[alsa-devel] Master Plan on rewinding

[Alexander E. Patrakov]

09.09.2014 14:43, Clemens Ladisch wrote:
> Alexander E. Patrakov wrote:
>> |---------|---------P----h----p---------|-a-------|---------|
>>
>> So, what should alsa-lib return for snd_pcm_avail() and snd_pcm_rewind()?
>> The driver only knows that "P" is already used, can infer that "p" isn't
>> used yet, and knows nothing about samples in the middle.
> Indeed.  However, the DMA pointer moves asynchronously, so it is possible
> that it has already moved beyond p when snd_pcm_rewindable() returns.
> For the samples between P and p, the risk is larger than for those after
> p, but p is not a boundary where the risk abruptly decreases.
>
> It would make sense to report the pointer update granularity, but not
> to adjust the return value of snd_pcm_avail/rewindable().

OK, I understand your viewpoint, and the phrase "some indicator of the 
actual rewind granularity and/or safeguard ... should be enough for PA 
to be able to pick a suitable default latency" from David indicates that 
he has a similar opinion.

Now the remaining question is: can the proposed heuristic (minimum 
period size for a given sample rate, number of channels and sample 
format) be useful as an upper-bound approximation of the pointer update 
granularity for cards that are "rewindable even further than the nearest 
period"?

-- 
Alexander E. Patrakov