On Tue, 8 Apr 2025, Volker Rümelin wrote:
Am 08.04.25 um 14:55 schrieb Christian Schoenebeck:
On Friday, April 4, 2025 1:34:27 PM CEST BALATON Zoltan wrote:
On Fri, 4 Apr 2025, Christian Schoenebeck wrote:
On Monday, March 31, 2025 3:05:24 PM CEST BALATON Zoltan wrote:
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 <bala...@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?
Well, I understand that if fixes the misbehaviour you encountered. But how
helpful would it be if it then breaks behaviour for other people instead?
What behaviour would it break and how?
There are only a bunch of ALSA states handled right now in the QEMU Alsa
driver (see alsa_poll_handler()):
state = snd_pcm_state (hlp->handle);
switch (state) {
case SND_PCM_STATE_SETUP:
alsa_recover (hlp->handle);
break;
case SND_PCM_STATE_XRUN:
alsa_recover (hlp->handle);
break;
case SND_PCM_STATE_SUSPENDED:
alsa_resume (hlp->handle);
break;
case SND_PCM_STATE_PREPARED:
audio_run(hlp->s, "alsa run (prepared)");
break;
case SND_PCM_STATE_RUNNING:
audio_run(hlp->s, "alsa run (running)");
break;
For instance in poll mode it recovers in case of an xrun, which happens on
audio output if the audio output data was not delivered by the application in
time.
The other case is when the system was suspended (standby). It should also
recover the audio session here.
Hi Christian,
I think the timer based mode works fine. snd_pcm_readi() and
snd_pcm_writei() return -EPIPE in case of a xrun and -ESTRPIPE if a
suspend event occurred. Both cases are handled in alsa_write(). The
-ESTRPIPE case is missing in alsa_read(), which may be a mistake. I
don't think it's possible alsa_read() and alsa_write() get called if the
ALSA system is in state SND_PCM_STATE_SETUP.
The write_loop() example code at
https://www.alsa-project.org/alsa-doc/alsa-lib/examples.html in file
test/pcm.c also doesn't use the snd_pcm_state() function. Please have a
look at write_loop() in test/pcm.c and compare it with
write_and_poll_loop() in the same file.
With best regards
Volker
Could we get back to this and either accept this patch or find another
solution? From the above, it looks like this could be an acceptable fix
unless we can prove it would break something (but then it's already
possible to break it from the command line option so it's preexisting
problem).
Now I haven't tested whether these would work in callback mode right now, but
looking at the code suggests that they might not.
I think it would be better to add a 2nd patch that would handle state changes
in callback mode. That would satisfy both groups of people. AFAICS
snd_pcm_state() can be called both in polling mode and callback mode.
I can't do that because I don't quite know neither alsa nor audio in QEMU
so I have no idea what to do. Can you give more clues?
Well, as a starting point you might try whether these cases described above
would still work in callback mode. Maybe it is even working, who knows.
Can you suggest how can I test that? I'm not sure what to try and sound
works with this patch for the cases I use.
Regards,
BALATON Zoltan
