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kbuild: add fine grained build dependencies for exported symbols
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:Nicolas Pitre <nico@linaro.org> Acked-by: