[PATCH 04/32] dmaengine: ste_dma40: Amalgamate DMA source and destination channel numbers

[Linus Walleij]

On Thu, Apr 25, 2013 at 3:09 PM, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:

> There's a problem with device to device transfers though - you have to
> consider the rate at which the devices produce and consume data, and
> whether they both can cope with differing data rates.
>
> Take for instance your audio in to audio out idea - even if they are
> both operating at the same bits per sample and sample rate, if they are
> independently clocked, chances are that the clocks are not exactly the
> same, which means you will either underrun or overrun one of the FIFOs
> in the system.

This magically works for us, but yes, I think they had to use the same
clock (simplest solution, anyway).

It is/was a very real usecase, IIRC coming from roundtrip constraints
in the GSM 3GPP spec, wanting to keep the latency as low as possible.
With this scheme there is no more than a FIFO on each end of delay,
which is nice.

Yours,
Linus Walleij