Functionality of pcm_notify in snd-aloop?

[Jaroslav Kysela]

Dne 28. 04. 20 v 18:19 Kai Vehmanen napsal(a):
> Hey,
> 
> On Tue, 28 Apr 2020, Takashi Iwai wrote:
> 
>> I believe the missing piece here is a generic way to tell user-space
>> that the stream got invalidated.  This would be useful not only for
>> aloop but can be applied in general when a stream becomes temporarily
>> unavailable (e.g. the HDMI monitor disconnected or the DSP route
>> switched).
> 
> ack on that. I've been preparing this patch to add -ENODATA to alsa-lib
> documentation:
> "[RFC] pcm: add documentation for -ENODATA error code handling"
> https://github.com/kv2019i/alsa-lib/commit/87b298106e04054489ee93b26a610e37f99a3171
> 
> Have not yet had time to send a proper version to the list, but it's
> addressing specifically this need. This would serve as the interface for
> SOF DSP to tell that a given PCM node will not be providing data (as the
> DSP topology is not fully connected) [1].
> 
> To test the above, I've used a small hack to aplay/arecord that keeps
> trying to restart the PCM after a delay, in case -ENODATA is returned:
> https://github.com/kv2019i/alsa-utils/commit/a2ba541ea0b3e86a65687de88a41f10cf0a8ddc2

This code calls wrongly snd_pcm_resume() here. Also, we should really think 
about the whole mechanism when the stream cannot be activated (also for 
playback - Intel HDMI LPE driver [2]). I mean that the poll multiplexer should 
handle this state, too. If we don't find a way to notify the user space that 
the stream can be started using the poll(), we can use another interface like 
control API for the notifications.

We may also instroduce a new PCM state "WAIT" to wait on the hardware / link 
availability (SETUP -> -ENODATA -> WAIT -> use poll() for the link status -> 
success -> SETUP -> PREPARE) - talking about SNDRV_PCM_STATE states here.

					Jaroslav


> [1] https://github.com/thesofproject/sof/issues/2564

[2] 
https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/blob/36a4923f9bd05d4c5e2e8b1765f18edf452b9251/src/modules/alsa/module-alsa-card.c#L928-933


-- 
Jaroslav Kysela <perex at perex.cz>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.