[PATCH v4 3/3] kbuild: generate autoksyms.h early

[Quentin Perret]

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.

Signed-off-by: Quentin Perret <qperret@xxxxxxxxxx>
---
 Makefile                 | 7 +++++--
 scripts/gen_autoksyms.sh | 3 ++-
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/Makefile b/Makefile
index 84b71845c43f..17b7e7f441bd 100644
--- a/Makefile
+++ b/Makefile
@@ -1062,9 +1062,12 @@ 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)
 
diff --git a/scripts/gen_autoksyms.sh b/scripts/gen_autoksyms.sh
index 2cea433616a8..f52b93ad122c 100755
--- a/scripts/gen_autoksyms.sh
+++ b/scripts/gen_autoksyms.sh
@@ -32,7 +32,8 @@ 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 |
-- 
2.25.0.225.g125e21ebc7-goog