Re: RFC: Add support for monotonic clock for 2.6

[James Courtier-Dutton <James@superbug.co.uk>]

Takashi Iwai wrote:
> At Wed, 07 Mar 2007 22:33:25 +0000,
> James Courtier-Dutton wrote:
>> Takashi Iwai wrote:
>>> At Tue, 27 Feb 2007 15:52:41 +0100 (CET),
>>> Jaroslav Kysela wrote:
>>>> Hi,
>>>>
>>>> 	in 2.6 kernels, we can use ktime_get_ts() function to get 
>>>> monotonic time which can be used for PCM timestamps. I would propose to 
>>>> reuse the SNDRV_PCM_IOCTL_TSTAMP ioctl to enable using of monotonic clock.
>>>> The "enable" (int) argument might mean:
>>>>
>>>> 0: standard clock - getnstimeofday()
>>>> 1: standard clock - getnstimeofday() - for "compatibility"
>>>> 2: monotonic clock - ktime_get_ts()
>>> Good to add a new feature...  but is there any user of this PCM
>>> timestamp so far?  I see some good purposes but rarely seen in the
>>> reality, unfortunately...
>>>
>>>
>>> Takashi
>>>
>> There are some good applications for it, but I have not got round to 
>> writing them yet!
> 
> Me too ;)  But, I remember some difficulties regarding the
> timestamping at the last time I tried (quite ago).  The timestamp is
> somewhat hard to use because it's updated at each pointer or status
> update although I wanted to get the exact time only at the period
> boundary.  Maybe these usability issues should be sorted out all
> together.
> 
> 
> Takashi

Yes, timestamp at period boundary would be more useful. I had not looked 
at the current timestamp in enough details. A timestamp generated in the 
IRQ routine would probably be the best. Could the timestamp be attached 
to a particular PCM sample in the buffer. I.e. The app would retrieve 
the timestamp, and be told that that timestamp applies to the sample X 
samples from now, if using the snd_pcm_read() function.

James


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