[Buildroot] BR2_EXTERNAL and linux-ext-*.mk

[Yann E. MORIN <yann.morin.1998@free.fr>]

On 2020-01-23 22:08 +0100, Heiko Thiery spake thusly:
> Hi All,
> 
> I recently recognized that there is the support to have linux patches
> located in the BR2_EXTERNAL/linux directory with
> c26eafa96cabd597a5cce534133ee0ff996b800c.
> 
> While using this feature I noticed that in there seems to go something
> wrong. Once a file that matchs linux/linux-ext-*.mk is detected the
> LINUX_PKGDIR is set to the BR2_EXTERNAL/linux path and for example the
> "0001-timeconst.pl-Eliminate-Perl-warning.patch.conditional" patch
> fails because the path is no longer valid. Also the hash check for the
> linux sources fails due to the same reason.
> 
> Following 2 "make-linux-patch" tries, the first ends without error and
> the second show the error:
> 
> 
> 
> ---------- without BR2_EXTERNAL/linux/linux-ext-xxx.mk:
[--SNIP--]
> # make printvars VARS=LINUX_PKGDIR
> LINUX_PKGDIR=linux/
> --------------------
> 
> ----------  with BR2_EXTERNAL/linux/linux-ext-xxx.mk:
[--SNIP--]
> # make printvars VARS=LINUX_PKGDIR
> LINUX_PKGDIR=/home/hthiery/sources/test-linux/buildroot-external-linux-test/linux/
> ----------
> 
> Do I miss something here or is this a bug?

This is a bug, but not a simple one.

FOO_PKGDIR is set based on the last component of the $(MAKEFILES)
variable. That variables contains the list of Makefiles that have been
included, in the order they've been included, with the top-level
Makefile first (left-most), and the last one last (right-most).

As long as we did support only in-tree linux extensions, all was OK,
because we would only include Makefiles from the same directory as the
main linux.mk one, so LINUX_PKGDIR was set to the directory of the last
linux extension included, which was OK.

But now, we can include extensions from a br2-external tree, and that
last assumption breaks down.

When we added that support, we only thought about the name of the
package, which is set based on the latest component of the directory, so
it had to be 'linux'. But we completely forgot about the _PKGDIR
variable. The existing comment only spoke about the directory name, not
the full path, because FOO_PKGCDIR was only introduced after linux
extensions were.

It turns out that we may have a way out: we do not really care about the
order the extensions are included, since all they do is register post
hooks. So, if we were to include the extensions from the br2-external
trees, then the in-tree ones, we'd be back to the original situation.

This is still a bit flaky, because we may end up breaking this again in
the future, but I don;t have a better solution (well, we could also have
a dummy empty makefile fragment that we explicitly include just right
before callign the pkg-generic infra, but that's still fragile).

Care to test and properly send: http://code.bulix.org/4fcw7l-1104853?raw

Regards,
Yann E. MORIN.

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