[PATCH 2/3] dmaengine: stm32-mdma: use Link Address Register to compute residue

[Amelie Delaunay <amelie.delaunay@foss.st.com>]

Current implementation relies on curr_hwdesc index. But to keep this index
up to date, Block Transfer interrupt (BTIE) has to be enabled.
If it is not, curr_hwdesc is not updated, and then residue is not reliable.
Rely on Link Address Register instead. And disable BTIE interrupt
in stm32_mdma_setup_xfer() because it is no more needed in case of
_prep_slave_sg() to maintain curr_hwdesc up to date.
It avoids extra interrupts and also ensures a reliable residue. These
improvements are required for STM32 DCMI camera capture use case, which
need STM32 DMA and MDMA chaining for good performance.

Fixes: 696874322771 ("dmaengine: stm32-mdma: add support to be triggered by STM32 DMA")
Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Cc: stable@vger.kernel.org
---
 drivers/dma/stm32-mdma.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/drivers/dma/stm32-mdma.c b/drivers/dma/stm32-mdma.c
index cc6f4b00091f..da73e13b8c9d 100644
--- a/drivers/dma/stm32-mdma.c
+++ b/drivers/dma/stm32-mdma.c
@@ -777,8 +777,6 @@ static int stm32_mdma_setup_xfer(struct stm32_mdma_chan *chan,
 	/* Enable interrupts */
 	ccr &= ~STM32_MDMA_CCR_IRQ_MASK;
 	ccr |= STM32_MDMA_CCR_TEIE | STM32_MDMA_CCR_CTCIE;
-	if (sg_len > 1)
-		ccr |= STM32_MDMA_CCR_BTIE;
 	desc->ccr = ccr;
 
 	return 0;
@@ -1324,12 +1322,21 @@ static size_t stm32_mdma_desc_residue(struct stm32_mdma_chan *chan,
 {
 	struct stm32_mdma_device *dmadev = stm32_mdma_get_dev(chan);
 	struct stm32_mdma_hwdesc *hwdesc;
-	u32 cbndtr, residue, modulo, burst_size;
+	u32 cisr, clar, cbndtr, residue, modulo, burst_size;
 	int i;
 
+	cisr = stm32_mdma_read(dmadev, STM32_MDMA_CISR(chan->id));
+
 	residue = 0;
-	for (i = curr_hwdesc + 1; i < desc->count; i++) {
+	/* Get the next hw descriptor to process from current transfer */
+	clar = stm32_mdma_read(dmadev, STM32_MDMA_CLAR(chan->id));
+	for (i = desc->count - 1; i >= 0; i--) {
 		hwdesc = desc->node[i].hwdesc;
+
+		if (hwdesc->clar == clar)
+			break;/* Current transfer found, stop cumulating */
+
+		/* Cumulate residue of unprocessed hw descriptors */
 		residue += STM32_MDMA_CBNDTR_BNDT(hwdesc->cbndtr);
 	}
 	cbndtr = stm32_mdma_read(dmadev, STM32_MDMA_CBNDTR(chan->id));
-- 
2.25.1