs $(AR) cDPrST $@
>  
>  $(obj)/built-in.a: $(real-obj-y) FORCE
>  	$(call if_changed,ar_builtin)
> -- 
> 2.32.0

Reviewed-by: Nicolas Schier <nicolas at fjasle.eu>