Re: [PATCH 7/8] dmaengine/dw_dmac: Unmap all memory buffers after completion of slave transfers

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Mon, Jan 16, 2012 at 03:20:35PM +0530, Viresh Kumar wrote:
> Currently, after completion of transfer, source address or destination address
> of only the first LLI descriptor is unmapped. And length passed for unmap is
> total length of all descriptors in the list. Which means unmapping code assumed
> that the memory buffers pointed to by the descriptors will be physically
> contiguous, which might not be the case. It is possible for other drivers to
> pass sglist to slave_sg(), in which all buffers are scattered throughout the
> memory.
> 
> This patch intends to fix this wrong expectation of dw_dmac. Now, first desc
> will not contain total length of transfer. But individual descriptors will
> contain their individual lengths. Finally, we will call unmap for all
> descriptors.

Note that DMA engine drivers are not responsible for unmapping the buffers
when the transfer completes - that is the responsibility of the caller.

The automatic buffer unmapping is required for the async_tx APIs and
offload APIs.