[alsa-devel] Driver - PCM MMAP mode timestamps

[Takashi Iwai]

At Tue, 8 Jan 2008 12:24:31 +0100 (CET),
Jaroslav Kysela wrote:
> 
> On Tue, 8 Jan 2008, Takashi Iwai wrote:
> 
> > At Tue, 8 Jan 2008 10:45:34 +0100 (CET),
> > Jaroslav Kysela wrote:
> > > 
> > > Hi,
> > > 
> > > 	it seems that we have another inconsistency with PCM MMAP mode
> > > timestamps. If I read code correctly, snd_pcm_status() returns timestamp 
> > > from last interrupt BUT updates hw_ptr to current (calling 
> > > snd_pcm_update_hw_ptr() again).
> > > 	It means, that application will get unrelated timestamp to 
> > > hw position returned in snd_pcm_status() in case of mmap mode. I don't see 
> > > any way to use this timestamp now.
> > > 	To fix this issue, I would like to create a next status ioctl 
> > > which will return only mmap timestamp plus last related interrupt ring 
> > > buffer position and remove 'if (runtime->tstamp_mode == 
> > > SNDRV_PCM_TSTAMP_MMAP)' condition from snd_pcm_status() in driver 
> > > completely.
> > > 	Any opinion? Other idea?
> > 
> > It'd be OK to remove the tstamp_mode check there.  It was newly added
> > after discussions.  We are still in the development phase about this,
> > and removing is no regression.
> > 
> > However, I'm against adding new ioctls.  The same thing can be simply
> > done by reading the mmapped status struct, as snd_pcm_hw_fast_tstamp()
> > in alsa-lib pcm_hw.c.  If the mmaped status isn't available, it can
> > fallback to the normal snd_pcm_status().
> 
> You're right. I forgot on mmaped status and PCM_IOCTL_SYNC_PTR for 
> non-mmap architectures.
> 
> > The real problem is that the behavior of TSTAMP_MMAP isn't strictly
> > defined.  It's just a TSTAMP_NORMAL in the end unless the new if check
> > there.  I think we may get rid of this mode if the tstamp via mmap is
> > available.
> 
> Actually, after condition removal, TSTAMP_MMAP will just enable 
> timestamping in interrupt update. No other tests are in the driver.
> I commited change to HG.

Thanks.  Then the missing piece is the new functions in alsa-lib.
snd_pcm_status_get_tstamp() should be as is now...


Takashi