[pulseaudio-discuss] underrun behavior with alsa-plugins

[Maarten Lankhorst]

Hi David,

Op 20-04-11 09:33, David Henningsson schreef:
> On 2011-04-19 18:12, Maarten Lankhorst wrote:
>> Hi all,
>>
>> For wine I was investigating a bug with pulseaudio, it seems
>> alsa-plugins' pulse driver ignores underruns.
>
> Hmm, doesn't wine come with a native pulse driver these days?
Nope, but distros patch in a buggy winepulse driver, wine is working on 
rearchitecting its driver model, so that a pulseaudio driver might be 
added after that, but the current winepulse driver was a bad 
copy/replace job of the wrong sound driver. :)

>> This is probably because
>> an underrun will force you to call snd_pcm_prepare, this will destroy
>> the stream and set up a similar new one again.
>
> ...whereas the proper way to resolve the underrun in the pulse plugin is
> just to feed it with more data, which is likely what would happen if you
> do not report the underrun.
>
>> This causes more
>> susceptibility to underruns, so that code was disabled.
>
> Takashi added some kind of option so you can turn the reporting of
> underruns on again, if that helps.
If you read my patch, that's what it does.

>> However if I
>> force underruns to occur,the state will stay running and it appears
>> there is still some data left in the buffer, so sound stalls entirely.
>
> This is nothing I've heard of.
Sadly all too common here, if I feed data and it underruns, it may 
appear to be running but is stalled entirely.

>> The latency gets updated to something like 0x7bdXXXXXXXXXXXXX which
>> looks suspiciously much like a pointer to me, which may be a bug. If I
>> instead make pulse_prepare call pulse_start on underrun, it's handled
>> properly and sound will continue to work.
>>
>> So my questions are.
>> 1. Is the weird latency value update a bug?
>
> I guess so, but what sense would it make to read the latency if you're
> in an underrun condition - and btw, exactly what latency variable is this?
In a response to a latency event I printf the latency inside the 
callback, with pa_stream_get_latency.

>> Digging it up I can only
>> assume it's a bug in src/stream.c , but I haven't figured out why yet.
>> Tested with pulseaudio.c
>> 2. Any comments on this patch for alsa-plugins?
>
> IIRC, by setting handle_underrun to 1, you're reopening bugs of
> broken/skipping audio for mpg123 and other programs, which get stuck in
> an infinite loop of "write one sample, force start the stream, write
> more samples, asynchronusly get an underrun, drop the stream and drop
> samples already written".
>
> There should be threads on at least alsa-devel about this, from a year
> back or so.
This is exactly what my patch was addressing, I was fixing that bug by 
handling underrun correctly. snd_pcm_prepare() is called when an 
underrun occurs, so instead I make it only restart the stream if 
underrun. Not handling underruns at all seems to allow it to stall on 
xrun, while claiming to be running. This sometimes appears to happen in 
other programs too though, like mplayer. Could it be because of the 
weird latency value?

Cheers,
Maarten