On 2019-09-25 11:49, Markus Armbruster wrote:
"Zoltán Kővágó" <address@hidden> writes:
On 2019-09-23 15:08, Markus Armbruster wrote:
"Kővágó, Zoltán" <address@hidden> writes:
This will allow us to disable mixeng when we use a decent backend.
Disabling mixeng have a few advantages:
* we no longer convert the audio output from one format to another, when
the underlying audio system would just convert it to a third format.
We no longer convert, only the underlying system, when needed.
* the underlying system probably has better resampling and sample format
converting methods anyway...
* we may support formats that the mixeng currently does not support (S24
or float samples, more than two channels)
* when using an audio server (like pulseaudio) different sound card
outputs will show up as separate streams, even if we use only one
backend
Disadvantages:
* audio capturing no longer works (wavcapture, and vnc audio extension)
* some backends only support a single playback stream or very picky
about the audio format. In this case we can't disable mixeng.
However mixeng is not removed, only made optional, so this shouldn't be
a big concern.
Signed-off-by: Kővágó, Zoltán <address@hidden>
---
Notes:
Changes from v1:
* renamed mixeng to mixing-engine
qapi/audio.json | 5 +++++
qemu-options.hx | 6 ++++++
2 files changed, 11 insertions(+)
diff --git a/qapi/audio.json b/qapi/audio.json
index 9fefdf5186..0535eff794 100644
--- a/qapi/audio.json
+++ b/qapi/audio.json
@@ -11,6 +11,10 @@
# General audio backend options that are used for both playback and
# recording.
#
+# @mixing-engine: use QEMU's mixing engine to mix all streams inside QEMU. When
+# set to off, fixed-settings must be also off. Not every
backend
+# compatible with the off setting (default on, since 4.2)
+#
Last sentence no verb.
Which backends are compatible?
Actually that's a simplification, it depends on a few things. When
mixeng is off, qemu will try to use the same format as the emulated
sound card, and if the backend doesn't support that format, it won't
work (no audio). Also attaching multiple sound cards to the same
audiodev might not work, if the backend doesn't support multiple
playback streams. If you use pulseaudio, it'll work without problems,
if you use alsa, it depends on your device. If you use a hw: device
directly, you'll likely only be able to use one emulated sound card
with a few selected audio formats. If you use dmix: (and plug), alsa
will handle the conversion and mixing, so it will work no matter what
format the emulated sound card uses. With OSS the situation is
probably similar, it depends on the kernel/hw what works and what not.
wav and spice certainly doesn't support multiple streams. I'm not
completely sure about the other backends right now, but I think dsound
and coreaudio can handle the necessary sample format conversions and
mixing.
What happens when you try the off setting with incompatible backends?
See above.
What happens *exactly*?
I'm asking because I'm concerned about the user experience. When a user
asks for a combination of things QEMU can't provide, such as mixeng off
with an incompatible backend, QEMU should fail with a suitable error
message. Does it?
Error handling is not the best in the audio subsystem, if something
fails it generally just prints a warning to the console and continues,
and something will happen... For example, this is what happens when I
try to open one hw device twice. I ran qemu with:
-audiodev
alsa,id=foo,in.dev=hw:1,,0,out.mixing-engine=off,out.dev=hw:1,,0
-device piix4-usb-uhci -device usb-audio,audiodev=foo -device
AC97,audiodev=foo
When the guest tried to initialize the AC97 card, I got an error:
alsa: Could not initialize DAC
alsa: Failed to open `hw:1,0':
alsa: Reason: Device or resource busy
And it just continued. And the sound worked, but with wrong sample
rate (AC97 wants 44100 Hz, but USB audio previously opened the alsa
device with 48000 Hz). I'll fix this bug in the next revision,
audio_pcm_hw_add_* shouldn't fall back to other HWs without mixeng.
But even with that, the result will be that one emulated sound card
will work and the other won't.
It's not ideal, but fixing it would require a lot of effort. Right
now, if you specify an invalid audiodev for alsa (even with mixeng),
it'll just print an error to the console and continue without audio.