kbuild: update config_data.gz only when the content of .config is changed

BugLink: https://bugs.launchpad.net/bugs/1928825
commit 46b41d5dd8019b264717978c39c43313a524d033 upstream.

If the timestamp of the .config file is updated, config_data.gz is
regenerated, then vmlinux is re-linked. This occurs even if the content
of the .config has not changed at all.

This issue was mitigated by commit 67424f61f813 ("kconfig: do not write
.config if the content is the same"); Kconfig does not update the
.config when it ends up with the identical configuration.

The issue is remaining when the .config is created by *_defconfig with
some config fragment(s) applied on top.

This is typical for powerpc and mips, where several *_defconfig targets
are constructed by using merge_config.sh.

One workaround is to have the copy of the .config. The filechk rule
updates the copy, kernel/config_data, by checking the content instead
of the timestamp.

With this commit, the second run with the same configuration avoids
the needless rebuilds.

  $ make ARCH=mips defconfig all
   [ snip ]
  $ make ARCH=mips defconfig all
  *** Default configuration is based on target '32r2el_defconfig'
  Using ./arch/mips/configs/generic_defconfig as base
  Merging arch/mips/configs/generic/32r2.config
  Merging arch/mips/configs/generic/el.config
  Merging ./arch/mips/configs/generic/board-boston.config
  Merging ./arch/mips/configs/generic/board-ni169445.config
  Merging ./arch/mips/configs/generic/board-ocelot.config
  Merging ./arch/mips/configs/generic/board-ranchu.config
  Merging ./arch/mips/configs/generic/board-sead-3.config
  Merging ./arch/mips/configs/generic/board-xilfpga.config
  #
  # configuration written to .config
  #
    SYNC    include/config/auto.conf
    CALL    scripts/checksyscalls.sh
    CALL    scripts/atomic/check-atomics.sh
    CHK     include/generated/compile.h
    CHK     include/generated/autoksyms.h

Reported-by: Elliot Berman <eberman@codeaurora.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

diff --git a/kernel/.gitignore b/kernel/.gitignore
index 34d1e77ee9df85e98cceffcd93cf4ebe978f9e26..0f0dba6c1eedc2912f4933d2658387cf1580af70 100644 (file)
--- a/kernel/.gitignore
+++ b/kernel/.gitignore
@@ -1,4 +1,5 @@
 #
+/config_data
 # Generated files
 #
 kheaders.md5

diff --git a/kernel/Makefile b/kernel/Makefile
index 6aef13053a5c41914a01da9e4b6b8692feb2719f..d038b0de886e97a0351fcb8459512f9fe738630f 100644 (file)
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -122,10 +122,15 @@ KCOV_INSTRUMENT_stackleak.o := n
 
 $(obj)/configs.o: $(obj)/config_data.gz
 
-targets += config_data.gz
-$(obj)/config_data.gz: $(KCONFIG_CONFIG) FORCE
+targets += config_data config_data.gz
+$(obj)/config_data.gz: $(obj)/config_data FORCE
        $(call if_changed,gzip)
 
+filechk_cat = cat $<
+
+$(obj)/config_data: $(KCONFIG_CONFIG) FORCE
+       $(call filechk,cat)
+
 $(obj)/kheaders.o: $(obj)/kheaders_data.tar.xz
 
 quiet_cmd_genikh = CHK     $(obj)/kheaders_data.tar.xz