[alsa-devel] Breakage in pcm_lib.c

[Takashi Iwai]

At Sun, 26 Apr 2009 12:45:32 -0400,
Jon Smirl wrote:
> 
> On Sun, Apr 26, 2009 at 12:31 PM, Jaroslav Kysela <perex at perex.cz> wrote:
> > On Sun, 26 Apr 2009, Jon Smirl wrote:
> >
> >> On Sun, Apr 26, 2009 at 4:12 AM, Jaroslav Kysela <perex at perex.cz> wrote:
> >>>
> >>> On Sat, 25 Apr 2009, Jon Smirl wrote:
> >>>
> >>>> This appears to be a bug in this code...
> >>>>
> >>>>        if (delta < 0) {
> >>>>                delta += runtime->period_size * runtime->periods;
> >>>>
> >>>> it was adding, buffer_size. But buffer_size is not correct when the
> >>>> periods don't evenly fit into the buffer
> >>>
> >>> Could you show real values what you're getting here with your driver?
> >>
> >> psc_dma_pcm_pointer pos 0
> >> hw_ptr_interrupt 22052 new_hw_ptr 0
> >
> > pos should be >2 here for buffer_size = 22050 and period_size = 5513.. Your
> > driver code is broken - the routine must be called when period_size is
> > processed - not before and .pointer callback must return correct value at
> > this point..
> >
> > So pos at interrupt time should be equal or slightly greater than:
> >
> > 5513, 11026, 16540, 22054 % 22050 = 4, 5517 etc..
> >
> > pos is position from  range 0 .. (buffer_size - 1) not from range 0 ..
> > (periods * period_size).
> 
> So the DMA hardware needs to be able to wrap that last packet in the
> buffer? I will need to turn that into two DMA operations which will
> cause two interrupts, is that ok?
> 
> This code was originally written with the assumption that an integral
> number of periods would fit into the DMA buffer.

It's assured only when you set up the proper hw_params constraints.
Namely, call snd_pcm_hw_constraint_integer() with
SNDRV_PCM_HW_PARAM_PERIODS parameter.


Takashi