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 -- To unsubscribe from this list: send the line "unsubscribe dmaengine" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
