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

Nicolas Schier
2022-05-05T20:36:59Z

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@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@fjasle.eu>