Re: [PATCH 2/2] ASoC: max98090: fix lockdep warning

[Tzung-Bi Shih]

On Thu, Jan 9, 2020 at 7:09 PM Marek Szyprowski
<m.szyprowski@xxxxxxxxxxx> wrote:
> On 09.01.2020 06:36, Tzung-Bi Shih wrote:
> > On Wed, Jan 8, 2020 at 7:50 PM Marek Szyprowski
> > <m.szyprowski@xxxxxxxxxxx> wrote:
> >> Fix this by introducing a separate mutex only for serializing the SHDN
> >> hardware register related operations.
> > This fix makes less sense to me.  We used dapm_mutex intentionally
> > because: both DAPM and userspace mixer control would change SHDN bit
> > at the same time.

We should not use a separate lock.  Either mixer control or DAPM would
change the SHDN bit.  The patch overlooks the calling path from DAPM.
As a result, DAPM can change the bit in the middle of mixer control.

> Nope. This is just a lockdep warning about possible hypothetical
> situation on the test system during the normal boot. It doesn't mean
> that the circular dependency actually happens (if so, it would end in
> deadlock). It also doesn't mean that such circular dependency can be
> really triggered, because some other dependencies that not known to
> lockdep engine might protect against it. However the easiest way to fix
> it is to use fine-grained locking instead of reusing some framework
> locks for other purposes. Such approach is also usually a good practice.

If the possible circular locking is a hypothetical situation, shall we
ignore it since we are very sure userspace cannot see the control
devices when building the sound card?