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Date:   Thu, 22 Oct 2020 15:31:47 +0200
Message-ID: <s5h7dribd4s.wl-tiwai@suse.de>
From:   Takashi Iwai <tiwai@suse.de>
To:     Maxime Ripard <maxime@cerno.tech>
Cc:     Jaroslav Kysela <perex@perex.cz>, Takashi Iwai <tiwai@suse.com>,
        Liam Girdwood <lgirdwood@gmail.com>,
        Mark Brown <broonie@kernel.org>, alsa-devel@alsa-project.org,
        linux-kernel@vger.kernel.org, Dom Cobley <dom@raspberrypi.com>,
        Dave Stevenson <dave.stevenson@raspberrypi.com>,
        Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Subject: Re: Context expectations in ALSA
In-Reply-To: <20201022132412.vntap4kfb2aj24qy@gilmour.lan>
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        <20201022132412.vntap4kfb2aj24qy@gilmour.lan>
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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