[PATCH 5/5] kbuild: update config_data.gz only when the content of .config is changed

[Masahiro Yamada <masahiroy@xxxxxxxxxx>]

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 actually not changed.

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 (ab)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@xxxxxxxxxxxxxx>
Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
---

 kernel/.gitignore | 1 +
 kernel/Makefile   | 9 +++++++--
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/.gitignore b/kernel/.gitignore
index 4dc1ffe9770b..c6b299a6b786 100644
--- a/kernel/.gitignore
+++ b/kernel/.gitignore
@@ -1,2 +1,3 @@
 # SPDX-License-Identifier: GPL-2.0-only
+/config_data
 /kheaders.md5
diff --git a/kernel/Makefile b/kernel/Makefile
index 320f1f3941b7..605ec3e70cb7 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -138,10 +138,15 @@ obj-$(CONFIG_SCF_TORTURE_TEST) += scftorture.o
 
 $(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
-- 
2.27.0