"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? Sometimes rejecting non-working configurations is impractical. Is it here? If yes, we should call out the problematic configurations in documentation. [...]
