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.
Unified · Split
| @@ -377,9 +377,14 @@ $(subdir-modorder): $(obj)/%/modules.order: $(obj)/% ; · 376 unmodified lines | ||
| 377 | 377 | # |
| 378 | 378 | # Rule to compile a set of .o files into one .a file (without symbol table) |
| 379 | 379 | # |
| 380 | +# To make this rule robust against "Argument list too long" error, | |
| 381 | +# remove $(obj)/ prefix, and restore it by a shell command. | |
| 380 | 382 | |
| 381 | 383 | quiet_cmd_ar_builtin = AR $@ |
| 382 | − cmd_ar_builtin = rm -f $@; $(AR) cDPrST $@ $(real-prereqs) | |
| 384 | + cmd_ar_builtin = rm -f $@; \ | |
| 385 | + echo $(patsubst $(obj)/%,%,$(real-prereqs)) | \ | |
| 386 | + sed -E 's:([^ ]+):$(obj)/\1:g' | \ | |
| 387 | + xargs $(AR) cDPrST $@ | |
| 383 | 388 | |
| 384 | 389 | $(obj)/built-in.a: $(real-obj-y) FORCE |
| 385 | 390 | $(call if_changed,ar_builtin) |