On 4/2/2021 10:11 PM, Masahiro Yamada wrote:
On Fri, Apr 2, 2021 at 7:45 AM Elliot Berman <eberman@xxxxxxxxxxxxxx> wrote:
If you update the timestamp of KCONFIG_CONFIG without actually changing
anything, config_data.gz is re-generated and causes vmlinux to re-link.
When Link Time Optimization is enabled, unnecessary re-linking of
vmlinux is highly desirable since it adds several minutes to build time.
Avoid touching config_data.gz by using filechk to compare the existing
config_data.gz and update only if it changed.
The .config can be touched, for instance, by a build script which
installs the default defconfig and then applies a defconfig fragment on
top.
For a simple example on my x86 machine, I modified x86 default defconfig to set
CONFIG_IKCONFIG=y and run:
make -j50 defconfig tiny.config vmlinux
make -j50 defconfig tiny.config vmlinux
With this patch, vmlinux is not re-built as a result of config_data.gz
change.
Signed-off-by: Elliot Berman <eberman@xxxxxxxxxxxxxx>
---
kernel/Makefile | 2 +-
scripts/Makefile.lib | 2 ++
2 files changed, 3 insertions(+), 1 deletion(-)
diff --git a/kernel/Makefile b/kernel/Makefile
index 320f1f3..bd4e558 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -140,7 +140,7 @@ $(obj)/configs.o: $(obj)/config_data.gz
targets += config_data.gz
$(obj)/config_data.gz: $(KCONFIG_CONFIG) FORCE
- $(call if_changed,gzip)
+ $(call filechk,gzip)
I do not think this is the right approach
because gzip is executed every time, even
if the time stamp is not changed.
Since .config is relatively small, gzip was quickly producing the same
binary in multiple runs on my host. I thought about following
gen_ikheaders.sh approach of comparing the md5sum, but I felt it was
more complex than using filechk and re-compressing every time. I'll send
a v2 tomorrow which follows gen_ikheaders.sh approach.
$(obj)/kheaders.o: $(obj)/kheaders_data.tar.xz
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index a4fbaf8..81d3ec1 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -282,6 +282,8 @@ cmd_objcopy = $(OBJCOPY) $(OBJCOPYFLAGS) $(OBJCOPYFLAGS_$(@F)) $< $@
quiet_cmd_gzip = GZIP $@
cmd_gzip = cat $(real-prereqs) | $(KGZIP) -n -f -9 > $@
+filechk_gzip = cat $(real-prereqs) | $(KGZIP) -n -f -9
+
# DTC
# ---------------------------------------------------------------------------
DTC ?= $(objtree)/scripts/dtc/dtc
--
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