• Nicolas Pitre's avatar
    kbuild: add fine grained build dependencies for exported symbols · c1a95fda
    Nicolas Pitre authored
    Like with kconfig options, we now have the ability to compile in and
    out individual EXPORT_SYMBOL() declarations based on the content of
    include/generated/autoksyms.h.  However we don't want the entire
    world to be rebuilt whenever that file is touched.
    
    Let's apply the same build dependency trick used for CONFIG_* symbols
    where the time stamp of empty files whose paths matching those symbols
    is used to trigger fine grained rebuilds. In our case the key is the
    symbol name passed to EXPORT_SYMBOL().
    
    However, unlike config options, we cannot just use fixdep to parse
    the source code for EXPORT_SYMBOL(ksym) because several variants exist
    and parsing them all in a separate tool, and keeping it in synch, is
    not trivially maintainable.  Furthermore, there are variants such as
    
    	EXPORT_SYMBOL_GPL(pci_user_read_config_##size);
    
    that are instanciated via a macro for which we can't easily determine
    the actual exported symbol name(s) short of actually running the
    preprocessor on them.
    
    Storing the symbol name string in a special ELF section doesn't work
    for targets that output assembly or preprocessed source.
    
    So the best way is really to leverage the preprocessor by having it
    output actual symbol names anchored by a special sequence that can be
    easily filtered out. Then the list of symbols is simply fed to fixdep
    to be merged with the other dependencies.
    
    That implies the preprocessor is executed twice for each source file.
    A previous attempt relied on a warning pragma for each EXPORT_SYMBOL()
    instance that was filtered apart from stderr by the build system with
    a sed script during the actual compilation pass. Unfortunately the
    preprocessor/compiler diagnostic output isn't stable between versions
    and this solution, although more efficient, was deemed too fragile.
    
    Because of the lowercasing performed by fixdep, there might be name
    collisions triggering spurious rebuilds for similar symbols. But this
    shouldn't be a big issue in practice. (This is the case for CONFIG_*
    symbols and I didn't want to be different here, whatever the original
    reason for doing so.)
    
    To avoid needless build overhead, the exported symbol name gathering is
    performed only when CONFIG_TRIM_UNUSED_KSYMS is selected.
    Signed-off-by: default avatarNicolas Pitre <nico@linaro.org>
    Acked-by: default avatarRusty Russell <rusty@rustcorp.com.au>
    c1a95fda
Kbuild.include 14.4 KB