Subject: [PATCH v4 3/4] dma: mmp_pdma: add support for residue reporting

From: [email-masked] (Robert Jarzmik)
Date: Mon, 26 Aug 2013 08:07:34 +0200

Hi Russell,

Russell King - ARM Linux <[email-masked]> writes:
> Even when there is a callback set, there's no requirement for the DMA
> engine driver to keep the descriptor around after it has been submitted -
> it is free to copy that data into whatever internal representation it
> requires and then discard the descriptor if it so wishes.

That means that a driver which wished to "resubmit" the descriptor has to
"rebuild it" again, no ?

I was under the impression that a descriptor had the guarantee to survive even
after completion. This comes from include/linux/dmaengine.h:168, in enum
dma_ctrl_flags comment :
 * @DMA_CTRL_ACK - if clear, the descriptor cannot be reused until the client
 *  acknowledges receipt, i.e. has has a chance to establish any dependency
 *  chains

The sentence "cannot be reused until" meant that after it could be reused.

And there are usecases for that, for example a video capture buffer. The source
for DMA is always the same (camera FIFO), the destination is always the same
(ie. the same scatter-gather). Why rebuild all the info to resubmit the same
descriptor ?

So is my understand incorrect, and in that case is there a way with dmaengine to
"resubmit" a descriptor, without going through the whole preparation ?

Cheers.

-- 
Robert