Re: pointer callback in pcm

[Adrian McMenamin <adrian@mcmen.demon.co.uk>]

On Sat, 2006-03-11 at 11:16 +0000, James Courtier-Dutton wrote:
> Adrian,
> 
> Here is a quick summary of what I think you have been saying.
> 1) Your hardware has min and max periods set to 8.
> 2) Your hardware can do DMA, but not periodic DMA.   <- This is really 
> the crux of the problem.
> 3) You wish to use DMA.

I really need to because transfers between the sound memory and the main
memory are very slow otherwise. When I wrote the OSS driver for this a
few years ago I simply copied stuff over, but it would effectively lock
the machine up at high sample rates.

> 4) Unknown:  What triggers the audio interrupt? It should be triggered 
> once each period is completed.


This is done by the ARM7 spu that controls the sound output. I write the
"firmware" for that so I have written some code that puts an IRQ on the
SH4 (main CPU) bus every time a period has been played out.

> 
> Now, here is what the ALSA model assumes:
> 1) hardware period sizes are exactly the same size as the memory mapped 
> DMA pages.

I wasn't aware of that but I think I've done that anyway (4k period
size)

> 2) snd_period_ellapsed() is called at periodic intervals dependent on 
> where the ADC is playing samples.
> 3) If the copy callback is NULL, ALSA assumes the DMA has already completed.
> 4) If the copy callback is not NULL, ALSA calls it, but when the copy 
> callback returns, the copy has been completed.

I haven't got a copy callback - should I write one?


> 5) The pointer callback is then called that should return the position 
> of the ADC in frames (not bytes) within the audio buffer.
> 
> So, if you wish to use DMA, you should be calling snd_period_ellapsed() 
> only after the DMA has completed, alternatively, the copy() callback 
> should only return after the DMA transaction has completed, but I don't 
> know if delaying the copy() return is allowed.
> Can you get it to interrupt on each DMA completion?
> 

In theory yes, but nobody seems have managed to reverse that bit of the
spu function effectively yet, so I'd have to use polling on the progress
of the transfer.


> I would recommend you trying to do the following:
> 1) Discover when the ADC has played the entire period.
> 2) start the DMA transfer.
> 3) At the DMA transfer complete interrrupt, call period_ellapsed().
> 
> So, the problem might be how to discover that the ADC has played the 
> entire period.

I've got that covered (see above).

How do I start? I am copying over 8 periods via DMA on the trigger, is
that ok? (These are all I ever hear getting played). What about the ack
callback that was recommended to me yesterday?

> You might have to use some sort of timer based polling to discover this.
> 
> James



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