[alsa-devel] [PATCH] ALSA: pcm_lib: avoid timing jitter in snd_pcm_read/write()

[Jassi Brar]

On Sun, Jun 27, 2010 at 7:13 AM, David Dillow <dave at thedillows.org> wrote:
> When using poll() to wait for the next period -- or avail_min samples --
> one gets a consistent delay for each system call that is usually just a
> little short of the selected period time. However, When using
> snd_pcm_read/write(), one gets a jittery delay that alternates between
> less than a millisecond and approximately two period times. This is
> caused by snd_pcm_lib_{read,write}1() transferring any available samples
> to the user's buffer and adjusting the application pointer prior to
> sleeping to the end of the current period. When the next period
> interrupt occurs, there is then less than avail_min samples remaining to
> be transferred in the period, so we end up sleeping until a second
> period occurs.
>
> 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