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

Jon Smirl jonsmirl at gmail.com
Mon May 11 20:00:59 CEST 2009

On Mon, May 11, 2009 at 1:37 PM, Jaroslav Kysela <perex at perex.cz> wrote:
> On Mon, 11 May 2009, Jon Smirl wrote:
>> Checking for over/under run in software is not reliable since the DMA
>> hardware runs asynchronously with the CPU. There will always be
>> variable latencies between when the CPU detects the condition and when
>> it can control the DMA hardware.  The only reliable way to do this is
>> to program the DMA hardware to do itself. AFAIK all DMA modern
>> hardware can be programed to do this if the right information is made
>> available. Programming the DMA hardware to do this is a 100% reliable
>> solution and not subject to random latency problems.
> I comment playback for simplicity.
> The key point is when you fill DMA engine. Nothing forces you to queue whole
> ring buffer in the scatter-gather list. You may queue just one period and
> then next one (including partial). The available samples can be determined
> using snd_pcm_playback_hw_avail() function at any time. If you have large

I didn't know about this function. I can use it to replace the direct
manipulation of appl_ptr.

> FIFO and you receive interrupt before FIFO goes empty, you can fill partial
> period. The elapsed notifier does not require any change.
> The underruns should be avoided as first step, because it's really unwanted
> system behaviour. All methods to fix underruns fails in some respects.
> Also note that we have also mode when appl_ptr is not updated at all (when
> stop_threshold == boundary).

How does this mode work? Could it be hidden inside
snd_pcm_playback_hw_avail() such that low level drivers don't have to
worry about it?

>                                                Jaroslav
> -----
> Jaroslav Kysela <perex at perex.cz>
> Linux Kernel Sound Maintainer
> ALSA Project, Red Hat, Inc.

Jon Smirl
jonsmirl at gmail.com

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