kbuild: use -fmacro-prefix-map to make __FILE__ a relative path
The __FILE__ macro is used everywhere in the kernel to locate the file printing the log message, such as WARN_ON(), etc. If the kernel is built out of tree, this can be a long absolute path, like this: WARNING: CPU: 1 PID: 1 at /path/to/build/directory/arch/arm64/kernel/foo.c:... This is because Kbuild runs in the objtree instead of the srctree, then __FILE__ is expanded to a file path prefixed with $(srctree)/. Commit 9da0763b ("kbuild: Use relative path when building in a subdir of the source tree") improved this to some extent; $(srctree) becomes ".." if the objtree is a child of the srctree. For other cases of out-of-tree build, __FILE__ is still the absolute path. It also means the kernel image depends on where it was built. A brand-new option from GCC, -fmacro-prefix-map, solves this problem. If your compiler supports it, __FILE__ is the relative path from the srctree regardless of O= option. This provides more readable log and more reproducible builds. Please note __FILE__ is always an absolute path for external modules. Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
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