[PATCH v2 23/26] kbuild: make built-in.a rule robust against too long argument error

[Masahiro Yamada <masahiroy@kernel.org>]

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>
---

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 cc8f362f58e2..a9236add4864 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