• Paul Burton's avatar
    compiler.h: Allow arch-specific asm/compiler.h · 3c3bec81
    Paul Burton authored
    [ Upstream commit 04f264d3 ]
    
    We have a need to override the definition of
    barrier_before_unreachable() for MIPS, which means we either need to add
    architecture-specific code into linux/compiler-gcc.h or we need to allow
    the architecture to provide a header that can define the macro before
    the generic definition. The latter seems like the better approach.
    
    A straightforward approach to the per-arch header is to make use of
    asm-generic to provide a default empty header & adjust architectures
    which don't need anything specific to make use of that by adding the
    header to generic-y. Unfortunately this doesn't work so well due to
    commit 28128c61 ("kconfig.h: Include compiler types to avoid missed
    struct attributes") which caused linux/compiler_types.h to be included
    in the compilation of every C file via the -include linux/kconfig.h flag
    in c_flags.
    
    Because the -include flag is present for all C files we compile, we need
    the architecture-provided header to be present before any C files are
    compiled. If any C files can be compiled prior to the asm-generic header
    wrappers being generated then we hit a build failure due to missing
    header. Such cases do exist - one pointed out by the kbuild test robot
    is the compilation of arch/ia64/kernel/nr-irqs.c, which occurs as part
    of the archprepare target [1].
    
    This leaves us with a few options:
    
      1) Use generic-y & fix any build failures we find by enforcing
         ordering such that the asm-generic target occurs before any C
         compilation, such that linux/compiler_types.h can always include
         the generated asm-generic wrapper which in turn includes the empty
         asm-generic header. This would rely on us finding all the
         problematic cases - I don't know for sure that the ia64 issue is
         the only one.
    
      2) Add an actual empty header to each architecture, so that we don't
         need the generated asm-generic wrapper. This seems messy.
    
      3) Give up & add #ifdef CONFIG_MIPS or similar to
         linux/compiler_types.h. This seems messy too.
    
      4) Include the arch header only when it's actually needed, removing
         the need for the asm-generic wrapper for all other architectures.
    
    This patch allows us to use approach 4, by including an asm/compiler.h
    header from linux/compiler_types.h after the inclusion of the
    compiler-specific linux/compiler-*.h header(s). We do this
    conditionally, only when CONFIG_HAVE_ARCH_COMPILER_H is selected, in
    order to avoid the need for asm-generic wrappers & the associated build
    ordering issue described above. The asm/compiler.h header is included
    after the generic linux/compiler-*.h header(s) for consistency with the
    way linux/compiler-intel.h & linux/compiler-clang.h are included after
    the linux/compiler-gcc.h header that they override.
    
    [1] https://lists.01.org/pipermail/kbuild-all/2018-August/051175.htmlSigned-off-by: 's avatarPaul Burton <paul.burton@mips.com>
    Reviewed-by: Masahiro Yamada's avatarMasahiro Yamada <yamada.masahiro@socionext.com>
    Patchwork: https://patchwork.linux-mips.org/patch/20269/
    Cc: Arnd Bergmann <arnd@arndb.de>
    Cc: James Hogan <jhogan@kernel.org>
    Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
    Cc: Ralf Baechle <ralf@linux-mips.org>
    Cc: linux-arch@vger.kernel.org
    Cc: linux-kbuild@vger.kernel.org
    Cc: linux-mips@linux-mips.org
    Signed-off-by: 's avatarSasha Levin <sashal@kernel.org>
    3c3bec81