kbuild: use -fmacro-prefix-map to make __FILE__ a relative path

Linux commit a73619a845d5625079cc1b3b820f44c899618388

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 9da0763bdd82 ("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: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

1 files changed, 3 insertions, 0 deletions

diff --git a/Makefile b/Makefile
index 08ca8e2dac..e817771b25 100644
--- a/Makefile
+++ b/Makefile
@@ -486,6 +486,9 @@ CFLAGS += $(call cc-option,-Wdeclaration-after-statement,)
 # disable pointer signed / unsigned warnings in gcc 4.0
 CFLAGS += $(call cc-option,-Wno-pointer-sign,)
 
+# change __FILE__ to the relative path from the srctree
+CFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
+
 # KBUILD_IMAGE: Default barebox image to build
 # Depending on the architecture, this can be either compressed or not.
 # It will also include any necessary headers to be bootable.