[alsa-devel] SALSA-Lib: Playback overrun on initial PCM start

[J. Scott Merritt]

On Fri, 29 Jun 2007 18:07:20 +0200
Takashi Iwai <tiwai at suse.de> wrote:

> At Fri, 29 Jun 2007 11:24:09 -0400,
> J. Scott Merritt wrote:
> > 
> > On Fri, 29 Jun 2007 11:25:16 +0200
> > Takashi Iwai <tiwai at suse.de> wrote:
> > 
> > > At Thu, 28 Jun 2007 15:01:08 -0400,
> > > J. Scott Merritt wrote:
> > > > 
> > > > Dear List,
> > > > 
> > > > Using SALSA-Lib 0.0.3, I call:	
> > > > 
> > > > snd_pcm_open (&playback_handle, argv[1], SND_PCM_STREAM_PLAYBACK, 0);
> > > > snd_pcm_hw_params_malloc (&hw_params);
> > > > snd_pcm_hw_params_any (playback_handle, hw_params);
> > > > snd_pcm_hw_params_set_access (playback_handle, hw_params, SND_PCM_ACCESS_RW_INTERLEAVED);
> > > > snd_pcm_hw_params_set_format (playback_handle, hw_params, SND_PCM_FORMAT_S16_LE));
> > > > snd_pcm_hw_params_set_rate_near (playback_handle, hw_params, 44100, 0);
> > > > snd_pcm_hw_params_set_channels (playback_handle, hw_params, 2);
> > > > snd_pcm_hw_params_set_buffer_size_last (playback_handle, hw_params, &bfrsize);
> > > > snd_pcm_hw_params_get_buffer_size (hw_params, &bfrsize);
> > > > snd_pcm_hw_params (playback_handle, hw_params);
> > > > snd_pcm_hw_params_free (hw_params);
> > > > 	
> > > > snd_pcm_sw_params_malloc (&sw_params);
> > > > snd_pcm_sw_params_current (playback_handle, sw_params);
> > > > snd_pcm_sw_params_set_avail_min (playback_handle, sw_params, 4096);
> > > > snd_pcm_sw_params_set_start_threshold (playback_handle, sw_params, 10000U);
> > > > snd_pcm_sw_params_set_xfer_align (playback_handle, sw_params, 1);
> > > > snd_pcm_sw_params (playback_handle, sw_params);
> > > > 	
> > > > state = snd_pcm_state (playback_handle);
> > > > frames_to_deliver = snd_pcm_avail_update (playback_handle);
> > > > 
> > > > while ((frames_to_deliver = snd_pcm_avail_update (playback_handle)) > 4096) {
> > > >    snd_pcm_writei (playback_handle, &buf, 4096) }
> > > > 
> > > > 
> > > > As soon as the buffer start threshold is reached, or alternatively
> > > > if I manually start PCM stream, the PCM stream reports an overrun
> > > > (i.e. "Broken Pipe").  If I then examine the PCM state, it is in the
> > > > overrun (XRUN) state.
> > > 
> > > What are the buffer and period sizes?  Did you try alsa-lib with hw,
> > > too, right?
> > 
> > In sample program above, buffer size is set to "last", which results
> > in a buffer size of 32768 (which matches the allocation in the kernel driver).
> > The sample programs above does not establish a period size and is unknown.
> > 
> > I added code to set the period size to 1024 frames and experienced the
> > same (overrun) result.   I also forced the buffer size to 8192 and lowered
> > the sample rate to 16 kHz and did not see any improvement.
> > 
> > Yes, the same exact code runs properly on the target hardware with
> > alsa-lib 1.0.13.
> 
> OK, looks like a bug in salsa-lib, then.
> I fixed some remaining bugs and release 0.0.4 now.  Please give it a
> try.  (Note that it might take some time until the ftp server is
> exported / mirrored.)

Much better :)

I no longer get the PlayBack overrun on PCM start.  However, it appears
that "poll" or the snd_pcm_sw_params_set_avail_min (=4096) is not being
properly honored.  With the normal alsa-lib, when the "poll" returns
snd_pcm_avail_update reports that space is available for 4096 frames.
However, in my initial testing, when running with SALSA lib, only about
100 frames or so are available when the poll returns.

I will examine this more closely.