[alsa-devel] Driver - PCM MMAP mode timestamps
tiwai at suse.de
Tue Jan 8 12:41:38 CET 2008
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...
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