"Zoltán Kővágó" <dirty.ice...@gmail.com> writes: > On 2019-09-25 11:49, Markus Armbruster wrote: >> "Zoltán Kővágó" <dirty.ice...@gmail.com> writes: >> >>> On 2019-09-23 15:08, Markus Armbruster wrote: >>>> "Kővágó, Zoltán" <dirty.ice...@gmail.com> 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 <dirty.ice...@gmail.com> >>>>> --- >>>>> >>>>> 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.
Should we document this general error handling deficiency somehow? >> Sometimes rejecting non-working configurations is impractical. Is it >> here? > > I think it is. It depends on the backend, its settings, the frontend > (emulated sound card), and how the guest uses it. We currently don't > know what formats does a backend support, what formats can a frontend > produce, and even if we would know that, just because a frontend can > produce a format that the backend doesn't understand doesn't mean that > it will actually do it. For example, right now with this patch series > applied, usb-audio can produce 7.1 audio. If we want to be strict, it > means we can only use it with backends that support at least 8 > channels, even if the user only wants to use stereo audio. > >> If yes, we should call out the problematic configurations in >> documentation. > > I think we should rather list known working configurations, and leave > the others as "try at your own risk" because there's too many things > that can go wrong. (pulseaudio will work, alsa with dmix too. Need > to check coreaudio, dsound and oss. spice and wavcapture won't work.) Far from ideal, but better than nothing. Possibly naive idea: what about automatically falling back to mixeng on when mixeng off doesn't work? Requires detecting "doesn't work", which I understand just isn't there. Any other reasons why this couldn't be done? Way out of scope for this series, of course.
