[pulseaudio-discuss] snd_pcm_avail no reliable

[Alexander E. Patrakov]

09.06.2015 11:27, Arun Raghavan wrote:
>
>
> On 09-Jun-2015 11:26 am, "Raymond Yau" <superquad.vortex2 at gmail.com 
> <mailto:superquad.vortex2 at gmail.com>> wrote:
> >
> > >>
> > >>
> > >>  >
> > >>  >   below is what the terminate shows when running pcm_avail.c
> > >>  >
> > >>  >   uid=0 gid=1007 at nutshell:/ # alsactl_test
> > >>  > min_period_size: 8 frames, dir: 0
> > >>  > Playback hwparams: FIFO size is 8
> > >>  > Hardware PCM card 0 'rsnd-dai.0-dirana3.0' device 0 subdevice 0
> > >>  > Its setup is:
> > >>  >   stream       : PLAYBACK
> > >>  >   access       : RW_INTERLEAVED
> > >>  >   format       : S16_LE
> > >>  >   subformat    : STD
> > >>  >   channels     : 2
> > >>  >   rate         : 48000
> > >>  >   exact rate   : 48000 (48000/1)
> > >>  >   msbits       : 16
> > >>  >   buffer_size  : 4096
> > >>  >   period_size  : 1024
> > >>  >   period_time  : 21333
> > >>  >   tstamp_mode  : NONE
> > >>  >   period_step  : 1
> > >>  >   avail_min    : 1024
> > >>  >   period_event : 0
> > >>  >   start_threshold  : 1024
> > >>  >   stop_threshold   : 4096
> > >>  >   silence_threshold: 0
> > >>  >   silence_size : 0
> > >>  >   boundary     : 1073741824
> > >>  >   appl_ptr     : 0
> > >>  >   hw_ptr       : 0
> > >>  > Playing silence
> > >>  > Available: 0, loop iteration: 0
> > >>  > Available: 1024, loop iteration: 1469
> > >>  > Available: 2048, loop iteration: 5609
> > >>  > Available: 3072, loop iteration: 9667
> > >>  >
> > >>  >  All I got is just the 4 lines.
> > >>
> > >> If your sound card only increment hw_ptr only at interrupt occur, you
> > >> need to increase default_rewind_safeguard from 256 bytes to your
> > >> selected period size
> > >
> > >
> > > No. PulseAudio, in timer-scheduling mode, does not use periods at 
> all. You need to change the driver so that it reports 
> SNDRV_PCM_INFO_BATCH, so that PulseAudio does not try to use this mode.
> > >
> > >
> > >>
> > >> This mean that  your sound card won't work with timer scheduling or
> > >> dynamic latency, you can only archieve low latency by decrease 
> period size
> > >> Why do pulseaudio enable timer scheduling when most sound card 
> use IRQ ?
> > >
> > >
> > > Because most broken sound cards driver authors forget to report 
> SNDRV_PCM_INFO_BATCH?
> >
> > Why pulseaudio rely on the flag if your program can find out the 
> granulatity ?
>
> AFAIK, there isn't a way to figure out granularity. Having this would 
> be nice as we could be more intelligent about our tsched behaviour.
>

There is not only no way to query granularity, in some cases it is 
simply unknown. As for my approach (of measuring it directly), I 
currently think (but do not insist) that it is not suitable for 
inclusion into PulseAudio, because it is based on using a silent "test 
sound", busy-looping and repeatedly querying the position until it plays 
out. This would be unreliable if there is an unrelated CPU usage spike, 
and I think that busy-looping in general is not welcome.

-- 
Alexander E. Patrakov
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