[alsa-devel] appl_ptr and DMA overrun at end of stream

[Takashi Iwai]

At Mon, 11 May 2009 11:50:22 -0400,
Jon Smirl wrote:
> 
> On Mon, May 11, 2009 at 11:40 AM, Takashi Iwai <tiwai at suse.de> wrote:
> > At Mon, 11 May 2009 11:11:55 -0400,
> > Jon Smirl wrote:
> >>
> >> On Mon, May 11, 2009 at 9:45 AM, Jaroslav Kysela <perex at perex.cz> wrote:
> >> > On Mon, 11 May 2009, Jon Smirl wrote:
> >> >
> >> >>> Right.  This is the value to check in your case.
> >> >>
> >> >> What do think about redesigning the ALSA DMA interface to support
> >> >> detection of over and under run? Leaving the DMA engine in a loop and
> >> >> not coordinating with ALSA as to where the valid data is does not seem
> >> >> to be a safe way of exchanging data. That interface may be a source of
> >> >> the problems pulseaudio is encountering.
> >> >>
> >> >> A simple solution would be for snd_pcm_period_elapsed() to return
> >> >> physical address of the last valid sample. That would let me avoid
> >> >> playing with  s->runtime->control->appl_ptr. You could provide the
> >> >> same data in the pointer() function.
> >> >
> >> > More simpler solution is to check the stream state in the low level driver.
> >> > If it's in DRAINING state, then end of stream is signaled from the
> >> > application and driver might not queue next buffer. We may also add another
> >> > callback (or use ioctl callback) to pass this stream state change to the
> >> > lowlevel driver immediately, so the driver might react more quickly on this
> >> > situation.
> >> >
> >>
> >> Quickness is the wrong way to think about this problem. ALSA knows exactly
> >> when it has placed valid data into the buffer.
> >
> > Not really.  When the mmap mode is used, the update isn't always
> > notified to the driver and the transfer can be completely
> > asynchronous.
> 
> This seems to me to be a broken design. ALSA is being put into the
> position of guessing when the application has supplied new data.
> Shouldn't the app be required to make a commit() call after filling in
> the data? Without commit it is impossible to detect over/underrun.

The commit updates the mmapped control data (so that it works even
without the context switch) if the architecture supports.  In other
cases, a commit issues an explicit sync ioctl.

Actually it should be possible to disable the mmap-control mode
explicitly, but right now it's not done from the driver side but only
checks the architecture.


Takashi