Re: [Qemu-devel] [PATCH] Document Qemu coding style

[malc <av1474@comtv.ru>]

On Wed, 1 Apr 2009, David Turner wrote:

> On Wed, Apr 1, 2009 at 12:38 AM, malc <av1474@comtv.ru> wrote:
>
> >
> > For starters you could have asked about things you believe are subtle.
> >
>
> True enough, but for a long time the project was confidential and alluding
> to it publicly
> was not recommended. I'll try to break the habit.
>
>
> >
> > Now to the part i do not understand: what do you mean by time-based(sic)?
>
>
> > There's one clock involved, as far as audio is concerned, and it's the
> > one dervied from the speed with which host audio can consume/produce
> > audio, anything else just doesn't work (for scenarios i was interested
> > in anyway)
> >
>
> I had some problems when running qemu on low-powered computers. If I
> remember correctly, the emulation part was taking so much of the CPU
> that audio could very well stutter in strange ways on some
> platforms. One of the reason for it was that the host audio output
> (e.g. esd) stopped consuming samples at a consistent rate, which
> forced SWVoiceOut buffers to fill up and delay audio production in
> the emulated system even more.

Hmm.. please define low-powered.. Audio started being developed on a
1Ghz Athlon, granted the machine was equipped with Aureal Vortex 2 and
i had a luxory of using [ADA]C_FIXED_SETTINGS set to zero and
DAC_VOICES set to something reasonable thus basically avoiding
software mixing altogether, that said some of the old DOS games/demos
use Soundblaster/OPL[23] in such creative ways so doing things in
software even with au8830 was sometimes inevitable...

And FWIW nowadays it's MPC7447A at 1.3Ghz which is not speed demon
either. Not that i dont belive you, esp considering esd thrown into
equation, but it would be interesting to know what people deem
low-powered currently.

> In certain cases, there is also a non-trivial latency between the
> moment the emulated system produces audio-samples (e.g. sends them
> through dma to the emulated hardware), and the moment it is
> effectively sent to the host backend. This is mostly related to
> playing with the audio timer tick configuration which by default is
> so small that it should not matter.
>
> What I mean by time-based means a way to deal with varying latencies
> in both audio production and consumption. I agree it's a hard
> problem and is not totally required for QEMU. It's just that I spent
> some time trying to understand how the audio subsystem dealt with
> the problem, except that it didn't.

Well.. I certainly fail to see how adding some other clock would
overcome the fact that something can't keep up.. (rt priorties and
suchlike?)

>
> >
> > As for the question of why everything just calls audio_pcm_sw_write
> > raised in AUDIO.txt, the reason, if my memory serves and it might as
> > well not do that all that well, things are the way they are for the
> > reason that any given driver can, theoretically, use the respective
> > audio subsystems/libraries own, less naive, st_rate_flow equivalent.
> >
>
> Interesting, thanks for the explanation, this makes it clear.
>
>
> > P.S. Last/only time i looked at Android couldn't help but notice that,
> >     apart from other things, capture was implemented for coreaudio,
> >     it would have been nice, if for nothing else but the sake of
> >     completeness, to have that in main QEMU too.
> >
>
> Actually, the details of audio-related changes performed are:
>
> - adding audio input to the CoreAudio backend
>
> - adding dynamic linking support to the esd and alsa backends
> (using dlopen/dlsym allows the emulator to run on platforms where
> all the corresponding libraries / sound server are not available).

I don't think this is worth it for QEMU, after all shipping binaries
is not what it's best known for.

> - rewriting the sdl backend run_out() method. For some reason the old one
> tended to lock up QEMU in certain weird cases I could never
> completely understand.

Hmm..

> - modifying the sub-system to be able to use different backends for
>   audio input and output.

Yeah i recall seeing this, and was wondering why Android needed this
functionality.

> - adding a new "winaudio.c" backend for Windows that uses Wave
>   functions instead of DirectX (mainly to be able to build on old
>   versions of mingw that didn't provide directx-compatible headers,
>   I'm not sure if it's still needed these days, but at least the
>   code is 10x simpler than the dxaudio.c one, talk about a complex
>   sound API).
>

Complexity notwithstanding i happen to like the way things are done in
DSound, the conceptually simple approach that is.

> I plan to provide patches for all of these for upstream. Sorry if this
> hasn't been done yet.
>

Nice to hear that.

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