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kernel: configure the kernel only when an input changed

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The configure stage rewrites the kernel configuration on every build. It
runs kconfig.pl twice and package-metadata.pl once, which takes an eighth of
a second. target/compile and target/install both pay it.

The rule carries FORCE because the kernel configuration also depends on the
package metadata, which is not a prerequisite. Add a check file that the
rule creates only when the configure stage must run, and make the stamp
depend on it. The check covers the kernel configuration files, .config, the
package metadata, the scripts that read them, the two build system files
that hold the recipe, the target directory and the user headers.

This takes the time of target/linux install on an unchanged tree from 1.33 s
to 1.15 s, and the time of a full no-op build from 3.68 s to 3.40 s for the
OpenWrt One.

Signed-off-by: John Crispin <john@phrozen.org>
This commit is contained in:
John Crispin committed 2026-09-22 18:18:58 +02:00
1 parent 746c76168c
commit e97e3f36bd
1 file changed
+13 -1
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include/kernel-build.mk
+13 -1
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@@ -81,6 +81,11 @@ ifeq ($(DUMP)$(filter prereq clean refresh update,$(MAKECMDGOALS)),)
endif
endif
KERNEL_CONFIG_CHECK:=$(LINUX_DIR)/.configured-check
KERNEL_CONFIG_DEPENDS:=$(LINUX_KCONFIG_LIST) $(TOPDIR)/.config $(TMP_DIR)/.packageinfo \
$(SCRIPT_DIR)/kconfig.pl $(SCRIPT_DIR)/package-metadata.pl $(SCRIPT_DIR)/metadata.pm \
$(INCLUDE_DIR)/kernel-defaults.mk $(INCLUDE_DIR)/kernel-build.mk $(CURDIR)
IMAGE_INSTALL_STAMP:=$(KERNEL_BUILD_DIR)/.image_install
IMAGE_INSTALL_INPUTS:=$(LINUX_DIR) $(STAGING_DIR)/image \
$(STAGING_DIR_HOST)/bin $(CURDIR) $(INCLUDE_DIR) $(SCRIPT_DIR) $(TOPDIR)/.config
@@ -149,7 +154,14 @@ define BuildKernel
) > $$@
$(call BuildTimeLog,end,compile)
$(STAMP_CONFIGURED): $(STAMP_PREPARED) $(LINUX_KCONFIG_LIST) $(TOPDIR)/.config FORCE
$(KERNEL_CONFIG_CHECK): $(STAMP_PREPARED) $(LINUX_KCONFIG_LIST) $(TOPDIR)/.config FORCE
@mkdir -p $(LINUX_DIR)
@[ -f $(STAMP_CONFIGURED) ] && [ -d $(LINUX_DIR)/user_headers ] && \
[ -z "$$$$(find $(KERNEL_CONFIG_DEPENDS) ! -type d \
-newer $(STAMP_CONFIGURED) -print -quit 2>/dev/null)" ] || \
touch $$@
$(STAMP_CONFIGURED): $(KERNEL_CONFIG_CHECK)
$(call BuildTimeLog,begin,configure)
$(Kernel/Configure)
touch $$@