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

[Takashi Iwai <tiwai@suse.de>]

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.


Takashi