linux-kernel - Re: Context expectations in ALSA

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Message-ID: <20201022132412.vntap4kfb2aj24qy@gilmour.lan>
Date:   Thu, 22 Oct 2020 15:24:12 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     Takashi Iwai <tiwai@...e.de>
Cc:     Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>, alsa-devel@...a-project.org,
        linux-kernel@...r.kernel.org, Dom Cobley <dom@...pberrypi.com>,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Subject: Re: Context expectations in ALSA

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.

> 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

Maxime

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