From 6b8b5e7248fd8562e8cdd5ef6f3d52f51418fc3b Mon Sep 17 00:00:00 2001
From: Eneas U de Queiroz <cotequeiroz@gmail.com>
Date: Fri, 15 Oct 2021 10:30:05 -0300
Subject: [PATCH] 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@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
-- 
2.30.2