On Thu, 23 Feb 2012, Clemens Ladisch wrote:
Philip Spencer wrote:
3. What should happen? (a) The call returns immediately, since there's lots of room in the buffer. (b) The call returns after one I/O period's worth of data has been drained from the buffer.(a)
If the correct answer is (a) (ALSA's current behaviour), then is there any mechanism for an app to achieve (b) -- get woken up after one I/O period's worth of data have been drained (other than just doing some infinite loop of sleeping then checking snd_pcm_avail periodically)?
Adjust avail_min (but note that wakeups happen only at period boundaries).
That won't work if using the pulseaudio plugin as the backend -- it'll stop the first part of snd_pcm_wait from succeeding right away, but when it moves on to snd_pcm_wait_nocheck the poll call will return right away, because the pulseaudio plugin marks the file descriptor as ready whenever there are more than buffer_attr.minreq bytes available in the buffer.
Perhaps (though that won't help me, as I don't imagine I'll have any success getting Flash Player developers to change anything!, but it may help any other similar app) that should be fixed in the pulseaudio plugin:
- Implement sw_params
- When sw_params is called, check the avail_min value and record it as a new field avail_min (and if it's different from the previous recorded value and the stream is running, call update_active)
- In check_active, instead of "return wsize >= pcm->buffer_attr.minreq", "return wsize >= pcm->avail_min". (Question: should buffer_attr.minreq be changed too, or left alone?)
- At the same time the start_threshold parameter could be implemented too.
I could whip up a patch for that if needed, although of course it won't actually help my situation -- I may just have to be resigned to using non-live mode with about a 3/4 second latency for linux users.
Regards,
Philip
--------------------------------------------+------------------------------- Philip Spencer pspencer@fields.utoronto.ca | Director of Computing Services Room 336 (416)-348-9710 ext3036 | The Fields Institute for 222 College St, Toronto ON M5T 3J1 Canada | Research in Mathematical Sciences