Re: The ALSA Situation

[Takashi Iwai <tiwai@suse.de>]

At Thu, 11 Nov 2004 19:52:47 -0300,
Manuel Jander wrote:
> 
> Hi,
> 
> On Thu, 2004-11-11 at 17:34 +0100, Takashi Iwai wrote:
> > At Wed, 10 Nov 2004 22:42:20 -0800 (PST),
> > Linus Torvalds wrote:
> > > 
> > > On Thu, 11 Nov 2004, Jaroslav Kysela wrote:
> > > > 
> > > > Unfortunately, the application will have to settle other real-time
> > > > parameters (including stream parameters, i/o chunk sizes etc.), so we must
> > > > drain on close() the complete stream and not allow intermixing requests
> > > > from applications.
> > > 
> > > That seems to be a misdesign of the interfaces, but whatever.
> > 
> > Well, the contiguous use of a stream isn't easy as it sounds.
> > Since each app wants different buffer size, period (fragment) size,
> > sample format, sample rate etc, the configuration must be reset
> > totally at each time.  This eventually requires to stop of the DMA.
> 
> There 2 situations (AFAIK): 
> 
> 1) * Stinking intel8x0 or similar card with only one stereo stream:
>   - Device should be accessed as 48KHz 16 bit stereo (highest quality
> available), since it most probably does not even have a samplerate
> converter.

No, most of boards have SRC (on AC97 chip).  The board without SRC is
rare nowadays.

>  There should be no concern about latency/buffer size, thats
> just silly in such a kind of device. Just use the most efficient DMA
> setup.
>   - All applications using the device should be softmixed. It does not
> make any sense to give direct access to applications.

IMO, it's too narrow-sighted.  You can't say that no user wants the
low-latency system like JACK for an onboard chip.  There definitely
are (imagine laptop users).

But for _normal_ apps, yes, they should all use the softmix.

> 2) * Multichannel card:
>   - Use one channel for softmixing (reserved for ever for that purpose,
> at highest audio quality). Just the same as for the single channel card.
>   - Use a "Resource Manager" that assigns hardware channels as are
> available, and choose the softmixing engine coupled to the reserved
> channel when we get out of hardware channels.
>    - If we want to make this even nicer, and thinking in the future
> where hardware channels may have (they actually have, but are not
> supported) advanced processing features, the "Resource Manager" could
> assign hardware channels on behalf of the features the applications asks
> for. For example if a application does not ask for HRTF processing, give
> it a softmixing channel, or  a hardware channel lacking that feature.

The multichannel cards have usually no hardware mixing.  They provide
one DMA for all channels, either interleaved or non-interleaved.

So, decoupling these channels means that one has to grab all channels
exclusively and distribute them.  The situation is pretty similar like
the first case, anyway.


Takashi


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