Re: [Qemu-devel] [PATCH v2 1/6] qapi: qapi for audio backends

[Kővágó Zoltán]

2015-06-17 18:06 keltezéssel, Markus Armbruster írta:
> "Kővágó Zoltán" <address@hidden> writes:
>
> > 2015-06-17 15:37 keltezéssel, Markus Armbruster írta:
> > > "Kővágó Zoltán" <address@hidden> writes:
> > >
> > > > 2015-06-17 13:48 keltezéssel, Markus Armbruster írta:
> > > > > "Kővágó Zoltán" <address@hidden> writes:
> > > > >
> > > > > > 2015-06-17 09:46 keltezéssel, Markus Armbruster írta:
> [...]
> > > > > > > > +##
> > > > > > > > +# @AudiodevBackendOptions
> > > > > > > > +#
> > > > > > > > +# A discriminated record of audio backends.
> > > > > > > > +#
> > > > > > > > +# Since: 2.4
> > > > > > > > +##
> > > > > > > > +{ 'union': 'AudiodevBackendOptions',
> > > > > > > > +  'data': {
> > > > > > > > +    'none':      'AudiodevNoOptions',
> > > > > > > > +    'alsa':      'AudiodevAlsaOptions',
> > > > > > > > +    'coreaudio': 'AudiodevNoOptions',
> > > > > > > > +    'dsound':    'AudiodevDsoundOptions',
> > > > > > > > +    'oss':       'AudiodevOssOptions',
> > > > > > > > +    'pa':        'AudiodevPaOptions',
> > > > > > > > +    'sdl':       'AudiodevNoOptions',
> > > > > > > > +    'spice':     'AudiodevNoOptions',
> > > > > > > > +    'wav':       'AudiodevWavOptions' } }
> > > > > > > > +
> > > > > > > > +##
> > > > > > > > +# @AudioFormat
> > > > > > > > +#
> > > > > > > > +# An enumeration of possible audio formats.
> > > > > > > > +#
> > > > > > > > +# Since: 2.4
> > > > > > > > +##
> > > > > > > > +{ 'enum': 'AudioFormat',
> > > > > > > > +  'data': [ 'u8', 's8', 'u16', 's16', 'u32', 's32' ] }
> > > > > > > > +
> > > > > > > > +##
> > > > > > > > +# @AudiodevPerDirectionOptions
> > > > > > > > +#
> > > > > > > > +# General audio backend options that are used for both playback
> > > > > > > > and recording.
> > > > > > > > +#
> > > > > > > > +# @fixed-settings: #optional use fixed settings for host DAC/ADC
> > > > > > > > +#
> > > > > > > > +# @frequency: #optional frequency to use when using fixed settings
> > > > > > > > +#
> > > > > > > > +# @channels: #optional number of channels when using fixed settings
> > > > > > > > +#
> > > > > > > > +# @format: #optional sample format to use when using fixed settings
> > > > > > >
> > > > > > > Are these guys used when @fixed-settings is off?
> > > > > >
> > > > > > No.
> > > > >
> > > > > If @fixed-settings, are the other three all required?  If not, what are
> > > > > their defaults?
> > > >
> > > > No, they all have defaults: 44100 Hz, 2 channels and s16 format.
> > >
> > > Okay, this sort of explains why you have @fixed-settings.
> > >
> > > My first thought was that @fixed-settings is redundant, because we can
> > > have any of @frequency, @channels, @format imply fixed settings.  Except
> > > that doesn't let you ask for the *default* fixed settings, as you have
> > > to specify at least one.
> > >
> > > What's the default for @fixed-settings?
> >
> > It's on by default.
> >
> > > What if I specify frequency, channels or format together with explicit
> > > fixed-settings: false?
> >
> > They will be ignored.
> >
> > The audio system currently work like this: when an audio frontend
> > wants to open an output with some format (frequency, channels, format)
> > it checks fixed-settings. If it's false, it will just open the stream
> > with the frontend specified settings. If it's true, it'll convert it
> > into the format specified by @frequency, @channels, @format, then pass
> > this converted/recoded stream to the backend.
>
> So user typically specifies either fixed-settings=off, or any
> combination of the other three (including none of them).  Correct?
>
> We could reject the non-sensical combination of fixed-settings=off plus
> any of the other three instead of silently ignoring their values.
> Matter of taste, your choice.
>
> Whatever you do, make sure to document how these four work together.
>
> Thank you for educating me so patiently.

The audio backend currently works like that you can pass any 
non-sensical values to it, like negative frequency, or 'kdp' count of 
channels, it will silently fallback to some default value, or just fail, 
but qemu will continue to run. We can make the new config more strict 
(and we should, I think), so if you have any idea where should we be 
more strict (without creating a backward compatibility headache), don't 
hesitate to point it out.