RFC: changing DMA slave configuration API

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Mon, Jun 11, 2012 at 10:20:49AM +0530, Vinod Koul wrote:
> I think it is a good idea. And I would like to extend it even a little
> bit. Do we have any users of peripheral to peripheral slave dma?
> IIRC  that is not the case, or does anyone know of existence or plans
> for such a h/w?
> 
> If not, lets junk the src/dst fields and keep burst, length, addr fields
> which point to the peripheral values.
> 
> Alternatively if we need both, then we can't have union and Russell's
> idea seems good one :)

We don't need the union whatever way that goes.

The question over whether we have the src/dst fields is whether we want
to support a different configuration for DMA_DEV_TO_MEM/DMA_MEM_TO_DEV
without having to reconfigure the channel each time its direction is
switched.

Out of the following users:
drivers/mmc/host/mmci.c
drivers/spi/spi-pl022.c
drivers/tty/serial/amba-pl011.c

with amba-pl08x, I don't see any which set a different configuration
depending on direction, and sa11x0 and OMAP DMA engine drivers only
support one direction per channel.

So, the question really comes down to whether we _want_ to support this,
and how painful it would be to re-introduce this if we did need it.
Maybe the right answer is to use the control command value to sort that
out when we come to it.