Like built-in.a, the command length of the *.mod rule scales with
the depth of the directory times the number of objects in the Makefile.

Add $(obj)/ by the shell command (awk) instead of by Make's builtin
function.

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 a module (CONFIG_DRM_I915=m) and
compare drivers/gpu/drm/i915/.i915.mod.cmd with/without this commit.

The issue is more critical for external modules because the M= path
can be very long as Jeff Johnson reported before [1].

[1] https://lore.kernel.org/linux-kbuild/4c02050c4e95e4cb8cc04282695f8404@codeaurora.org/ (local)

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <redacted>
Tested-by: Nathan Chancellor <nathan@kernel.org>
---

(no changes since v2)

Changes in v2:
  - New patch

 scripts/Makefile.build | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 8f1a355df7aa..f546b5f1f33f 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build

@@ -270,8 +270,8 @@ $(obj)/%.o: $(src)/%.c $(recordmcount_source) FORCE
 	$(call if_changed_rule,cc_o_c)
 	$(call cmd,force_checksrc)
 
-cmd_mod = echo $(addprefix $(obj)/, $(call real-search, $*.o, .o, -objs -y -m)) | \
-	$(AWK) -v RS='( |\n)' '!x[$$0]++' > $@
+cmd_mod = echo $(call real-search, $*.o, .o, -objs -y -m) | \
+	$(AWK) -v RS='( |\n)' '!x[$$0]++ { print("$(obj)/"$$0) }' > $@
 
 $(obj)/%.mod: FORCE
 	$(call if_changed,mod)

-- 
2.32.0