Re: [PULL 8/9] alsaaudio: Set try-poll to false by default

[Markus Armbruster <armbru@redhat.com>]

marcandre.lureau@redhat.com writes:

> From: BALATON Zoltan <balaton@eik.bme.hu>
>
> 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>
> Acked-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> [ Marc-André - Updated QAPI and CLI doc ]
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> Message-Id: <20250316002046.D066A4E6004@zero.eik.bme.hu>
> ---
>  qapi/audio.json   | 2 +-
>  audio/alsaaudio.c | 2 +-
>  qemu-options.hx   | 4 ++--
>  3 files changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/qapi/audio.json b/qapi/audio.json
> index dd5a58d13e..49633cf317 100644
> --- a/qapi/audio.json
> +++ b/qapi/audio.json
> @@ -96,7 +96,7 @@
>  # @period-length: the period length in microseconds
>  #
>  # @try-poll: attempt to use poll mode, falling back to non-polling
> -#     access on failure (default true)
> +#     access on failure (default false)
>  #
>  # Since: 4.0
>  ##

Missed this when it was posted (it wasn't cc'ed to me).  Flipping the
default is technically an incompatible change.  I understand the
justification, and I'm not passing judgement on its validity; that's the
audio maintainers job.  I just want to ask: does this need a release
note?

We normally record incompatible changes in docs/about/deprecated.rst and
then docs/about/removed-features.rst, but these don't fit here.

[...]