kernel: copy kernel build dir on Per-Device Initramfs compilation

To speedup compilation of Per-Device Initramfs, copy the kernel build
directory for each rootfs ID.

This permits concurrent execution of kernel build without conflicting
with each other at the expense of additional disk space usage.

To limit disk space usage, the copied kernel directory is deleted after
the Per-Device Initramfs image is generated and saved.

Link: https://github.com/openwrt/openwrt/pull/16404
Signed-off-by: Christian Marangi <[email protected]>

include/kernel-defaults.mk

@@ -153,13 +153,13 @@ endif
 # $1: image suffix
 # $2: Per Device Rootfs ID
 define Kernel/CopyImage
-	cmp -s $(LINUX_DIR)/vmlinux $(KERNEL_BUILD_DIR)/vmlinux$(1).debug$(2) || { \
-		$(KERNEL_CROSS)objcopy -O binary $(OBJCOPY_STRIP) -S $(LINUX_DIR)/vmlinux $(LINUX_KERNEL)$(1)$(2); \
-		$(KERNEL_CROSS)objcopy $(OBJCOPY_STRIP) -S $(LINUX_DIR)/vmlinux $(KERNEL_BUILD_DIR)/vmlinux$(1).elf$(2); \
-		$(CP) $(LINUX_DIR)/vmlinux $(KERNEL_BUILD_DIR)/vmlinux$(1).debug$(2); \
+	cmp -s $(LINUX_DIR)$(2)/vmlinux $(KERNEL_BUILD_DIR)/vmlinux$(1).debug$(2) || { \
+		$(KERNEL_CROSS)objcopy -O binary $(OBJCOPY_STRIP) -S $(LINUX_DIR)$(2)/vmlinux $(LINUX_KERNEL)$(1)$(2); \
+		$(KERNEL_CROSS)objcopy $(OBJCOPY_STRIP) -S $(LINUX_DIR)$(2)/vmlinux $(KERNEL_BUILD_DIR)/vmlinux$(1).elf$(2); \
+		$(CP) $(LINUX_DIR)$(2)/vmlinux $(KERNEL_BUILD_DIR)/vmlinux$(1).debug$(2); \
 		$(foreach k, \
 			$(if $(KERNEL_IMAGES),$(KERNEL_IMAGES),$(filter-out vmlinux dtbs,$(KERNELNAME))), \
-			$(CP) $(LINUX_DIR)/arch/$(LINUX_KARCH)/boot/$(IMAGES_DIR)/$(k) $(KERNEL_BUILD_DIR)/$(k)$(1)$(2); \
+			$(CP) $(LINUX_DIR)$(2)/arch/$(LINUX_KARCH)/boot/$(IMAGES_DIR)/$(k) $(KERNEL_BUILD_DIR)/$(k)$(1)$(2); \
 		) \
 	}
 endef
@@ -173,7 +173,9 @@ endef
 define Kernel/PrepareConfigPerRootfs
 	{ \
 		[ ! -d "$(1)" ] || rm -rf $(1); \
-		mkdir $(1) && $(CP) $(LINUX_DIR)/.config $(1); \
+		mkdir $(1) && $(CP) -T $(LINUX_DIR) $(1); \
+		touch $(1)/.config; \
+		rm -rf $(1)/usr/initramfs_data.cpio*; \
 	}
 endef
 
@@ -209,10 +211,9 @@ define Kernel/CompileImage/Initramfs
 			$(if $(CONFIG_TARGET_INITRAMFS_COMPRESSION_ZSTD), \
 				$(STAGING_DIR_HOST)/bin/zstd -T0 -f -o $(if $(2),$(LINUX_DIR)$(2),$(KERNEL_BUILD_DIR))/initrd.cpio.zstd $(if $(2),$(LINUX_DIR)$(2),$(KERNEL_BUILD_DIR))/initrd.cpio;) \
 			$(if $(2),,$(KERNEL_MAKE) $(KERNEL_MAKEOPTS_IMAGE) $(if $(KERNELNAME),$(KERNELNAME),all);),\
-			$(call locked,$(if $(2),$(CP) $(LINUX_DIR)$(2)/.config* $(LINUX_DIR) && touch $(LINUX_DIR)/.config && ) \
-				rm -rf $(LINUX_DIR)/usr/initramfs_data.cpio* $(LINUX_DIR)/.config.prev && \
-				$(KERNEL_MAKE) $(KERNEL_MAKEOPTS_IMAGE) $(if $(KERNELNAME),$(KERNELNAME),all), compile-initramfs);) \
+			$(KERNEL_MAKE) $(if $(2),-C $(LINUX_DIR)$(2)) $(KERNEL_MAKEOPTS_IMAGE) $(if $(KERNELNAME),$(KERNELNAME),all);) \
 		$(call Kernel/CopyImage,-initramfs,$(2)); \
+		$(if $(2),rm -rf $(LINUX_DIR)$(2);) \
 	}, gen-initramfs$(if $(2),-$(2)));
 endef
 else