[PATCH 3/4] kbuild: sink stdout from cmd for silent build

[Masahiro Yamada <masahiroy@kernel.org>]

When building with 'make -s', no output to stdout should be printed.

As Arnd Bergmann reported [1], mkimage shows the detailed information
of the generated images.

I think this should be done by the 'cmd' macro instead of by individual
scripts.

Insert 'exec >/dev/null;' in order to redirect stdout to /dev/null for
silent builds.

[Note about this implementation]

'exec >/dev/null;' may look somewhat tricky, but this has a reason.

At first, I tried this:

  cmd = @set -e; $(echo-cmd) $(cmd_$(1)) >/dev/null

Appending '>/dev/null' is a commonly used way for redirection, but it
would not work if $(cmd_$(1)) itself contains a redirection.

For example, cmd_wrap in scripts/Makefile.asm-generic redirects the
output from the 'echo' command into the target file.

'$(cmd_$(1)) >/dev/null' would be expanded into:

  echo "#include <asm-generic/$*.h>" > $@ >/dev/null

Then, the target file gets empty because the string will go to /dev/null
instead of $@.

Next, I tried this:

  cmd = @set -e; $(echo-cmd) { $(cmd_$(1)); } >/dev/null

The form above would be expanded into:

  { echo "#include <asm-generic/$*.h>" > $@; } >/dev/null

This works as expected. However, it would be a syntax error if
$(cmd_$(1)) is empty.

When CONFIG_TRIM_UNUSED_KSYMS is disabled, $(call cmd,gen_ksymdeps) in
scripts/Makefile.build would be expanded into:

  set -e;  { ; } >/dev/null

..., which causes an syntax error.

The following does not work for the same reason.

  cmd = @set -e; $(echo-cmd) ( $(cmd_$(1)) ) >/dev/null

So, finally I adopted:

  cmd = @set -e; $(echo-cmd) exec >/dev/null; $(cmd_$(1))

[1]: https://lore.kernel.org/lkml/20210514135752.2910387-1-arnd@kernel.org/

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---

 scripts/Kbuild.include | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
index 82dd1b65b7a8..f247e691562d 100644
--- a/scripts/Kbuild.include
+++ b/scripts/Kbuild.include
@@ -90,8 +90,13 @@ clean := -f $(srctree)/scripts/Makefile.clean obj
 echo-cmd = $(if $($(quiet)cmd_$(1)),\
 	echo '  $(call escsq,$($(quiet)cmd_$(1)))$(echo-why)';)
 
+# sink stdout for 'make -s'
+       redirect :=
+ quiet_redirect :=
+silent_redirect := exec >/dev/null;
+
 # printing commands
-cmd = @set -e; $(echo-cmd) $(cmd_$(1))
+cmd = @set -e; $(echo-cmd) $($(quiet)redirect) $(cmd_$(1))
 
 ###
 # if_changed      - execute command if any prerequisite is newer than
-- 
2.27.0