[alsa-devel] [PATCH 2/2] pcm_pulse: set prebuf parameter according to software parameters

[Rémi Denis-Courmont]

Le jeudi 15 novembre 2012 10:00:03, David Henningsson a écrit :
> On 11/14/2012 07:07 PM, Rémi Denis-Courmont wrote:
> > The current default value for prebuf is very high, almost the full
> > virtual ALSA buffer. This breaks some application especially where
> > low latency is involved.
> > 
> > This patch makes pcm_pulse implement the sw_params callback and get
> > the prebuf value from the ALSA software parameters. Thus the
> > trigger latency is much more like what an ALSA application should
> > expect from an ALSA PCM device.
> 
> This seems reasonable I believe; see review comments below.
> 
> > ---
> > 
> >   pulse/pcm_pulse.c |   25 +++++++++++++++++++++++++
> >   1 file changed, 25 insertions(+)
> > 
> > diff --git a/pulse/pcm_pulse.c b/pulse/pcm_pulse.c
> > index 0165120..08b6cea 100644
> > --- a/pulse/pcm_pulse.c
> > +++ b/pulse/pcm_pulse.c
> > @@ -859,6 +859,30 @@ static int pulse_hw_params(snd_pcm_ioplug_t * io,
> > 
> >   	return err;
> >   
> >   }
> > 
> > +static int pulse_sw_params(snd_pcm_ioplug_t *io, snd_pcm_sw_params_t
> > *params) +{
> > +	snd_pcm_pulse_t *pcm = io->private_data;
> > +	snd_pcm_uframes_t start_threshold;
> > +
> > +	assert(pcm);
> > +
> > +	if (!pcm->p || !pcm->p->mainloop)
> > +		return -EBADFD;
> > +
> > +	pa_threaded_mainloop_lock(pcm->p->mainloop);
> > +
> > +	snd_pcm_sw_params_get_start_threshold(params, &start_threshold);
> > +
> > +	if (start_threshold < io->period_size)
> > +		start_threshold = io->period_size;
> 
> Why do we need the above constraint?

In my experience, PulseAudio really does not like having less than a period 
worth of buffered samples. I guess the code assumes there is always at least 
that much, and may underrun if not.

But sure enough, we could remove the constraint and blame the ALSA application 
for using inadequate parameters if a problem occurs. What do you prefer?
 
-- 
Rémi Denis-Courmont
http://www.remlab.net/