Re: PCM delay compensation

[Takashi Iwai <tiwai@suse.de>]

At Mon, 23 Feb 2009 17:24:32 +0100,
Lennart Poettering wrote:
> 
> On Mon, 23.02.09 08:45, Takashi Iwai (tiwai@suse.de) wrote:
> 
> > 
> > At Sun, 22 Feb 2009 00:45:04 +0100,
> > Lennart Poettering wrote:
> > > 
> > > On Tue, 07.10.08 17:32, Takashi Iwai (tiwai@suse.de) wrote:
> > > 
> > > > Hi,
> > > 
> > > Sorry for the overly long delay.
> > > > 
> > > > the patch below (to the latest sound git tree) adds the extra delay
> > > > count for USB-audio driver.  This change will appear as the return
> > > > value of snd_pcm_delay().
> > > > 
> > > > Could you check whether it's appropriate behavior you've wanted?
> > > 
> > > I have now tested this patch on the current 2.6.29-rc5 kernel. The
> > > effect is that snd_pcm_delay() returns always increasing values, as if
> > > the playback never proceeds. Most movie players which need that call
> > > to synchronize video frames hence stall completely.
> > 
> > OK, that's bad.  However, the increased value of snd_pcm_delay() is
> > exactly the effect.  The usb-audio always prefetch the buffer in
> > advance.
> > 
> > That means, changing (or "fixing") snd_pcm_delay() would cause many
> > regressions with many apps -- thus basically we are not allowed to
> > change the semantics any more at this too late point.
> > 
> > I feel it's better to introduce another kernel-side API to give a
> > better sync/timing information, and mark snd_pcm_delay as obsolete for
> > future (although we can never deprecate such a basic API).
> 
> No, snd_pcm_delay() with this patch is clearly broken: it apparently
> increases at the same speed as the hw ptr, ignoring the app
> ptr. i.e. after 5 min of playback the delay will be reported a 5 mins!
> After 10 min of playback the delay will be reported as 10 mins!

Ah, then it must be a buggy patch indeed.  It didn't appear in my
some test cases at that time, so didn't check after that.

Maybe some delay account in complete callback wasn't correct.
Another item to my TODO list. 


thanks,

Takashi