[alsa-devel] Driver - PCM MMAP mode timestamps

Takashi Iwai tiwai at suse.de
Tue Jan 8 12:02:12 CET 2008

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().

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


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