• Masahiro Yamada's avatar
    kbuild: build init/built-in.a just once · 2df8220c
    Masahiro Yamada authored
    Kbuild builds init/built-in.a twice; first during the ordinary
    directory descending, second from scripts/link-vmlinux.sh.
    
    We do this because UTS_VERSION contains the build version and the
    timestamp. We cannot update it during the normal directory traversal
    since we do not yet know if we need to update vmlinux. UTS_VERSION is
    temporarily calculated, but omitted from the update check. Otherwise,
    vmlinux would be rebuilt every time.
    
    When Kbuild results in running link-vmlinux.sh, it increments the
    version number in the .version file and takes the timestamp at that
    time to really fix UTS_VERSION.
    
    However, updating the same file twice is a footgun. To avoid nasty
    timestamp issues, all build artifacts that depend on init/built-in.a
    are atomically generated in link-vmlinux.sh, where some of them do not
    need rebuilding.
    
    To fix this issue, this commit changes as follows:
    
    [1] Split UTS_VERSION out to include/generated/utsversion.h from
        include/generated/compile.h
    
        include/generated/utsversion.h is generated just before the
        vmlinux link. It is generated under include/generated/ because
        some decompressors (s390, x86) use UTS_VERSION.
    
    [2] Split init_uts_ns and linux_banner out to init/version-timestamp.c
        from init/version.c
    
        init_uts_ns and linux_banner contain UTS_VERSION. During the ordinary
        directory descending, they are compiled with __weak and used to
        determine if vmlinux needs relinking. Just before the vmlinux link,
        they are compiled without __weak to embed the real version and
        timestamp.
    Signed-off-by: default avatarMasahiro Yamada <masahiroy@kernel.org>
    2df8220c
wrapper 14.1 KB