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  • Masahiro Yamada's avatar
    kbuild: link vmlinux only once for CONFIG_TRIM_UNUSED_KSYMS · 3fdc7d3f
    Masahiro Yamada authored
    If CONFIG_TRIM_UNUSED_KSYMS is enabled and the kernel is built from
    a pristine state, the vmlinux is linked twice.
    
    [1] A user runs 'make'
    
    [2] First build with empty autoksyms.h
    
    [3] adjust_autoksyms.sh updates autoksyms.h and recurses 'make vmlinux'
    
      --------(begin sub-make)--------
      [4] Second build with new autoksyms.h
    
      [5] link-vmlinux.sh is invoked because vmlinux is missing
      ---------(end sub-make)---------
    
    [6] link-vmlinux.sh is invoked again despite vmlinux is up-to-date.
    
    The reason of [6] is probably because Make already decided to update
    vmlinux at the time of [2] because vmlinux was missing when Make
    built up the dependency graph.
    
    Because if_changed is implemented based on $?, this issue can be
    narrowed down to how Make handles $?.
    
    You can test it with the following simple code:
    
    [Test Makefile]
      A: B
              @echo newer prerequisite: $?
              cp B A
    
      B: C
              cp C B
              touch A
    
    [Result]
      $ rm -f A B
      $ touch C
      $ make
      cp C B
      touch A
      newer prerequisite: B
      cp B A
    
    Here, 'A' has been touched in the recipe of 'B'.  So, the dependency
    'A: B' has already been met before the recipe of 'A' is executed.
    However, Make does not notice the fact that the recipe of 'B' also
    updates 'A' as a side-effect.
    
    The situation is similar in this case; the vmlinux has actually been
    updated in the vmlinux_prereq target.  Make cannot predict this, so
    judges the vmlinux is old.
    
    link-vmlinux.sh is costly, so it is better to not run it when unneeded.
    Split CONFIG_TRIM_UNUSED_KSYMS recursion to a dedicated target.
    
    The reason of commit 2441e78b ("kbuild: better abstract vmlinux
    sequential prerequisites") was to cater to CONFIG_BUILD_DOCSRC, but
    it was later removed by commit 18489292
    
     ("samples: move blackfin
    gptimers-example from Documentation").
    
    Signed-off-by: default avatarMasahiro Yamada <yamada.masahiro@socionext.com>
    Acked-by: default avatarNicolas Pitre <nico@linaro.org>
    3fdc7d3f