[alsa-devel] [PATCH 2/2] ALSA: hda: support for wallclock timestamps

Pierre-Louis Bossart pierre-louis.bossart at linux.intel.com
Thu Jun 14 19:01:47 CEST 2012

On 6/14/2012 3:15 AM, Clemens Ladisch wrote:
>> As long as you have 2 periods or events per 179s, the wrap-around can
>> be detected without any issues. It's explicitely handled in the code.
> AFAICS there is no code that enforces the 179s restriction.
179s corresponds to a 33MB buffer for stereo 48kHz 16bit. There is 
indeed nothing preventing the wrap-around at the moment but this could 
be achieved by limiting the buffer size.
> And why are you using a separate wallclock timer instead of the sample
> count?  Does the higher resolution result in a noticeable improvement?
The wallclock is common for each HDAudio controller, this helps you 
build _one_ estimator for the drift between audio time and system 
(monotonic) time. It'd help avoid what PulseAudio does today, ie a 
different drift estimate per sink/source. If you work with sample 
counts, you'll have separate results for each devices and possibly 
different ASRC in user-space. Also the precision of sample counters is 
limited to 10us for 48kHz, a lot higher than what we can get with 
PTP-based schemes. The accuracy of the wall clock is 41.6 ns, order of 
magnitude more precise.
> How should userspace detect streams whose sample clock is not
> synchronous with this wall clock, such as digital inputs?
Good point. I didn't think about this case, i need to look into it.
Thanks for your feedback, much appreciated

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