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SubjectRe: [PATCH 3/5] dmaengine: dw: Discard dlen from the dev-to-mem xfer width calculation
On Thu, Jul 30, 2020 at 06:45:43PM +0300, Serge Semin wrote:
> Indeed in case of the DMA_DEV_TO_MEM DMA transfers it's enough to take the
> destination memory address and the destination master data width into
> account to calculate the CTLx.DST_TR_WIDTH setting of the memory

> peripheral. According to the DW DMAC IP-core Databook (page 66, Example 5)

Always put a version of the Databook document. I have several and they may differ.

> at the and of a DMA transfer when the DMA-channel internal FIFO is left
> with data less than for a single destination burst transaction, the
> destination peripheral will enter the Single Transaction Region where the
> DW DMA controller can complete a block transfer to the destination using
> single transactions (non-burst transaction of CTLx.DST_TR_WIDTH bytes). If
> there is no enough data in the DMA-channel internal FIFO for even a single
> non-burst transaction of CTLx.DST_TR_WIDTH bytes, then the channel enters
> "FIFO flush mode". That mode is activated to empty the FIFO and flush the
> leftovers out to the memory peripheral. The flushing procedure is simple.
> The data is sent to the memory by means of a set of single transaction of
> CTLx.SRC_TR_WIDTH bytes. To sum up it's redundant to use the LLPs length
> to find out the CTLx.DST_TR_WIDTH parameter value, since each DMA transfer
> will be completed with the CTLx.SRC_TR_WIDTH bytes transaction if it is
> required.

> In this commit we remove the LLP entry length from the statement which

"In this commit" should be removed, see Submitting Patches ("This patch").

> calculates the memory peripheral DMA transaction width since it's
> redundant due to the feature described above. By doing so we'll improve
> the memory bus utilization and speed up the DMA-channel performance for
> DMA_DEV_TO_MEM DMA-transfers.

Okay, I have no objections.
Acked-by: Andy Shevchenko <>

> Signed-off-by: Serge Semin <>
> ---
> drivers/dma/dw/core.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> diff --git a/drivers/dma/dw/core.c b/drivers/dma/dw/core.c
> index 4700f2e87a62..3da0aea9fe25 100644
> --- a/drivers/dma/dw/core.c
> +++ b/drivers/dma/dw/core.c
> @@ -723,7 +723,7 @@ dwc_prep_slave_sg(struct dma_chan *chan, struct scatterlist *sgl,
> lli_write(desc, sar, reg);
> lli_write(desc, dar, mem);
> lli_write(desc, ctlhi, ctlhi);
> - mem_width = __ffs(data_width | mem | dlen);
> + mem_width = __ffs(data_width | mem);
> lli_write(desc, ctllo, ctllo | DWC_CTLL_DST_WIDTH(mem_width));
> desc->len = dlen;
> --
> 2.27.0

With Best Regards,
Andy Shevchenko

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