[alsa-devel] XRUN handling

melwyn lobo linux.melwyn at gmail.com
Wed Jul 7 10:11:53 CEST 2010

On Wed, Jul 7, 2010 at 12:09 AM, Jaroslav Kysela <perex at perex.cz> wrote:
> On Tue, 6 Jul 2010, melwyn lobo wrote:
>> done away with by using silence_size and silence_threshold. And if the
>> application still does not get processor time for some reason, the
>> hw_ptr will overrun the appl_ptr by more than buffer_size and trigger
>> a bug in snd_pcm_playback_silence().
> Could you fix (send us a patch) or describe exactly this problem?

Yes. It occurs in a loaded system. The hardware runs continously and
eventually underruns (stop_threshold trigger does not
occur because of its large value).
Eventually due to silence_threshold, silence_size setting,
snd_pcm_playback_silence() attempts to continously append
The function snd_pcm_playback_hw_avail(runtime) returns negative
values. Now consider the code fragment;
noise_dist = snd_pcm_playback_hw_avail(runtime) + runtime->silence_filled;

                if (noise_dist >= (snd_pcm_sframes_t)
                frames = runtime->silence_threshold - noise_dist;

which makes at some point frames > runtime->buffer_size and triggers

  if (snd_BUG_ON(frames > runtime->buffer_size))

This is my analysis as I can see the above statement getting executed.
For this, the solution implemented in the driver, is to eventually
stop the infinite DMA when appl_ptr <= hw_ptr.
>                                        Jaroslav
> -----
> Jaroslav Kysela <perex at perex.cz>
> Linux Kernel Sound Maintainer
> ALSA Project, Red Hat, Inc.


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