Re: [PATCH v6 9/9] ALSA: virtio: introduce device suspend/resume support

[Anton Yakovlev <anton.yakovlev@opensynergy.com>]

On 02.03.2021 10:11, Takashi Iwai wrote:
> On Tue, 02 Mar 2021 09:09:33 +0100,
> Anton Yakovlev wrote:
> > On 02.03.2021 07:48, Takashi Iwai wrote:
> > > On Tue, 02 Mar 2021 07:29:20 +0100,
> > > Anton Yakovlev wrote:
> > > > On 28.02.2021 13:05, Takashi Iwai wrote:
> > > > > On Sat, 27 Feb 2021 09:59:56 +0100,
> > > > > Anton Yakovlev wrote:
> > > > > >
> > > >
> > > > [snip]
> > > >
> > > > > > --- a/sound/virtio/virtio_pcm.c
> > > > > > +++ b/sound/virtio/virtio_pcm.c
> > > > > > @@ -109,6 +109,7 @@ static int virtsnd_pcm_build_hw(struct virtio_pcm_substream *vss,
> > > > > >                  SNDRV_PCM_INFO_BATCH |
> > > > > >                  SNDRV_PCM_INFO_BLOCK_TRANSFER |
> > > > > >                  SNDRV_PCM_INFO_INTERLEAVED |
> > > > > > +             SNDRV_PCM_INFO_RESUME |
> > > > > >                  SNDRV_PCM_INFO_PAUSE;
> > > > >
> > > > > Actually you don't need to set SNDRV_PCM_INFO_RESUME.
> > > > > This flag means that the driver supports the full resume procedure,
> > > > > which isn't often the case; with this, the driver is supposed to
> > > > > resume the stream exactly from the suspended position.
> > > > >
> > > > > Most drivers don't set this but implement only the suspend-stop
> > > > > action.  Then the application (or the sound backend) will re-setup the
> > > > > stream and restart accordingly.
> > > >
> > > > I tried to resume driver without SNDRV_PCM_INFO_RESUME, and alsa-lib
> > > > called only ops->prepare(). It makes sense for a typical hw, but we have
> > > > "clean" unconfigured device on resume. And we must set hw parameters as
> > > > a first step. It means, that code should be more or less the same. And
> > > > maybe it's better to set SNDRV_PCM_INFO_RESUME, since it allows us to
> > > > resume substream in any situation (regardless of application behavior).
> > > > I can refactor code to only send requests from trigger(RESUME) path and
> > > > not to call ops itself. It should make code more straitforward. What do
> > > > you say?
> > >
> > > How about calling hw_params(NULL) conditionally in the prepare?
> >
> > Then the question is that condition. When ops->prepare() is called, the
> > substream is in SUSPENDED state or not? If not then we need to track
> > this in some additional field (and it will make logic a little bit
> > clumsy, since that field is needed to be carefully handled in other
> > places).
>
> Yes, you'd need to have a suspend/resume PM callback in the driver
> that flips the internal flag to invalidate the hw_parmas, and in the
> prepare callback, just call hw_params(NULL) if that flag is set.
>
> > > Doing the full stack work in the trigger callback is bad from the API
> > > design POV; in general the trigger callback is supposed to be as short
> > > as possible.
> >
> > Yeah, but usually original subsystem design does not take into account
> > para-virtualized devices, which usually have it's own slightly different
> > reality. And we need to introduce some tricks.
>
> The hardware drivers do a lot of more things in either suspend/resume
> PM callbacks or prepare callback for re-setup of the hardware.  We can
> follow the similar pattern.  Heavy-lifting works in the trigger
> callbacks is really something to avoid.

Ok, I redone this part and now the driver sets parameters for the device
in ops->prepare() if the substream was suspended. And everything works
fine. Thanks! I will send a new patch set soon.


> Takashi

--
Anton Yakovlev
Senior Software Engineer

OpenSynergy GmbH
Rotherstr. 20, 10245 Berlin