Re: [PATCH] ALSA: aw2-alsa: select SND_PCM

[Jude Gyimah <jude.gyimah87@gmail.com>]

> Applied now.  It's interesting why this wasn't caught by build tests
> for long time...

Thanks for applying it.

Yes i agree. Quite interesting that this flew under the radar.
The short answer is that none of the stock configurations can express
the combination, and each one misses it for a different reason.  On
arm64 at v7.3:

  defconfig       SND_AW2 not set     SND_PCM=m   caller absent
  allmodconfig    SND_AW2=m           SND_PCM=m   caller is a module too
  allyesconfig    SND_AW2=y           SND_PCM=y   provider built in
  allnoconfig     neither set

The failure needs SND_AW2=y while SND_PCM=m, and allmodconfig and
allyesconfig are each wrong in exactly one coordinate.

randconfig can express it but is unlikely to find it. And the reason is
kinda counter-intuitive i.e., 110 stanzas in 44 Kconfig files select SND_PCM.  
Any one of them at =y pins SND_PCM=y and hides every caller that does not
select it.  So the configuration has to put all 110 at m or n and this
driver at y.  Treating the tristates as independent and uniform (which
randconfig and tristates are not, since most sit behind PCI or a
platform) that is on the order of (2/3)^110.  The real figure will be
different, but the shape is right. The more drivers select a provider,
the better hidden the one driver that forgot to.

So SND_PCM being selected by almost everything is what protected this
bug rather than what would have exposed it. A sampler would have to be 
lucky in 110 places at once. Solving for the assignment instead takes 
it in one step, which is what turned this up.

It is not a one-off. The same y/m shape found 14 missing selects in
sound/, all of them now applied i.e., five Rockchip, three fsl, sunxi,
four Amlogic, and this. Reproducer configurations and the failing
vmlinux link log for each are published at
https://github.com/JGyimah/kreach under results/, if the ALSA side is
ever interested in running it over sound/ before a merge window.

Best,
Jude