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

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

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().

/Christian