Re: PCM driver only plays tiny portion of clip

[Lee Revell <rlrevell@joe-job.com>]

On Wed, 2006-03-01 at 20:28 +0000, Adrian McMenamin wrote:
> On Wed, 2006-03-01 at 14:59 -0500, Lee Revell wrote:
> > On Wed, 2006-03-01 at 19:40 +0000, Adrian McMenamin wrote:
> > > On Wed, 2006-03-01 at 14:31 -0500, Lee Revell wrote:
> > > 
> > > > > What happens next?
> > > > 
> > > > The app(s) waiting to output audio (sleeping in poll() or write()) will
> > > > be woken up and based on the value returned by the pointer callback
> > > > above, informed that there is space in the buffer and how much space
> > > > there is, and will then write the next chunk of frames, and go back to
> > > > sleep.  Then we go back to "Hardware triggers interrupt" above and the
> > > > process repeats.
> > > 
> > > But something in the alsa subsystem determines whether there is any
> > > space before waking the app, yes?
> > > 
> > 
> > Yes - the value returned by your pointer callback minus the last value
> > returned by your pointer callback determines how much space is left.  So
> > if the app is waiting to write 512 frames, the pointer was 256 at the
> > last interrupt, and the current pointer is 512, the app will not wake up
> > until the next interrupt because only 256 frames can be written.
> > 
> 
> Presumably this is on the assumption that 512 frames is a period?

In this example the period (# of frames between interrupts) is 256.  So
if the app is waiting to write 512 frames (by setting the avail_min
software parameter) it will be woken up every other interrupt.  (This
example only makes sense if there are more than 2 periods per buffer,
otherwise the app is constantly overwriting the data that the hardware
is playing).

Lee



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