[alsa-devel] [PATCH 3/3] imx-ssi: Use a hrtimer in FIQ mode

Sascha Hauer s.hauer at pengutronix.de
Thu Apr 8 15:13:53 CEST 2010

On Thu, Apr 08, 2010 at 10:53:25AM +0100, Mark Brown wrote:
> On Thu, Apr 08, 2010 at 11:31:26AM +0200, Sascha Hauer wrote:
> > -	/* Restart the timer; if we didn't report we'll run on the next tick */
> > -	add_timer(&iprtd->timer);
> > +	hrtimer_forward_now(hrt, ns_to_ktime(iprtd->poll_time_ns));
> Hrm, this looks like it's going to have an issue with clock drift -
> we're now unconditionally advancing the timer every period, even if the
> data transfer hasn't pushed through a period of data.  This will cause
> problems on lengthy playbacks (and shorter ones if the clocks are
> sufficiently out of sync).

We are calling snd_pcm_period_elapsed when at least one period is over.
As I see it the worst thing that could happen is that we have not
transfered enough data for one period in the timer callback and thus we
call snd_pcm_period_elapsed in the next timer callback, so about one
period too late, but the comment in sound/core/pcm_lib.c says:

Even if more than one periods have elapsed since the last call, you
have to call this only once.

So I think this should be save.


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