• Masahiro Yamada's avatar
    kbuild: sink stdout from cmd for silent build · 174a1dcc
    Masahiro Yamada authored
    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 suppressed 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.
    
    Appending '>/dev/null' at the end of command line is a common way for
    redirection, so I first tried this:
    
      cmd = @set -e; $(echo-cmd) $(cmd_$(1)) >/dev/null
    
    ... 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.
    
    It 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.
    
    I also tried this:
    
      cmd = @set -e; $(echo-cmd) ( $(cmd_$(1)) ) >/dev/null
    
    ... but this causes a syntax error for the same reason.
    
    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: default avatarMasahiro Yamada <masahiroy@kernel.org>
    174a1dcc