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

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

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).


> 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.


> 
> Takashi
> 

-- 
Anton Yakovlev
Senior Software Engineer

OpenSynergy GmbH
Rotherstr. 20, 10245 Berlin