Re: POSIX clocks and ALSA

[Heikki Lindholm <holindho@cs.helsinki.fi>]

Jaroslav Kysela kirjoitti:
> On Mon, 26 Nov 2007, Heikki Lindholm wrote:
> 
>> Hello,
>>
>> Some years ago there was some talk about UST support in Linux, but the 
>> support never happened. With the hrtimers patch (and I'm not quite sure 
>> if even earlier?) CLOCK_MONOTONIC would seem like a fairly good UST time 
>> source. What I'd like to see, is a selectable clock for ALSA 
>> timestamping, e.g. something like snd_sw_params_clock(..., clockid_t 
>> clk). Would this seem plausible? I don't know that much about ALSA 
>> internals, so, no idea whether different clocks on different 
>> pcms/whatever would quickly turn into an unmanageable mess.
> 
> We are aware about this extension and I already proposed an 
> implementation. I hope to implement it soon. Timestamps are not used in 
> driver internally.

I had a look at the proposal and the thread at
http://thread.gmane.org/gmane.linux.alsa.devel/45237/focus=45573
I don't know about the history of SNDRV_PCM_IOCTL_TSTAMP, but since it 
currently seems unused I don't see a problem reusing it for something 
(else). However, I think that it'd be nicer to define the API to use a 
POSIX clock id instead of some arbitrary constants, because there have 
been/are other clocks on Linux, for example, CLOCK_SGI_CYCLE. An update 
to pcm_status might also be in order to tell which clock is being used.

-- Heikki Lindholm