The existing residual calculation returns an incorrect value when
bytes_xfer == bytes_req. This scenario occurs particularly with drivers
like UART where DMA is scheduled for maximum number of bytes and is
terminated when the bytes inflow stops. At higher baud rates, it could
request the tx_status while there is no bytes left to transfer. This will
lead to incorrect residual being set. Hence return residual as '0' when
bytes transferred equals to the bytes requested.

Fixes: ee17028009d4 ("dmaengine: tegra: Add tegra gpcdma driver")
Signed-off-by: Akhil R <akhilrajeev@nvidia.com>
Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
---
v1->v2:
* corrected typo - s/exisiting/existing/

 drivers/dma/tegra186-gpc-dma.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/dma/tegra186-gpc-dma.c b/drivers/dma/tegra186-gpc-dma.c
index 88547a23825b..3642508e88bb 100644
--- a/drivers/dma/tegra186-gpc-dma.c
+++ b/drivers/dma/tegra186-gpc-dma.c
@@ -746,6 +746,9 @@ static int tegra_dma_get_residual(struct tegra_dma_channel *tdc)
 	bytes_xfer = dma_desc->bytes_xfer +
 		     sg_req[dma_desc->sg_idx].len - (wcount * 4);
 
+	if (dma_desc->bytes_req == bytes_xfer)
+		return 0;
+
 	residual = dma_desc->bytes_req - (bytes_xfer % dma_desc->bytes_req);
 
 	return residual;
-- 
2.43.2

On Fri, 15 Mar 2024 18:14:11 +0530, Akhil R wrote:
> The existing residual calculation returns an incorrect value when
> bytes_xfer == bytes_req. This scenario occurs particularly with drivers
> like UART where DMA is scheduled for maximum number of bytes and is
> terminated when the bytes inflow stops. At higher baud rates, it could
> request the tx_status while there is no bytes left to transfer. This will
> lead to incorrect residual being set. Hence return residual as '0' when
> bytes transferred equals to the bytes requested.
> 
> [...]

Applied, thanks!

[1/1] dmaengine: tegra186: Fix residual calculation
      commit: 30f0ced9971b2d8c8c24ae75786f9079489a012d

Best regards,
-- 
~Vinod

On Fri Mar 15, 2024 at 1:44 PM CET, Akhil R wrote:
> The existing residual calculation returns an incorrect value when
> bytes_xfer == bytes_req. This scenario occurs particularly with drivers
> like UART where DMA is scheduled for maximum number of bytes and is
> terminated when the bytes inflow stops. At higher baud rates, it could
> request the tx_status while there is no bytes left to transfer. This will
> lead to incorrect residual being set. Hence return residual as '0' when
> bytes transferred equals to the bytes requested.
>
> Fixes: ee17028009d4 ("dmaengine: tegra: Add tegra gpcdma driver")
> Signed-off-by: Akhil R <akhilrajeev@nvidia.com>
> Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
> ---
> v1->v2:
> * corrected typo - s/exisiting/existing/
>
>  drivers/dma/tegra186-gpc-dma.c | 3 +++
>  1 file changed, 3 insertions(+)

Acked-by: Thierry Reding <treding@nvidia.com>