[alsa-devel] [PATCH v2] staging: bcm2835-audio: interpolate audio delay

[Mike Brady]

> On 25 Oct 2018, at 08:37, Takashi Iwai <tiwai at suse.de> wrote:
> 
> On Thu, 25 Oct 2018 00:02:34 +0200,
> Kirill Marinushkin wrote:
>> 
>>>> When you play sound - the pointer increments.
>>> 
>>> Unfortunately, when you play sound, the pointer does not actually increment, for up to about 10 milliseconds. I know of no way to actually access the true “live” position of the frame that is being played at any instant; hence the desire to estimate it.
>>> 
>> 
>> Your vision of situation in the opposite from my vision. What you see as a
>> symptom - I see as a root cause. As I see, you should fix the
>> pointer-not-incrementing. Why do you think that it's okay that the pointer is
>> not updating during sound play? Why do you insist that there is a delay? I don't
>> understand why we are so stuck here.
> 
> Well, in the API POV, it's nothing wrong to keep hwptr sticking while
> updating only delay value.  It implies that the hardware chip doesn't
> provide the hwptr update.
> 
> Though, usually the delay value is provided also from the hardware,
> e.g. reading the link position or such.  It's a typical case like
> USB-audio, where the hwptr gets updated and the delay indicates the
> actual position *behind* hwptr.  That works because hwptr shows the
> position in the ring buffer at which you can access the data.  And it
> doesn't mean that hwptr is the actually playing position, but it can
> be ahead of the current position, when many samples are queued on
> FIFO.  The delay is provided to correct the position back to the
> actual point.
> 
> But, this also doesn't mean that the delay shouldn't be used for the
> purpose like this patchset, either.  OTOH, providing a finer hwptr
> value would be likely more apps-friendly; there must be many programs
> that don't evaluate the delay value properly.
> 
> So, I suppose that hwptr update might be a better option if the code
> won't become too complex.  Let's see.

Indeed. It will take me a few days to look into this…

Regards
Mike

> One another thing I'd like to point out is that the value given in the
> patch is nothing but an estimated position, optimistically calculated
> via the system timer.  Mike and I had already discussion in another
> thread, and another possible option would be to provide the proper
> timestamp-vs-hwptr pair, instead of updating the timestamp always at
> the status read.
> 
> Maybe it's worth to have a module option to suppress this optimistic
> hwptr update behavior, in case something went wrong with clocks?
> 
> 
> thanks,
> 
> Takashi