[alsa-devel] maintaining fixed latency between output and input?
what's the most straight-forward way to maintain fixed latency between output and input without sitting in loop and continuously checking snd_pcm_avail_update() for the output and input descriptors?
I was originally going to use callbacks, but I've recently read that is not a good idea. poll() seems to be the reccomended method.
I could use separate threads for output and input, but then I have to perform synchronization between them, marshal external requests between the two threads, and make a lot more work for myself.
I'm visualizing something like this:
for(;;) { poll()
if (output_is_ready) { /* write output buffer */ /* do output-side housekeeping */ }
if (input_is_ready) { /* read input buffer */ /* do input-side housekeeping */ }
/* communicate with other threads */ }
combining the output and input snd_pcm_t into a single descriptor list for poll() seems straight-forward, but can I pass this superset list to snd_pcm_poll_descriptors_revents() without it getting confused?
Aaron J. Grier wrote:
what's the most straight-forward way to maintain fixed latency between output and input without sitting in loop and continuously checking snd_pcm_avail_update() for the output and input descriptors?
I was originally going to use callbacks, but I've recently read that is not a good idea. poll() seems to be the reccomended method.
Yes.
I'm visualizing something like this:
for(;;) { poll()
if (output_is_ready) { /* write output buffer */ /* do output-side housekeeping */ }
if (input_is_ready) { /* read input buffer */ /* do input-side housekeeping */ }
/* communicate with other threads */ }
This is exactly how a full-duplex application should be structured.
To communicate with other threads, you can use a pipe so that you have a file descriptor that can be used with the poll().
combining the output and input snd_pcm_t into a single descriptor list for poll() seems straight-forward, but can I pass this superset list to snd_pcm_poll_descriptors_revents() without it getting confused?
No, snd_pcm_poll_descriptors_revents() looks at the file descriptors of a single device and returns the ready status of that device.
HTH Clemens
On Fri, Sep 26, 2008 at 08:33:19AM +0200, Clemens Ladisch wrote:
To communicate with other threads, you can use a pipe so that you have a file descriptor that can be used with the poll().
now how to get posix message queues available to poll()? (=
Aaron J. Grier wrote:
combining the output and input snd_pcm_t into a single descriptor list for poll() seems straight-forward, but can I pass this superset list to snd_pcm_poll_descriptors_revents() without it getting confused?
No, snd_pcm_poll_descriptors_revents() looks at the file descriptors of a single device and returns the ready status of that device.
to follow up to myself, I was able to allocate a contiguous array of struct pollfd, and point ALSA at different sections of it. the entire array was handed to poll(). it seems to work as described, and my timing is now a bit tighter than it previously was.
participants (2)
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Aaron J. Grier -
Clemens Ladisch