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kbuild: make *.mod rule robust against too long argument error

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/

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com> # LLVM-14 (x86-64)

Masahiro Yamada c6031b1d cd968b97

+4 -2
+4 -2
scripts/Makefile.build
··· 275 275 $(call if_changed_rule,cc_o_c) 276 276 $(call cmd,force_checksrc) 277 277 278 - cmd_mod = echo $(addprefix $(obj)/, $(call real-search, $*.o, .o, -objs -y -m)) | \ 279 - $(AWK) -v RS='( |\n)' '!x[$$0]++' > $@ 278 + # To make this rule robust against "Argument list too long" error, 279 + # ensure to add $(obj)/ prefix by a shell command. 280 + cmd_mod = echo $(call real-search, $*.o, .o, -objs -y -m) | \ 281 + $(AWK) -v RS='( |\n)' '!x[$$0]++ { print("$(obj)/"$$0) }' > $@ 280 282 281 283 $(obj)/%.mod: FORCE 282 284 $(call if_changed,mod)