[PATCH 1/1] dmaengine: at_xdmac: fix transfer data width in at_xdmac_prep_slave_sg()

[Cyrille Pitchen]

This patch adds a missing part in commit
6d3a7d9e3ada345948f72564ce638c412ccd8c4a:
dmaengine: at_xdmac: allow muliple dwidths when doing slave transfers

Indeed, for each item in the scatter-gather list, we check whether the
transfer length is aligned with the data width provided by
dmaengine_slave_config(). If so, we directly use this data width for the
current part of the transfer we are preparing. Otherwise, the data width
is reduced to 8 bits (1 byte). Of course, the actual number of register
accesses must also be updated to match the new data width.

So one chunk was missing in the original patch: the number of register
accesses was correctly set to (len >> fixed_dwidth) in mbr_ubc but the
real data width was not updated in mbr_cfg. Since mbr_cfg may change for
each part of the scatter-gather transfer this also explains why the
original patch used the Descriptor View 2 instead of the Descriptor
View 1.

Let's take the example of a DMA transfer to write 8bit data into an Atmel
USART with FIFOs. When FIFOs are enabled in the USART, its Transmit
Holding Register (THR) works in multidata mode, that is to say that up to
4 8bit data can be written into the THR in a single 32bit access and it is
still possible to write only one data with a 8bit access. To take
advantage of this new feature, the DMA driver was modified to allow
multiple dwidths when doing slave transfers.
For instance, when the total length is 22 bytes, the USART driver splits
the transfer into 2 parts:

First part: 20 bytes transferred through 5 32bit writes into THR
Second part: 2 bytes transferred though 2 8bit writes into THR

For the second part, the data width was first set to 4_BYTES by the USART
driver thanks to dmaengine_slave_config() then at_xdmac_prep_slave_sg()
reduces this data width to 1_BYTE because the 2 byte length is not aligned
with the original 4_BYTES data width. Since the data width is modified,
the actual number of writes into THR must be set accordingly.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@xxxxxxxxx>
---
 drivers/dma/at_xdmac.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/dma/at_xdmac.c b/drivers/dma/at_xdmac.c
index cf1213d..931b218 100644
--- a/drivers/dma/at_xdmac.c
+++ b/drivers/dma/at_xdmac.c
@@ -681,15 +681,16 @@ at_xdmac_prep_slave_sg(struct dma_chan *chan, struct scatterlist *sgl,
 			desc->lld.mbr_sa = mem;
 			desc->lld.mbr_da = atchan->sconfig.dst_addr;
 		}
-		desc->lld.mbr_cfg = atchan->cfg;
-		dwidth = at_xdmac_get_dwidth(desc->lld.mbr_cfg);
+		dwidth = at_xdmac_get_dwidth(atchan->cfg);
 		fixed_dwidth = IS_ALIGNED(len, 1 << dwidth)
-			       ? at_xdmac_get_dwidth(desc->lld.mbr_cfg)
+			       ? dwidth
 			       : AT_XDMAC_CC_DWIDTH_BYTE;
 		desc->lld.mbr_ubc = AT_XDMAC_MBR_UBC_NDV2			/* next descriptor view */
 			| AT_XDMAC_MBR_UBC_NDEN					/* next descriptor dst parameter update */
 			| AT_XDMAC_MBR_UBC_NSEN					/* next descriptor src parameter update */
 			| (len >> fixed_dwidth);				/* microblock length */
+		desc->lld.mbr_cfg = (atchan->cfg & ~AT_XDMAC_CC_DWIDTH_MASK) |
+				    AT_XDMAC_CC_DWIDTH(fixed_dwidth);
 		dev_dbg(chan2dev(chan),
 			 "%s: lld: mbr_sa=%pad, mbr_da=%pad, mbr_ubc=0x%08x\n",
 			 __func__, &desc->lld.mbr_sa, &desc->lld.mbr_da, desc->lld.mbr_ubc);
-- 
1.8.2.2

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