On Sun, 27 Jun 2010, David Dillow wrote:
On Sun, 2010-06-27 at 12:29 +0900, Jassi Brar wrote:
On Sun, Jun 27, 2010 at 7:13 AM, David Dillow dave@thedillows.org wrote:
This is solved by using runtime->twake as the number of samples needed for a wakeup in addition to selecting the proper wait queue to wake in snd_pcm_update_state(). This requires twake to be non-zero when used by snd_pcm_lib_{read,write}1() even if avail_min is zero.
http://mailman.alsa-project.org/pipermail/alsa-devel/2010-June/028786.html
Hmm, yes, I should have search the archives a bit. I originally tuned out the thread you listed as a request for parameter help, and missed your original postings.
I think my patch is pretty close to preserving the existing semantics as it doesn't change poll() at all, but I do see a case where the user could get a read/write back prior to avail_min samples being ready. I think that's fixable -- if the user is requesting a read/write of less than avail_min samples, then we have to wait for avail_min regardless.
Takashi, is your concern about semantics the proper honoring of avail_min in all cases, or preserving the current behavior of waiting for two periods when avail_min is set to the size of one period?
I think that the avail_min semantics is quite clear. The problem is caused by the hw transfer acknowledge interrupt jitter. It seems that the only good solution is to postpone the wake_up() call to time when the avail_min condition becomes true - using an extra timing source (system timer for example).
Another possibility is to keep thing asis and keep to applications to handle this situation - use a different timing source than interrupts from soundcard for scheduling of I/O operations. But it's right that most of simple applications and use cases expect that I/O transfers will work even with 2 periods.
Jaroslav
----- Jaroslav Kysela perex@perex.cz Linux Kernel Sound Maintainer ALSA Project, Red Hat, Inc.