On Thu, Dec 19, 2019 at 08:51:00PM +0900, Masahiro Yamada wrote: > Kbuild descends into a directory by either 'y' or 'm', but there is an > important difference. > > Kbuild combines the built-in objects into built-in.a in each directory. > The built-in.a in the directory visited by obj-y is merged into the > built-in.a in the parent directory. This merge happens recursively when > Kbuild is ascending back towards the top directory, so built-in objects > are linked into vmlinux eventually. This works properly only when the > Makefile that specifies obj-y is reachable by the chain of obj-y. > > On the other hand, Kbuild does not take built-in.a from the directory > visited by obj-m. This it, all the objects in that directory are supposed > to be modular. If Kbuild descends into a directory by obj-m, but the > Makefile in the sub-directory specifies obj-y, those objects are just > left orphan. > > The current statement "Kbuild only uses this information to decide that > it needs to visit the directory" is misleading. Clarify the difference. > + Kbuild uses this information not only to decide that it needs to visit > + the directory, but also to decide whether or not to link objects from > + the directory into vmlinux. > + > + When Kbuild descends into the directory with 'y', all built-in objects > + from that directory are combined into the built-in.a, which will be > + eventually linked into vmlinux. > + > + When Kbuild descends into the directory with 'm', in contrast, nothing > + from that directory will be linked into vmlinux. If the Makefile in > + that directory specifies obj-y, those objects will be left orphan. > + It is very likely a bug of the Makefile or of dependencies in Kconfig. Perhaps the implementation should be changed to match the documentation? This seems like a very subtle point for people to know.
