Re: [PATCH] alsaaudio: Set try-poll to false by default

[BALATON Zoltan <balaton@eik.bme.hu>]

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On Sun, 23 Mar 2025, Christian Schoenebeck wrote:
> On Sunday, March 16, 2025 1:20:46 AM CET BALATON Zoltan wrote:
>> Quoting Volker Rümelin: "try-poll=on tells the ALSA backend to try to
>> use an event loop instead of the audio timer. This works most of the
>> time. But the poll event handler in the ALSA backend has a bug. For
>> example, if the guest can't provide enough audio frames in time, the
>> ALSA buffer is only partly full and the event handler will be called
>> again and again on every iteration of the main loop. This increases
>> the processor load and the guest has less processor time to provide
>> new audio frames in time. I have two examples where a guest can't
>> recover from this situation and the guest seems to hang."
>>
>> One reproducer I've found is booting MorphOS demo iso on
>> qemu-system-ppc -machine pegasos2 -audio alsa which should play a
>> startup sound but instead it freezes. Even when it does not hang it
>> plays choppy sound. Volker suggested using command line to set
>> try-poll=off saying: "The try-poll=off arguments are typically
>> necessary, because the alsa backend has a design issue with
>> try-poll=on. If the guest can't provide enough audio frames, it's
>> really unhelpful to ask for new audio frames on every main loop
>> iteration until the guest can provide enough audio frames. Timer based
>> playback doesn't have that problem."
>>
>> But users cannot easily find this option and having a non-working
>> default is really unhelpful so to make life easier just set it to
>> false by default which works until the issue with the alsa backend can
>> be fixed.
>>
>> Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
>> ---
>> This fixes my issue but if somebody has a better fix I'm open to that
>> too.
>>
>>  audio/alsaaudio.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/audio/alsaaudio.c b/audio/alsaaudio.c
>> index cacae1ea59..9b6c01c0ef 100644
>> --- a/audio/alsaaudio.c
>> +++ b/audio/alsaaudio.c
>> @@ -899,7 +899,7 @@ static void alsa_enable_in(HWVoiceIn *hw, bool enable)
>>  static void alsa_init_per_direction(AudiodevAlsaPerDirectionOptions *apdo)
>>  {
>>      if (!apdo->has_try_poll) {
>> -        apdo->try_poll = true;
>> +        apdo->try_poll = false;
>>          apdo->has_try_poll = true;
>>      }
>>  }
>>
>
> Correct me if I am wrong, but AFAICS if polling is not used then no state
> changes would be handled, no? At least I don't see any snd_pcm_state() call
> outside of alsa_poll_handler().

I have no idea but this fixes the problem (and does the same that can be 
also done from command line but nobody can find that command line option) 
so unless somebody has a better idea could this be merged as a fix for 
now?

Regards,
BALATON Zoltan