Re: Context expectations in ALSA

[Takashi Iwai]

On Thu, 22 Oct 2020 15:24:12 +0200,
Maxime Ripard wrote:
> 
> Hi Takashi,
> 
> On Thu, Oct 22, 2020 at 03:20:49PM +0200, Takashi Iwai wrote:
> > On Thu, 22 Oct 2020 14:57:41 +0200,
> > Maxime Ripard wrote:
> > > 
> > > On Thu, Oct 22, 2020 at 12:03:19PM +0200, Jaroslav Kysela wrote:
> > > > Dne 22. 10. 20 v 11:50 Maxime Ripard napsal(a):
> > > > 
> > > > > 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?
> > > > 
> > > > I think that you should set the non-atomic flag and wake up the workqueue or
> > > > so from interrupt handler in this case. Call snd_pcm_period_elapsed() from the
> > > > workqueue not the interrupt handler context.
> > > 
> > > Yeah, that was my first guess too. However, the DMA driver uses some
> > > kind of generic helpers using a tasklet, so getting rid of it would take
> > > some work and would very likely not be eligible for stable.
> > 
> > Who sets the nonatomic flag for vc4?  I couldn't find the relevant
> > code in the latest upstream.
> 
> Sorry if this wasn't clear enough, it's not there at the moment, ALSA
> takes a spinlock and lockdep complains that we're sleeping in an atomic
> context.
> 
> I tried to add the nonatomic flag in my tree to see if it was fixing the
> issue, but ran into another lockdep complain now with ALSA taking a
> mutex in a tasklet.

I see, thanks for clarification.

> > Ideally dmaengine PCM helper should support the nonatomic mode, but
> > until then, the other side needs to drop the nonatomic flag, I
> > suppose.
> 
> In this case, I'm not sure the blame is in the PCM helper but if there's
> any blame, I guess it's the virt-chan layer inside dmaengine (so for
> providers) that use a tasklet instead of something that allows sleeping

Well, we have to align either to atomic or non-atomic operation.
If we want to solve solely in vc4, the fix would be to make the trigger
action into some own work.


Takashi