[openwrt/openwrt] build: kernel: avoid unnecessary package rebuilds

[LEDE Commits]

nbd pushed a commit to openwrt/openwrt.git, branch master:
https://git.openwrt.org/6b8b5e7248fd8562e8cdd5ef6f3d52f51418fc3b

commit 6b8b5e7248fd8562e8cdd5ef6f3d52f51418fc3b
Author: Eneas U de Queiroz <cotequeiroz at gmail.com>
AuthorDate: Fri Oct 15 10:30:05 2021 -0300

    build: kernel: avoid unnecessary package rebuilds
    
    The $(LINUX_DIR)/.config timesptamp changes between runs of
    make target/compile and make target/install (which builds the image).
    
    Kernel-dependent packages and out of tree modules are built in between
    those runs, and they check the .config timestamp to decide if they need
    to be rebuilt.
    
    Save the target/compile .config to use its timestamp if the file does
    not change between runs.  That way the subsequent kernel packages are
    not unnecessarily rebuilt when you run 'make' back to back.
    
    Signed-off-by: Eneas U de Queiroz <cotequeiroz at gmail.com>
---
 include/kernel-defaults.mk | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/include/kernel-defaults.mk b/include/kernel-defaults.mk
index f2864d6a10..77a612dbe1 100644
--- a/include/kernel-defaults.mk
+++ b/include/kernel-defaults.mk
@@ -128,6 +128,10 @@ endef
 define Kernel/CompileModules/Default
 	rm -f $(LINUX_DIR)/vmlinux $(LINUX_DIR)/System.map
 	+$(KERNEL_MAKE) $(if $(KERNELNAME),$(KERNELNAME),all) modules
+	# If .config did not change, use the previous timestamp to avoid package rebuilds
+	cmp -s $(LINUX_DIR)/.config $(LINUX_DIR)/.config.modules.save && \
+		mv $(LINUX_DIR)/.config.modules.save $(LINUX_DIR)/.config; \
+	$(CP) $(LINUX_DIR)/.config $(LINUX_DIR)/.config.modules.save
 endef
 
 OBJCOPY_STRIP = -R .reginfo -R .notes -R .note -R .comment -R .mdebug -R .note.gnu.build-id