On Wed, 7 May 2025, BALATON Zoltan wrote:
On Wed, 7 May 2025, Christian Schoenebeck wrote:
On Tuesday, May 6, 2025 4:34:42 PM CEST BALATON Zoltan wrote:
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().
Yeah, I think you are right Volker. The already existing -EPIPE and
-ESTRPIPE
error cases should handle the "xrun" and "suspended" conditions
equivalently
in poll mode.
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.
Agreed, -ESTRPIPE is missing in alsa_read(), and -EBADF (audio device in
invalid state) is missing in both alsa_read() and alsa_write(). That could
probably happen when you unplug an USB audio device. On macOS applications
also need to recover when plugging in/out headphones, but IIRC that's not
necessary with ALSA.
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).
Well, I wouldn't say it's exactly the same thing to experience a
misbehaviour
with a specific, custom supplied option vs. experiencing the misbehaviour
when
running with default options. And I am pretty sure you know why. ;-)
Considering that most people use distros that have pulsaudio or pipewire or
some other sound server they likely already need a command line option to
enable alsa but needing an additional obscure option to make that work that's
not really documented at any easy to find place makes that much worse. So a
mostly working default is still better than a surely broken one.
But anyway, I won't object this change:
Acked-by: Christian Schoenebeck <qemu_...@crudebyte.com>
Thank you, hope some maintainer sees this and pick up this patch.
If there will be a pull request with Volker's fixes could this be also
included? Link:
https://patchew.org/QEMU/20250316002046.d066a4e6...@zero.eik.bme.hu/
Regards,
BALATON Zoltan
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.
A suspend condition is simply putting the host machine asleep, i.e. closing
the lid of your Linux notebook, or selecting suspend from the menu of your
desktop environment.
I have a desktop machine that I don't normally suspend and the menu item does
not seem to work so I can't test that. Is just stopping QEMU process enough
or the host has to be restarted while QEMU is running to test this?
For testing for xruns (a.k.a. buffer overrun / buffer underrun) you could
simply insert a short sleep in the render code to simulate the audio render
function to take too long to deliver the audio data to the ALSA subsystem.
I don't know audio in QEMU so don't know where to insert such sleep. Maybe I
could test this if you can give more specific advice.
Regards,
BALATON Zoltan
In both cases QEMU should recover audio. If it doesn't, you will simply
continue to hear silence with QEMU for good.
/Christian
