Re: [PATCH 04/32] dmaengine: ste_dma40: Amalgamate DMA source anddestination channel numbers

From: Linus Walleij
Date: Thu Apr 25 2013 - 09:21:13 EST

On Thu, Apr 25, 2013 at 3:09 PM, Russell King - ARM Linux
<linux@xxxxxxxxxxxxxxxx> 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.

Linus Walleij
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