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

[Sascha Hauer <s.hauer@pengutronix.de>]

On Thu, Apr 08, 2010 at 02:30:13PM +0100, Liam Girdwood wrote:
> On Thu, 2010-04-08 at 15:13 +0200, Sascha Hauer wrote:
> > 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.
> 
> Yeah agreed, as long as it's called at least once then we should be safe
> here. 
> 
> Btw, how does this driver behave under moderate -> heavy IO load. I do
> remember the SDMA based driver occasionally starved the SSI FIFO under
> moderate IO load (i.e. aplay reading wav file over NFS).

aplay works fine with files over nfs and survives moderate hackbench
attacks. I do not check the underrun bit though so there might be
glitches I did not hear.

Sascha


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