[alsa-devel] [PATCH 1/2] [ALSA] add SNDRV_PCM_TRIGGER_DRAIN trigger

[Takashi Iwai]

At Fri, 19 Dec 2014 06:29:33 +0000,
Yang, Libin wrote:
> 
> Hi Takashi,
> 
> Thanks for review, please see my comments.
> 
> 
> > -----Original Message-----
> > From: Takashi Iwai [mailto:tiwai at suse.de]
> > Sent: Thursday, December 18, 2014 4:30 PM
> > To: Yang, Libin
> > Cc: broonie at kernel.org; alsa-devel at alsa-project.org;
> > liam.r.girdwood at linux.intel.com; Jie, Yang
> > Subject: Re: [PATCH 1/2] [ALSA] add SNDRV_PCM_TRIGGER_DRAIN trigger
> > 
> > At Thu, 18 Dec 2014 15:36:03 +0800,
> > libin.yang at intel.com wrote:
> > >
> > > From: Libin Yang <libin.yang at intel.com>
> > >
> > > Add SNDRV_PCM_TRIGGER_DRAIN trigger for pcm drain.
> > >
> > > Some audio devices require notification of drain events in order to
> > > properly drain and shutdown an audio stream.
> > >
> > > Signed-off-by: Libin Yang <libin.yang at intel.com>
> > > ---
> > >  include/sound/pcm.h     | 1 +
> > >  sound/core/pcm_native.c | 5 +++++
> > >  2 files changed, 6 insertions(+)
> > >
> > > diff --git a/include/sound/pcm.h b/include/sound/pcm.h index
> > > 8bb00a2..bb77aa3 100644
> > > --- a/include/sound/pcm.h
> > > +++ b/include/sound/pcm.h
> > > @@ -109,6 +109,7 @@ struct snd_pcm_ops {
> > >  #define SNDRV_PCM_TRIGGER_PAUSE_RELEASE	4
> > >  #define SNDRV_PCM_TRIGGER_SUSPEND	5
> > >  #define SNDRV_PCM_TRIGGER_RESUME	6
> > > +#define SNDRV_PCM_TRIGGER_DRAIN		7
> > >
> > >  #define SNDRV_PCM_POS_XRUN		((snd_pcm_uframes_t)-1)
> > >
> > > diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c index
> > > 166d59c..15fe682 100644
> > > --- a/sound/core/pcm_native.c
> > > +++ b/sound/core/pcm_native.c
> > > @@ -1517,6 +1517,11 @@ static int snd_pcm_do_drain_init(struct
> > > snd_pcm_substream *substream, int state)  {
> > >  	struct snd_pcm_runtime *runtime = substream->runtime;
> > >  	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK) {
> > > +
> > > +		if (substream->runtime->trigger_master == substream)
> > > +			substream->ops->trigger(substream,
> > > +					SNDRV_PCM_TRIGGER_DRAIN);
> > 
> > The return value check is missing.
> 
> Actually, the code doesn't care the return value and I don't want to let the
> trigger return value impact the snd_pcm_do_drain_init() flow just like
> snd_pcm_do_stop(). 

Well, the fundamental problem in this patch is that it was written
*just for your hardware* although this is a public API extension.
Consider again about it: if the return value of this trigger doesn't
matter for your hardware, the driver can simply return zero always.
Why do you need to drop the check in the common code?  Wouldn't any
driver return any error there?


> > Besides, this patch will break many drivers.  Many of them will spew errors if
> > an unexpected trigger command is issued.  So, you have to limit this trigger
> > only to some drivers.  A typical workaround is to add a new
> > SNDRV_PCM_INFO_* and issue the trigger only when the flag is set, as done
> > for resume and pause operations.
> 
> Yes, that's a good suggestion. Thanks for reminding me.

OTOH, the info flag will be exposed to user-space while this is a
kernel-space-only information.  So, in the code copying the info flag
(snd_pcm_hw_refine()) would need to drop this bit, too.

Alternatively, we may add just a new flag in struct snd_pcm or struct
snd_pcm_substream and let the driver setting it manually.  This might
need more changes in ASoC side, though, as the PCM creation isn't
directly done by the driver.


> > Also, do you want to see this trigger even if the stream isn't started before
> > drain is issued?  Also, why no need for this trigger for capture streams?
> 
> In this case, the trigger action will be started. For the stream not started
> case, if I skip the trigger , I don't know where I can have the chance to 
> trigger later. And I didn't find the suitable place to trigger my code. 
> Put the trigger at the end of the function snd_pcm_do_drain_init() or 
> in the function snd_pcm_post_drain_init() will be OK? 
> 
> I didn't include the capture case because we found the capture currently 
> don't need the trigger.
> 
> Our issue is the fw don't know when to stop fetching the data. For playback
> we need fill silence data when draining, otherwise the fw will get the old data.
> But for capture, after the driver has fetched the data, it will stopped and no
> old data will be fetched.

A similar problem here.  If you extend the API, don't build up only
from the bottom.  Rather define from a higher POV.

If calling the drain trigger for capture stream doesn't make *any*
sense for all hardware, then we shouldn't add it.  This would be the
only reason not to do it.  What about empty streams...?

That being said, we need to define the proper abstraction, what this
trigger does for which purpose, not specific to a certain hardware.
That is, the requirement goes up from your hardware level while the
design comes down from the abstraction level.

So, in general I found it good to have this kind of API extension.
But let's add it a bit more carefully.


thanks,

Takashi