kbuild: generate autoksyms.h early

When doing a cold build, autoksyms.h starts empty, and is updated late
in the build process to have visibility over the symbols used by in-tree
drivers. But since the symbol whitelist is known upfront, it can be used
to pre-populate autoksyms.h and maximize the amount of code that can be
compiled to its final state in a single pass, hence reducing build time.

Do this by using gen_autoksyms.sh to initialize autoksyms.h instead of
creating an empty file.
Acked-by: Nicolas Pitre <nico@fluxnic.net>
Tested-by: Matthias Maennich <maennich@google.com>
Reviewed-by: Matthias Maennich <maennich@google.com>
Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -1066,9 +1066,12 @@ endif
 
 autoksyms_h := $(if $(CONFIG_TRIM_UNUSED_KSYMS), include/generated/autoksyms.h)
 
+quiet_cmd_autoksyms_h = GEN     $@
+      cmd_autoksyms_h = mkdir -p $(dir $@); \
+			$(CONFIG_SHELL) $(srctree)/scripts/gen_autoksyms.sh $@
+
 $(autoksyms_h):
-	$(Q)mkdir -p $(dir $@)
-	$(Q)touch $@
+	$(call cmd,autoksyms_h)
 
 ARCH_POSTLINK := $(wildcard $(srctree)/arch/$(SRCARCH)/Makefile.postlink)
 

scripts/gen_autoksyms.sh

@@ -39,7 +39,8 @@ cat > "$output_file" << EOT
 
 EOT
 
-sed 's/ko$/mod/' modules.order |
+[ -f modules.order ] && modlist=modules.order || modlist=/dev/null
+sed 's/ko$/mod/' $modlist |
 xargs -n1 sed -n -e '2{s/ /\n/g;/^$/!p;}' -- |
 cat - "$ksym_wl" |
 sort -u |