Re: How to deal with cards with a large on-board buffer?

[Nicolas Boullis <nboullis@debian.org>]

On Wed, May 03, 2006 at 02:26:58PM +0200, Clemens Ladisch wrote:
> > 
> > To make it short, the trouble is that, to get correct working, it seems 
> > to require that the audio buffer be kept at some minimal fill level. Is 
> > there a way I can enforce this?
> 
> The card should do this automatically as it initiates the DMA transfers
> by itself.

It does. But then I have problems if the transfered buffer has not been 
filled by the application...


> > Now, just in case someone has a better idea how to make things work, 
> > here is a more comprehensive explanation about how those cards work, *as 
> > far as I could understand* (since I got absolutely no explanation from 
> > the manufacturer):
> >    - there is a rather large on-chip buffer (0x3c00 bytes), that we can 
> >      neither write to or read directly
> 
> So it's just a FIFO that is more or less transparent to the computer?

I guess so.

> >    - we can read pointers to the current read and write positions in 
> >      this buffer
> >    - this buffer is fed by chip-initiated DMA transfers from the main 
> >      memory
> 
> How big are these transfers?

In the current code, they are 4kB (1ksample) or 8kB (2ksample). I can 
specify the size I want, but it does not work with large buffers (8kB is 
already more than half the size of the on-chip buffer). And it seems 
that there are problems when the buffers are not 4kB-aligned.


> >    - the buffers for DMA transfers are declared in a FIFO;
> 
> You mean this chip supports scatter/gather buffers?

I'm not sure what you mean exactly by scatter/gather buffers, but I 
guess it is something like that.


> >    - if the on-chip buffer is "too empty" (don't know exactly when the 
> >      chip decides it is too empty) and no buffer is available for 
> >      DMA transfer, it "sends" some junk to the internal buffer
> 
> Most sound card FIFOs operate like this.

OK, I did not know.


> >    - interruptions are not raised when buffers are DMA-transfered, but 
> >      at fixed 20.2ms interval (stable experimental value)
> 
> The ymfpci and usb drivers deal with a similar problem.

I'll try to give them a look.
But if I understand things correctly, I must ensure that the periods are 
large enough to be played in more than 20.2ms, if I want 
snd_pcm_period_elapsed to be called often enough. Is that right?


Cheers,

Nicolas

PS: sorry if I'm being stupid, I'm not used to writing audio drivers...


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