Re: [PATCH] ASoC: cs42l43: Move shutter IRQ handling into a worker thread

[Charles Keepax <ckeepax@xxxxxxxxxxxxxxxxxxxxx>]

On Mon, Jul 29, 2024 at 06:18:50PM +0200, Takashi Iwai wrote:
> On Mon, 29 Jul 2024 17:59:32 +0200,
> Charles Keepax wrote:
> > 
> > The microphone/speaker privacy shutter ALSA control handlers need to
> > call pm_runtime_resume, since the hardware needs to be powered up to
> > check the hardware state of the shutter. The IRQ handler for the
> > shutters also needs to notify the ALSA control to inform user-space
> > the shutters updated. However this leads to a mutex inversion, between
> > the sdw_dev_lock and the controls_rwsem.
> 
> That's bad, how does the mutex inversion look like?  Generally
> speaking, a call of snd_ctl_notify() from an irq handler is a very
> standard procedure, and it should work without too much workaround.
> 

SoundWire IRQs are called under the sdw_dev_lock, and then in the
IRQ handler we call snd_ctl_notify which takes controls_rwsem. On
the other side, in the ALSA control handler which is obviously
called under the controls_rwsem, we do a pm_runtime_resume, which
causes SoundWire stuff to happen that takes the sdw_dev_lock.

Thanks,
Charles