[PATCH] dmaengine: stm32-dma: fix residue calculation in stm32-dma

[Arnaud Pouliquen]

During residue calculation. the DMA can switch to the next sg. When
this race condition occurs, the residue returned value is not valid.
Indeed the position in the sg returned by the hardware is the position
of the next sg, not the current sg.
Solution is to check the sg after the calculation to verify it.
If a transition is detected we consider that the DMA has switched to
the beginning of next sg.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@xxxxxx>
Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mordret@xxxxxx>
---
 drivers/dma/stm32-dma.c | 70 ++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 57 insertions(+), 13 deletions(-)

diff --git a/drivers/dma/stm32-dma.c b/drivers/dma/stm32-dma.c
index 4903a40..30309d2 100644
--- a/drivers/dma/stm32-dma.c
+++ b/drivers/dma/stm32-dma.c
@@ -1038,33 +1038,77 @@ static u32 stm32_dma_get_remaining_bytes(struct stm32_dma_chan *chan)
 	return ndtr << width;
 }
 
+static bool stm32_dma_is_current_sg(struct stm32_dma_chan *chan)
+{
+	struct stm32_dma_device *dmadev = stm32_dma_get_dev(chan);
+	struct stm32_dma_sg_req *sg_req;
+	u32 dma_scr, dma_smar, id;
+
+	id = chan->id;
+	dma_scr = stm32_dma_read(dmadev, STM32_DMA_SCR(id));
+
+	if (!(dma_scr & STM32_DMA_SCR_DBM))
+		return true;
+
+	sg_req = &chan->desc->sg_req[chan->next_sg];
+
+	if (dma_scr & STM32_DMA_SCR_CT) {
+		dma_smar = stm32_dma_read(dmadev, STM32_DMA_SM0AR(id));
+		return (dma_smar == sg_req->chan_reg.dma_sm0ar);
+	}
+
+	dma_smar = stm32_dma_read(dmadev, STM32_DMA_SM1AR(id));
+
+	return (dma_smar == sg_req->chan_reg.dma_sm1ar);
+}
+
 static size_t stm32_dma_desc_residue(struct stm32_dma_chan *chan,
 				     struct stm32_dma_desc *desc,
 				     u32 next_sg)
 {
 	u32 modulo, burst_size;
-	u32 residue = 0;
+	u32 residue;
+	u32 n_sg = next_sg;
+	struct stm32_dma_sg_req *sg_req = &chan->desc->sg_req[chan->next_sg];
 	int i;
 
+	residue = stm32_dma_get_remaining_bytes(chan);
+
 	/*
-	 * In cyclic mode, for the last period, residue = remaining bytes from
-	 * NDTR
+	 * Calculate the residue means compute the descriptors
+	 * information:
+	 * - the sg currently transferred
+	 * - the remaining position in this sg (NDTR).
+	 *
+	 * The issue is that a race condition can occur if DMA is
+	 * running. DMA can have started to transfer the next sg before
+	 * the position in sg is read. In this case the remaing position
+	 * can correspond to the new sg position.
+	 * The strategy implemented in the stm32 driver is to check the
+	 * sg transition. If detected we can not trust the SxNDTR register
+	 * value, this register can not be up to date during the transition.
+	 * In this case we can assume that the dma is at the beginning of next
+	 * sg so we calculate the residue in consequence.
 	 */
-	if (chan->desc->cyclic && next_sg == 0) {
-		residue = stm32_dma_get_remaining_bytes(chan);
-		goto end;
+
+	if (!stm32_dma_is_current_sg(chan)) {
+		n_sg++;
+		if (n_sg == chan->desc->num_sgs)
+			n_sg = 0;
+		residue = sg_req->len;
 	}
 
 	/*
-	 * For all other periods in cyclic mode, and in sg mode,
-	 * residue = remaining bytes from NDTR + remaining periods/sg to be
-	 * transferred
+	 * In cyclic mode, for the last period, residue = remaining bytes
+	 * from NDTR,
+	 * else for all other periods in cyclic mode, and in sg mode,
+	 * residue = remaining bytes from NDTR + remaining
+	 * periods/sg to be transferred
 	 */
-	for (i = next_sg; i < desc->num_sgs; i++)
-		residue += desc->sg_req[i].len;
-	residue += stm32_dma_get_remaining_bytes(chan);
+	if (!chan->desc->cyclic || n_sg != 0)
+		for (i = n_sg; i < desc->num_sgs; i++)
+			residue += desc->sg_req[i].len;
 
-end:
 	if (!chan->mem_burst)
 		return residue;
 
-- 
2.7.4