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Subject[PATCH v3 13/18] dmaengine: dma-jz4780: Set DTCn register explicitly
Date
From: Daniel Silsby <dansilsby@gmail.com>

Normally, we wouldn't set the channel transfer count register directly
when using descriptor-driven transfers. However, there is no harm in
doing so, and it allows jz4780_dma_desc_residue() to report the correct
residue of an ongoing transfer, no matter when it is called.

Signed-off-by: Daniel Silsby <dansilsby@gmail.com>
Tested-by: Mathieu Malaterre <malat@debian.org>
---
drivers/dma/dma-jz4780.c | 9 +++++++++
1 file changed, 9 insertions(+)

v2: No change

v3: No change

diff --git a/drivers/dma/dma-jz4780.c b/drivers/dma/dma-jz4780.c
index cc2a86844db4..78849131c81d 100644
--- a/drivers/dma/dma-jz4780.c
+++ b/drivers/dma/dma-jz4780.c
@@ -530,6 +530,15 @@ static void jz4780_dma_begin(struct jz4780_dma_chan *jzchan)
jz4780_dma_chn_writel(jzdma, jzchan->id, JZ_DMA_REG_DRT,
jzchan->transfer_type);

+ /*
+ * Set the transfer count. This is redundant for a descriptor-driven
+ * transfer. However, there can be a delay between the transfer start
+ * time and when DTCn reg contains the new transfer count. Setting
+ * it explicitly ensures residue is computed correctly at all times.
+ */
+ jz4780_dma_chn_writel(jzdma, jzchan->id, JZ_DMA_REG_DTC,
+ jzchan->desc->desc[jzchan->curr_hwdesc].dtc);
+
/* Write descriptor address and initiate descriptor fetch. */
desc_phys = jzchan->desc->desc_phys +
(jzchan->curr_hwdesc * sizeof(*jzchan->desc->desc));
--
2.11.0
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