[alsa-devel] speaker-test stalls

[Takashi Iwai]

At Sat, 08 Dec 2007 15:58:40 -0500,
Dave Dillow wrote:
> 
> While working with Fernando to get the sis7019 driver working in his
> buildroot setup, we kept running into a lockup of the PCM stream when
> using speaker-test. aplay was fine, as was other I finally tracked it
> down to a combination of uclibc not doing block reads, which led to
> XRUNs, and what I thought was issue in snd_pcm_lib_write1() when trying
> to fill up the tail of the PCM buffer with a large write, when there was
> an intervening small write.
> 
> As it turns out, the lockup problem may simply be in speaker-write's
> sw_params settings -- it calls snd_pcm_sw_params_set_avail_min() to set
> the minium write to the period size, and
> snd_pcm_sw_params_set_start_threshold() to start once the buffer has
> been filled. This doesn't account for odd wave file lengths and XRUNs.
> 
> The problem arises when the wave data is not a multiple of the period
> size, and you get an XRUN when there is less than buffer_size samples
> remaining in the current loop. 
> 
> So for example, period size is 8192, buffer is 32768, and the wave file
> has 71042 samples (Front_Left.wav for speaker-test).
> 
> speaker-test will break that into 8 writes of 8192 samples, and one of
> 5506.
> 
> If you get a XRUN in the last 32k of the file, the PCM will be stopped,
> and prepared, and we'll start loading data into again. Because we have a
> tail of 5506 samples, we get to the point where the available space is
> less than our minimum, and then snd_pcm_lib_write1() will wait forever
> for the available space to be >= runtime->avail_min, even though the
> channel is stopped.
> 
> [snip from a trace, patch to pcm_lib.c attached]
> writei size 8192 avail 32768 min 8192 xfer 1 state 2
> writei size 8192 avail 24576 min 8192 xfer 1 state 2
> writei size 5506 avail 16384 min 8192 xfer 1 state 2
> writei size 8192 avail 10878 min 8192 xfer 1 state 2
> writei size 8192 avail 2686 min 8192 xfer 1 state 2
> writei loop avail 2686 min 8192
> writei loop avail 2686 min 8192
> writei loop avail 2686 min 8192
> writei loop avail 2686 min 8192
> 
> 
> Assuming that this is desired behavior from the kernel side, then I see
> three options to fix this in speaker-test:
> 1) snd_pcm_sw_params_set_avail_min() to 1 sample
> 2) snd_pcm_sw_params_set_start_threshold() to start when (nperiods - 1)
> are ready (or be even more aggressive)
> 3) call snd_pcm_drain() at the end of each loop to clear out the buffer.
> This won't help if the wave file has fewer samples than the buffer size.
> 
> Options 1 or 2 seem to be best.
> 
> Thoughts?

Just to be sure:  Is it with the latest alsa-lib and alsa-utils from
HG?


Takashi