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 scalability 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@xxxxxxxxxx>
Reviewed-by: Nicolas Schier <nicolas@xxxxxxxxx>
Tested-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Tested-by: Sedat Dilek <sedat.dilek@xxxxxxxxx> # LLVM-14 (x86-64)
---
Changes in v7:
- Add comment to the code
Changes in v2:
- New patch
scripts/Makefile.build | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 0135b4e7f78b..014ebee70fd6 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -376,9 +376,14 @@ $(subdir-modorder): $(obj)/%/modules.order: $(obj)/% ;
#
# Rule to compile a set of .o files into one .a file (without symbol table)
#
+# To make this rule robust against "Argument list too long" error,
+# remove $(obj)/ prefix, and restore it by a shell command.
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
