[alsa-devel] Query on Audio DMA using DMAEngine

[Joel Fernandes]

Hi Mike,

On 07/01/2013 01:10 AM, Mike Looijmans wrote:
[..]
> The trouble with the current davinci driver is that the IRQ handler has a
> real-time requirement, it must finish before the next DMA block completes. This

I looked into this a little more.

I think you are picturing the following:

DMA transfer -> IRQ has to complete -> DMA transfer -> IRQ has to complete.. etc.

This is not really true in the davinci-pcm driver, the normal case without IRAM
works more like..

DMA ----> DMA ---> DMA
 \        \        \
  \__ IRQ  \__ IRQ  \__ IRQ

The only hard requirement is the IRQ handler much finish updating before the
next DMA transfer, or we're in trouble. Is this what you mean by real-time
requirement, or did you mean something else?

Either way I'm sure your multi-slot approach is superior, but I don't see how
you can get away with not updating the DMA addresses on every IRQ with the
current davinci-pcm or EDMA controller (Unless you use a complicated mechanism
like ping-pong where the address updates take care of itself). If you are using
a set of chained slots, you only have so many slots so you have to continuously
change addresses of the slots at some point or the other for a large transfer.

Thanks,

-Joel