A-sync case in EDMA driver is tricky and not so obvious.
Document the reasons for the calculations and the scenarious
they are used.
Signed-off-by: Joel Fernandes <joelf@xxxxxx>
---
drivers/dma/edma.c | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)
diff --git a/drivers/dma/edma.c b/drivers/dma/edma.c
index e008ed2..a1d9f3785 100644
--- a/drivers/dma/edma.c
+++ b/drivers/dma/edma.c
@@ -284,8 +284,24 @@ static struct dma_async_tx_descriptor *edma_prep_slave_sg(
*/
if (burst == 1) {
edesc->absync = false;
+ /*
+ * For the A-sync case, bcnt and ccnt are the remainder
+ * and quotient respectively of the division of:
+ * (sg_dma_len(sg) / acnt) by (SZ_64K -1). This is so
+ * that in case bcnt over flows, we have ccnt to use.
+ * Note: In A-sync tranfer only, bcntrld is used, but it
+ * only applies for sg_dma_len(sg) >= SZ_64K.
+ * In this case, the best way adopted is- bccnt for the
+ * first frame will be the remainder below. Then for
+ * every successive frame, bcnt will be SZ_64K-1. This
+ * is assured as bcntrld = 0xffff in end of function.
+ */
ccnt = sg_dma_len(sg) / acnt / (SZ_64K - 1);
bcnt = sg_dma_len(sg) / acnt - ccnt * (SZ_64K - 1);
+ /*
+ * If bcnt is non-zero, we have a remainder and hence an
+ * extra frame to transfer, so increment ccnt.
+ */
if (bcnt)
ccnt++;
else
@@ -343,6 +359,12 @@ static struct dma_async_tx_descriptor *edma_prep_slave_sg(
edesc->pset[i].a_b_cnt = bcnt << 16 | acnt;
edesc->pset[i].ccnt = ccnt;
+ /*
+ * Only time when (bcntrld) auto reload is required is for
+ * A-sync case, and in this case, a requirement of reload value
+ * of SZ_64K-1 only is assured. 'link' is initially set to NULL
+ * and then later will be populated by edma_execute.
+ */
edesc->pset[i].link_bcntrld = 0xffffffff;
}
--
1.7.9.5
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