[PATCH V2] tty: serial: fsl_lpuart: use the sg count from dma_map_sg

[Peng Fan]

From: Peng Fan <peng.fan@xxxxxxx>

The dmaengine_prep_slave_sg needs to use sg count returned
by dma_map_sg, not use sport->dma_tx_nents, because the return
value of dma_map_sg is not always same with "nents".

When enabling iommu for lpuart + edma, iommu framework may concatenate
two sgs into one.

Fixes: 6250cc30c4c4e ("tty: serial: fsl_lpuart: Use scatter/gather DMA for Tx")
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Peng Fan <peng.fan@xxxxxxx>
---

V2:
 Assign ret to sport->dma_tx_nents, then we no need to fix dma_unmap_sg
 Hi Greg,
  I saw v1 patch merged to tty-next, please help to replace with V2 if this
  is ok for you, or you need I have a follow up fix for v1.

 drivers/tty/serial/fsl_lpuart.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
index 3e17bb8a0b16..ec5ea098669e 100644
--- a/drivers/tty/serial/fsl_lpuart.c
+++ b/drivers/tty/serial/fsl_lpuart.c
@@ -436,6 +436,7 @@ static void lpuart_dma_tx(struct lpuart_port *sport)
 		return;
 	}
 
+	sport->dma_tx_nents = ret;
 	sport->dma_tx_desc = dmaengine_prep_slave_sg(sport->dma_tx_chan, sgl,
 					sport->dma_tx_nents,
 					DMA_MEM_TO_DEV, DMA_PREP_INTERRUPT);
-- 
2.16.4