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

[Manivannan Sadhasivam]

On Fri, Aug 14, 2026 at 11:08:37AM -0500, Frank Li wrote:
> On Fri, Aug 14, 2026 at 03:58:48PM +0200, Manivannan Sadhasivam wrote:
> > 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 sumit.kumar@xxxxxxxxxxxxxxxx. 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
> 
> Define API is memcpy, not memory to peripherail Or peripherail to memory
> 
> dmaengine_prep_dma_sg() - Prepare a memory-to-memory scatter-gather DMA descriptor.
> 
> Most memcpy user need call dma_map_sg() to get sg.  And most likely two
> virutal address offset to page is difference.
> 
> So most likely return failure by this API.
> 

I agreed that using MEMCPY semantics is wrong for this API. So it should be
changed to peripheral DMA.

- Mani

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