[RFC] Using the DMAEngine API for scatter/gather operations

[Dan Williams]

On Thu, 2009-05-14 at 09:13 -0700, Ira Snyder wrote:
> Hello all.
> 
> I'm working on a driver for a custom board (based heavily on the
> MPC8349EMDS board) to dump data out of onboard FPGA's. I need to use the
> onboard DMA controller for this, mostly due to data transfer rate and
> timing requirements.
> 
> Of course, I'd like to "play nice" with the existing Linux
> infrastructure in my driver, even though I have no plans to submit the
> driver upstream (since it is only useful for our custom board).
> 
> I have been using the DMAEngine API to interact with the DMA controller,
> and it works without any problems. However it seems that it is missing
> one of the major features that is supported by almost all DMA
> controllers: scatter/gather support.
> 
> What I'd really like to do is give the controller a scatterlist and list
> of hardware addresses, and have it set up the transfer. The Freescale
> hardware can handle this, and I could program it by hand to do so, but I
> don't want to invent my own DMA API.
> 
> I'm aware that the DMAEngine API was designed to offload memcpy, and
> pretty much nothing else. Is there any chance that such a change would
> be accepted into the mainline kernel? Any opinions on what such an API
> should look like?
> 

Have you taken a look at the 'slave' interface implemented by dw_dmac
and other drivers?  It has the following interface:

       struct dma_async_tx_descriptor *(*device_prep_slave_sg)(
                struct dma_chan *chan, struct scatterlist *sgl,
                unsigned int sg_len, enum dma_data_direction direction,
                unsigned long flags);

Haavard uses this interface to do sgl transfers on behalf of the
atmel-mci driver (drivers/mmc/host/atmel-mci.c).

--
Dan