Re: Context expectations in ALSA

[Maxime Ripard <maxime@cerno.tech>]

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Hi Mark

On Thu, Oct 22, 2020 at 02:50:53PM +0100, Mark Brown wrote:
> On Thu, Oct 22, 2020 at 11:50:41AM +0200, Maxime Ripard wrote:
> 
> > This is caused by the HDMI driver polling some status bit that reports
> > that the infoframes have been properly sent, and calling usleep_range
> > between each iteration[1], and that is done in our trigger callback that
> > seems to be run with a spinlock taken and the interrupt disabled
> > (snd_pcm_action_lock_irq) as part of snd_pcm_start_lock_irq. This is the
> > entire stack trace:
> 
> That doesn't sound like something I would expect you do be doing in the
> trigger callback TBH - it feels like if this is something that could
> block then the setup should have been done during parameter
> configuration or something rather than in trigger.
> 
> > It looks like the snd_soc_dai_link structure has a nonatomic flag that
> > seems to be made to address more or less that issue, taking a mutex
> > instead of a spinlock. However setting that flag results in another
> > lockdep issue, since the dmaengine controller doing the DMA transfer
> > would call snd_pcm_period_elapsed on completion, in a tasklet, this time
> > taking a mutex in an atomic context which is just as bad as the initial
> > issue. This is the stacktrace this time:
> 
> Like Jaroslav says you could punt to a workqueue here.  I'd be more
> inclined to move the sleeping stuff out of the trigger operations but
> that'd avoid the issue too.  There are some drivers doing this already
> IIRC.
> 
> > So, I'm not really sure what I'm supposed to do here. The drivers
> > involved don't appear to be doing anything extraordinary, but the issues
> > lockdep report are definitely valid too. What are the expectations in
> > terms of context from ALSA when running the callbacks, and how can we
> > fix it?
> 
> To me having something in the trigger that needs waiting for is the bit
> that feels the most awkward fit here, trigger is supposed to run very
> quickly.

Indeed, other DRM devices seem to send the infoframes as part of
hw_params, and it solves our issue there too. I'll send a patch

Thanks for the suggestion!
Maxime


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