Re: [PATCH v2 2/5] dmaengine: dw-edma: Add DMA_SG support

On Thu, Aug 06, 2026 at 02:06:30PM -0500, Frank Li wrote:
> On Mon, Aug 03, 2026 at 04:01:44PM +0530, Sumit Kumar wrote:
> > [You don't often get email from [email protected]. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ]
> >
> > Synopsys DesignWare eDMA supports a linked-list mode where each list item
> > carries independent source and destination addresses, letting multiple
> > independent memory transfers be described in one linked list and submitted
> > to the hardware as a single DMA transaction. The IP processes list items
> > strictly in order, so paired scatter-gather entries are never reordered.
> >
> > Implement the DMA_SG capability by adding a new EDMA_XFER_DUAL_SG transfer
> > type and a corresponding struct dw_edma_dual_sg carrying the paired source
> > and destination SG lists. dw_edma_device_transfer() walks both lists in
> > lockstep, building a single hardware linked-list; a per-entry length
> > mismatch or premature list end fails the whole request.
> 
> It is not as simple as it.  for example, if you want to transfer 9k data
> from src to dest
> 
> src virtual addr 0x1004,        dest 0xA0001008
> 
> when map_sg, src's sg maybe
> 
> 0x1000  .. 4k   offset 4,
> 0x9000  .. 4k   offset 0
> 0xA000  .. 4k   offset 0
> 
> or
> 0x1000 .. 8k  offset 4
> 0xA000 .. 4k  offset 0
> 
> which totally depend on physical address allocation although most likely
> first case happen
> 
> dest sg
> 0xA001000 .. 4k offset 8
> 0xA008000 .. 4k offset 0
> 0xA00E000 .. 4k offset 0
> 
> descriptors
> 1 	transfer 4k-8
> 2       transfer tail 4 byte
> 3 	transfer 4k-8
> 4       transfer tail 4 byte
> ...
> 
> the start address of src and dest is highly possible differences. So it
> is very hard to match your requirement, both sg's structure is the same.
> 

That's the API limitation, isn't it? The callers have to supply the same length
src/dst pairs to work with this API. But they don't necessarily need to use
dma_map_sg() which can provide fragmented mappings as you described. They can
use dma_map_single() or even the bus addresses directly as the MHI client is
already doing.

- Mani

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