[PATCH 3/5] dmaengine: dw: Discard dlen from the dev-to-mem xfer width calculation

Serge Semin
2020-07-30T15:46:21Z

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)
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
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.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
---
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