I'm investigating an issue in Firefox's audio code when the PulseAudio ALSA plugin is in use. It wasn't clear if PulseAudio ALSA plugin issues should be raised here or on alsa-devel, so please let me know if this is the wrong forum for this issue. Firefox requests a particular latency (100ms, 4410 frames at 44.1kHz) via snd_pcm_set_params. Inside the plugin (pcm_pulse.c:pulse_hw_params), that value is used to set up buffer_attr. When the PA stream is connected in pcm_pulse.c:pulse_prepare, PA may increase the buffer_attr values, but this isn't reflected in pcm->buffer_attr or higher layers in ALSA. Would a possible fix for this be to query pa_stream_get_buffer_attr after connecting and update pcm->buffer_attr and the higher (ioplug/etc.) layers that the buffer size has increased? The problem I'm faced with is that there doesn't appear to be a way to detect and handle this issue at the ALSA API level, and requesting a too low latency results in broken audio playback rather than a PCM setup failure or a larger buffer than requested being used. I've attached a simple testcase that uses snd_pcm_wait, snd_pcm_avail_update, and snd_pcm_writei. Run it with a latency argument specified in milliseconds on the command line. For my local machine, 45ms works and 44ms fails immediately like so: snd_pcm_wait wakes snd_pcm_avail_update returns 4410 snd_pcm_writei writes 4410 snd_pcm_wait wakes immediately snd_pcm_avail_update returns -EPIPE I'd expect to see one of the following behaviours instead: 1. PCM setup fails due to requesting a too small buffer. 2. Buffer is silently raised during setup and snd_pcm_avail_update requests the correct number of frames. 3. PCM wakes from snd_pcm_wait for refill enough times to fill the buffer. As a side note: this seems to be related to the PA server increasing the minimum latency over time. Once the PA server is in this state, restarting it (in non-daemon mode with debug logging or via pulseaudio -k and autospawn) causes the problem to disappear. After restarting the server, I can run the testcase with 44ms and lower latency successfully. Thanks, -mjg -------------- next part -------------- A non-text attachment was scrubbed... Name: latency.c Type: text/x-csrc Size: 1500 bytes Desc: not available URL: <http://lists.freedesktop.org/archives/pulseaudio-discuss/attachments/20120711/f38c0dcb/attachment.c>
