quoted

When DMA is used for TX and RX by serial driver, it should pass the
DMA device pointer to DMA API instead of UART device pointer. DMA
device should be used for DMA API because only the DMA device is aware
of how the device connects to the memory.
There might be an extra level of address translation due to a SMMU
attached to the DMA device. When serial device pointer device is used
for DMA API, the DMA API will have no clue of the SMMU attached to the
DMA device.

This patch is necessary to fix the SMMU page faults which is observed
when a DMA(with SMMU enabled) is attached to UART for transfer.

Signed-off-by: Tamseel Shams <redacted>
Signed-off-by: Ajay Kumar <redacted>
---
Changes since v1:
1. Rebased the patch on "tty-next" branch of TTY driver tree

Changes since v2:
1. Updated the commit message.
2. Changed the comment description

 drivers/tty/serial/samsung_tty.c | 62
+++++++++++++++++++++++++-------
 1 file changed, 50 insertions(+), 12 deletions(-)

diff --git a/drivers/tty/serial/samsung_tty.c

b/drivers/tty/serial/samsung_tty.c
index 9fbc61151c2e..7dd648cb7237 100644

--- a/drivers/tty/serial/samsung_tty.c
+++ b/drivers/tty/serial/samsung_tty.c

@@ -284,8 +284,13 @@ static void s3c24xx_serial_stop_tx(struct uart_port

*port)

quoted

 	struct s3c24xx_uart_dma *dma = ourport->dma;
 	struct circ_buf *xmit = &port->state->xmit;
 	struct dma_tx_state state;
+	struct device *dma_map_ops_dev = ourport->port.dev;
 	int count;

+	/* Null pointer check for "dma" and "dma->tx_chan" */

The comment is not useful.

I will remove these comments.

quoted

+	if (dma && dma->tx_chan)
+		dma_map_ops_dev = dma->tx_chan->device->dev;
+
 	if (!ourport->tx_enabled)
 		return;

@@ -305,7 +310,7 @@ static void s3c24xx_serial_stop_tx(struct uart_port

*port)

quoted

 		dmaengine_pause(dma->tx_chan);
 		dmaengine_tx_status(dma->tx_chan, dma->tx_cookie, &state);
 		dmaengine_terminate_all(dma->tx_chan);
-		dma_sync_single_for_cpu(ourport->port.dev,
+		dma_sync_single_for_cpu(dma_map_ops_dev,
 			dma->tx_transfer_addr, dma->tx_size,

DMA_TO_DEVICE);

quoted

 		async_tx_ack(dma->tx_desc);
 		count = dma->tx_bytes_requested - state.residue; @@ -331,14

+336,19

quoted

@@ static void s3c24xx_serial_tx_dma_complete(void *args)
 	struct circ_buf *xmit = &port->state->xmit;
 	struct s3c24xx_uart_dma *dma = ourport->dma;
 	struct dma_tx_state state;
+	struct device *dma_map_ops_dev = ourport->port.dev;
 	unsigned long flags;
 	int count;

+	/* Null pointer check for "dma" and "dma->tx_chan" */
+	if (dma && dma->tx_chan)

What is the "else" here? How is it even possible? Why having this "if"
at all?

As mentioned in my reply (https://lkml.org/lkml/2021/6/23/6)  to
Robin's comments for patch version 2, I will be removing these
"if" conditions, and directly assign DMA device pointers.

quoted

+		dma_map_ops_dev = dma->tx_chan->device->dev;
+
 	dmaengine_tx_status(dma->tx_chan, dma->tx_cookie, &state);
 	count = dma->tx_bytes_requested - state.residue;
 	async_tx_ack(dma->tx_desc);

-	dma_sync_single_for_cpu(ourport->port.dev, dma->tx_transfer_addr,
+	dma_sync_single_for_cpu(dma_map_ops_dev, dma->tx_transfer_addr,
 				dma->tx_size, DMA_TO_DEVICE);

 	spin_lock_irqsave(&port->lock, flags); @@ -436,6 +446,11 @@ static
int s3c24xx_serial_start_tx_dma(struct s3c24xx_uart_port *ourport,
 	struct uart_port *port = &ourport->port;
 	struct circ_buf *xmit = &port->state->xmit;
 	struct s3c24xx_uart_dma *dma = ourport->dma;
+	struct device *dma_map_ops_dev = ourport->port.dev;
+
+	/* Null pointer check for "dma" and "dma->tx_chan" */
+	if (dma && dma->tx_chan)
+		dma_map_ops_dev = dma->tx_chan->device->dev;

 	if (ourport->tx_mode != S3C24XX_TX_DMA)
 		enable_tx_dma(ourport);

@@ -443,7 +458,7 @@ static int s3c24xx_serial_start_tx_dma(struct

s3c24xx_uart_port *ourport,

quoted

 	dma->tx_size = count & ~(dma_get_cache_alignment() - 1);
 	dma->tx_transfer_addr = dma->tx_addr + xmit->tail;

-	dma_sync_single_for_device(ourport->port.dev, dma-
tx_transfer_addr,
+	dma_sync_single_for_device(dma_map_ops_dev, dma-
tx_transfer_addr,
 				dma->tx_size, DMA_TO_DEVICE);

 	dma->tx_desc = dmaengine_prep_slave_single(dma->tx_chan,

@@ -510,12 +525,17 @@ static void s3c24xx_uart_copy_rx_to_tty(struct

s3c24xx_uart_port *ourport,

quoted

 		struct tty_port *tty, int count)
 {
 	struct s3c24xx_uart_dma *dma = ourport->dma;
+	struct device *dma_map_ops_dev = ourport->port.dev;
 	int copied;

+	/* Null pointer check for "dma" and "dma->rx_chan" */
+	if (dma && dma->rx_chan)
+		dma_map_ops_dev = dma->rx_chan->device->dev;
+
 	if (!count)
 		return;

-	dma_sync_single_for_cpu(ourport->port.dev, dma->rx_addr,
+	dma_sync_single_for_cpu(dma_map_ops_dev, dma->rx_addr,
 				dma->rx_size, DMA_FROM_DEVICE);

 	ourport->port.icount.rx += count;

@@ -635,8 +655,13 @@ static void s3c24xx_serial_rx_dma_complete(void

*args)  static void s3c64xx_start_rx_dma(struct s3c24xx_uart_port
*ourport)  {
 	struct s3c24xx_uart_dma *dma = ourport->dma;
+	struct device *dma_map_ops_dev = ourport->port.dev;
+
+	/* Null pointer check for "dma" and "dma->rx_chan" */
+	if (dma && dma->rx_chan)
+		dma_map_ops_dev = dma->rx_chan->device->dev;

-	dma_sync_single_for_device(ourport->port.dev, dma->rx_addr,
+	dma_sync_single_for_device(dma_map_ops_dev, dma->rx_addr,
 				dma->rx_size, DMA_FROM_DEVICE);

 	dma->rx_desc = dmaengine_prep_slave_single(dma->rx_chan,

@@ -1045,6 +1070,7 @@ static int s3c24xx_serial_request_dma(struct

s3c24xx_uart_port *p)

quoted

 	struct s3c24xx_uart_dma	*dma = p->dma;
 	struct dma_slave_caps dma_caps;
 	const char *reason = NULL;
+	struct device *dma_map_ops_dev = p->port.dev;
 	int ret;

 	/* Default slave configuration parameters */ @@ -1102,18 +1128,25

@@

quoted

static int s3c24xx_serial_request_dma(struct s3c24xx_uart_port *p)
 		goto err_release_tx;
 	}

-	dma->rx_addr = dma_map_single(p->port.dev, dma->rx_buf,
+	/* Null pointer check for "dma" and "dma->rx_chan" */
+	if (dma && dma->rx_chan)
+		dma_map_ops_dev = dma->rx_chan->device->dev;
+
+	dma->rx_addr = dma_map_single(dma_map_ops_dev, dma->rx_buf,
 				dma->rx_size, DMA_FROM_DEVICE);
-	if (dma_mapping_error(p->port.dev, dma->rx_addr)) {
+	if (dma_mapping_error(dma_map_ops_dev, dma->rx_addr)) {
 		reason = "DMA mapping error for RX buffer";
 		ret = -EIO;
 		goto err_free_rx;
 	}

+	/* Null pointer check for "dma" and "dma->tx_chan" */
+	if (dma && dma->tx_chan)
+		dma_map_ops_dev = dma->tx_chan->device->dev;
 	/* TX buffer */
-	dma->tx_addr = dma_map_single(p->port.dev, p->port.state->xmit.buf,
+	dma->tx_addr = dma_map_single(dma_map_ops_dev,
+p->port.state->xmit.buf,
 				UART_XMIT_SIZE, DMA_TO_DEVICE);
-	if (dma_mapping_error(p->port.dev, dma->tx_addr)) {
+	if (dma_mapping_error(dma_map_ops_dev, dma->tx_addr)) {
 		reason = "DMA mapping error for TX buffer";
 		ret = -EIO;
 		goto err_unmap_rx;

@@ -1122,7 +1155,9 @@ static int s3c24xx_serial_request_dma(struct

s3c24xx_uart_port *p)

quoted

 	return 0;

 err_unmap_rx:
-	dma_unmap_single(p->port.dev, dma->rx_addr, dma->rx_size,
+	if (dma->rx_chan)
+		dma_map_ops_dev = dma->rx_chan->device->dev;
+	dma_unmap_single(dma_map_ops_dev, dma->rx_addr, dma->rx_size,
 			 DMA_FROM_DEVICE);
 err_free_rx:
 	kfree(dma->rx_buf);

@@ -1139,10 +1174,12 @@ static int s3c24xx_serial_request_dma(struct

s3c24xx_uart_port *p)  static void s3c24xx_serial_release_dma(struct
s3c24xx_uart_port *p)  {
 	struct s3c24xx_uart_dma	*dma = p->dma;
+	struct device *dma_map_ops_dev = p->port.dev;

 	if (dma->rx_chan) {
+		dma_map_ops_dev = dma->rx_chan->device->dev;
 		dmaengine_terminate_all(dma->rx_chan);
-		dma_unmap_single(p->port.dev, dma->rx_addr,
+		dma_unmap_single(dma_map_ops_dev, dma->rx_addr,
 				dma->rx_size, DMA_FROM_DEVICE);
 		kfree(dma->rx_buf);
 		dma_release_channel(dma->rx_chan);

@@ -1150,8 +1187,9 @@ static void s3c24xx_serial_release_dma(struct

s3c24xx_uart_port *p)

quoted

 	}

 	if (dma->tx_chan) {
+		dma_map_ops_dev = dma->tx_chan->device->dev;
 		dmaengine_terminate_all(dma->tx_chan);
-		dma_unmap_single(p->port.dev, dma->tx_addr,
+		dma_unmap_single(dma_map_ops_dev, dma->tx_addr,
 				UART_XMIT_SIZE, DMA_TO_DEVICE);
 		dma_release_channel(dma->tx_chan);
 		dma->tx_chan = NULL;

Best regards,
Krzysztof

Thanks & Regards,
Tamseel Shams