Re: DMA support for CF

[geomatsi <geomatsi@gmail.com>]

On 12/18/2012 08:16 AM, Matej Kupljen wrote:
> Dear Sergey and Jeff,

> I have a board based on AT91SAM9263, with the CF connected to
> the EBI0 and I am using the pata-at91.c driver.
>
> My question is, can I use DMA (MWDMA and/or UDMA) for the CF card,
> of course, if I add support to the driver?
> Basically, the question is, can we use the HDMAC (DMA controller)
> to support the DMA transfers?
> Please note that the DMARQ and DMACK are connected from CF to SoC.
>
>
> I've been looking for this in the documentation, and I cannot find the
> answer. Considering that the SAM9263 is considered legacy, I have
> checked the documentation for AT91SAM9R/RL and AT91SAM9G and I couldn't
> find the answer there either.
>
> Can you answer this question, please?
> If not, to whom I should send it?
>
> Thank you and Best Regards,
> Matej Kupljen

Hi,

In the future could you please consider sending your questions to a 
proper kernel mail list cc-ing authors/maintainers if necessary.


As you noted, driver pata-at91 does not have dma support. This is 
because originally it has been developed for a board based on 
at91sam9260 chip which has no dma engine. More recent at91 chips 
(including at91sam9263) do have dma engines. It should be possible to 
add dma support to pata-at91 driver and use dma transfers for CF.

A couple of notes regarding at91sam9263:

1. DMA driver
IIRC at91sam9263 chip has a different dma engine than all the other 
chips of at91 family. More recent chips have HDMAC: 
drivers/dma/at_hdmac.c while at91sam9263 has DWDMA: drivers/dma/dw_dmac.c

2. Size of dma transfers
IIRC there is a problem with dma engine on at91sam9263 chip: it has 
significantly reduced block size for transfers. This leads into a fairly 
small max transfer size in a single block transfer. As a result, it 
might not be practical to use this particular dma engine for CF.

Thanks,
Sergey