Re: [PATCH] kbuild: clarify the difference between obj-y and obj-m w.r.t. descending

[Matthew Wilcox]

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.