Re: [PATCH v4 15/24] audio: add mixing-engine option (documentation)

["Zoltán Kővágó" <dirty.ice.hu@gmail.com>]

On 2019-09-23 15:08, Markus Armbruster wrote:
> "Kővágó, Zoltán" <dirty.ice.hu@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.hu@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.

> 
>>   # @fixed-settings: use fixed settings for host input/output. When off,
>>   #                  frequency, channels and format must not be
>>   #                  specified (default true)
>> @@ -31,6 +35,7 @@
>>   ##
>>   { 'struct': 'AudiodevPerDirectionOptions',
>>     'data': {
>> +    '*mixing-engine':  'bool',
>>       '*fixed-settings': 'bool',
>>       '*frequency':      'uint32',
>>       '*channels':       'uint32',
>> diff --git a/qemu-options.hx b/qemu-options.hx
>> index bbfd936d29..395427422a 100644
>> --- a/qemu-options.hx
>> +++ b/qemu-options.hx
>> @@ -433,6 +433,7 @@ DEF("audiodev", HAS_ARG, QEMU_OPTION_audiodev,
>>       "                specifies the audio backend to use\n"
>>       "                id= identifier of the backend\n"
>>       "                timer-period= timer period in microseconds\n"
>> +    "                in|out.mixing-engine= use mixing engine to mix streams inside QEMU\n"
>>       "                in|out.fixed-settings= use fixed settings for host audio\n"
>>       "                in|out.frequency= frequency to use with fixed settings\n"
>>       "                in|out.channels= number of channels to use with fixed settings\n"
>> @@ -503,6 +504,11 @@ Identifies the audio backend.
>>   Sets the timer @var{period} used by the audio subsystem in microseconds.
>>   Default is 10000 (10 ms).
>>   
>> +@item in|out.mixing-engine=on|off
>> +Use QEMU's mixing engine to mix all streams inside QEMU.  When off,
>> +@var{fixed-settings} must be off too.  Not every backend is fully
>> +compatible with the off setting.  Default is on.
>> +
>>   @item in|out.fixed-settings=on|off
>>   Use fixed settings for host audio.  When off, it will change based on
>>   how the guest opens the sound card.  In this case you must not specify