[pulseaudio-discuss] snd_pcm_avail no reliable

[Raymond Yau]

>> >
>> > >>
>> > >>
>> > >>  >
>> > >>  >   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.

https://bugs.freedesktop.org/show_bug.cgi?id=86262#c19

Seem hwptr of snd-usb-audio are  not that bad around 240 to 288 frames
(less than period size)  but not as good as snd-hda-intel 32 frames or
oxygen 8 frames

How accurate do pulseaudio need to use timer base scheduling ?
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