[PATCH] serial: qcom-geni: Drop unsafe rx_buf realloc from setup_fifos()

[PATCH] serial: qcom-geni: Drop unsafe rx_buf realloc from setup_fifos()

[Praveen Talari]

The RX buffer is allocated once during probe using a fixed DMA_RX_BUF_SIZE
and is DMA-mapped for the lifetime of the port. However, setup_fifos()
attempts to reallocate rx_buf whenever the reported RX FIFO depth changes.
Since the DMA mapping is not re-established after reallocation, the buffer
pointer may change while the DMA engine continues using the stale DMA
address. This can result in RX DMA targeting memory that no longer
corresponds to the active buffer, leading to invalid DMA accesses and
potential memory corruption.

The RX FIFO depth is unrelated to the size of rx_buf. The buffer is
allocated independently using DMA_RX_BUF_SIZE and all RX DMA paths consume
it at that fixed size. As such, resizing the buffer based on FIFO depth
changes provides no functional benefit.

Signed-off-by: Praveen Talari <ptalari@qualcomm.com>
---
Signed-off-by: Praveen Talari <praveen.talari@oss.qualcomm.com>
---
 drivers/tty/serial/qcom_geni_serial.c | 14 --------------
 1 file changed, 14 deletions(-)

diff --git a/drivers/tty/serial/qcom_geni_serial.c b/drivers/tty/serial/qcom_geni_serial.c
index 3633723acef8..75b2e0b77d05 100644
--- a/drivers/tty/serial/qcom_geni_serial.c
+++ b/drivers/tty/serial/qcom_geni_serial.c
@@ -1291,7 +1291,6 @@ static irqreturn_t qcom_geni_serial_isr(int isr, void *dev)
 static int setup_fifos(struct qcom_geni_serial_port *port)
 {
 	struct uart_port *uport;
-	u32 old_rx_fifo_depth = port->rx_fifo_depth;
 
 	uport = &port->uport;
 	port->tx_fifo_depth = geni_se_get_tx_fifo_depth(&port->se);
@@ -1300,19 +1299,6 @@ static int setup_fifos(struct qcom_geni_serial_port *port)
 	uport->fifosize =
 		(port->tx_fifo_depth * port->tx_fifo_width) / BITS_PER_BYTE;
 
-	if (port->rx_buf && (old_rx_fifo_depth != port->rx_fifo_depth) && port->rx_fifo_depth) {
-		/*
-		 * Use krealloc rather than krealloc_array because rx_buf is
-		 * accessed as 1 byte entries as well as 4 byte entries so it's
-		 * not necessarily an array.
-		 */
-		port->rx_buf = devm_krealloc(uport->dev, port->rx_buf,
-					     port->rx_fifo_depth * sizeof(u32),
-					     GFP_KERNEL);
-		if (!port->rx_buf)
-			return -ENOMEM;
-	}
-
 	return 0;
 }
 

---
base-commit: 903c1cf6dff9964e71eda98a39e2e5d442050472
change-id: 20260822-drop-unsafe-rx-buf-realloc-from-setup-fifos-2b82eb0cb5d4

Best regards,
--  
Praveen Talari <praveen.talari@oss.qualcomm.com>

Re: [PATCH] serial: qcom-geni: Drop unsafe rx_buf realloc from setup_fifos()

[Greg Kroah-Hartman]

On Mon, Aug 24, 2026 at 04:15:14PM +0530, Praveen Talari wrote:
> The RX buffer is allocated once during probe using a fixed DMA_RX_BUF_SIZE
> and is DMA-mapped for the lifetime of the port. However, setup_fifos()
> attempts to reallocate rx_buf whenever the reported RX FIFO depth changes.
> Since the DMA mapping is not re-established after reallocation, the buffer
> pointer may change while the DMA engine continues using the stale DMA
> address. This can result in RX DMA targeting memory that no longer
> corresponds to the active buffer, leading to invalid DMA accesses and
> potential memory corruption.
> 
> The RX FIFO depth is unrelated to the size of rx_buf. The buffer is
> allocated independently using DMA_RX_BUF_SIZE and all RX DMA paths consume
> it at that fixed size. As such, resizing the buffer based on FIFO depth
> changes provides no functional benefit.
> 
> Signed-off-by: Praveen Talari <ptalari@qualcomm.com>
> ---
> Signed-off-by: Praveen Talari <praveen.talari@oss.qualcomm.com>

Why is this signed-off-by twice, with different email addresses?

confused,

greg k-h

Re: [PATCH] serial: qcom-geni: Drop unsafe rx_buf realloc from setup_fifos()

[Praveen Talari]

Hi

On 24-08-2026 18:26, Greg Kroah-Hartman wrote:
> On Mon, Aug 24, 2026 at 04:15:14PM +0530, Praveen Talari wrote:
>> The RX buffer is allocated once during probe using a fixed DMA_RX_BUF_SIZE
>> and is DMA-mapped for the lifetime of the port. However, setup_fifos()
>> attempts to reallocate rx_buf whenever the reported RX FIFO depth changes.
>> Since the DMA mapping is not re-established after reallocation, the buffer
>> pointer may change while the DMA engine continues using the stale DMA
>> address. This can result in RX DMA targeting memory that no longer
>> corresponds to the active buffer, leading to invalid DMA accesses and
>> potential memory corruption.
>>
>> The RX FIFO depth is unrelated to the size of rx_buf. The buffer is
>> allocated independently using DMA_RX_BUF_SIZE and all RX DMA paths consume
>> it at that fixed size. As such, resizing the buffer based on FIFO depth
>> changes provides no functional benefit.
>>
>> Signed-off-by: Praveen Talari <ptalari@qualcomm.com>
>> ---
>> Signed-off-by: Praveen Talari <praveen.talari@oss.qualcomm.com>
> Why is this signed-off-by twice, with different email addresses?

Apologies, that was a mistake while updating the commit message. Only 
praveen.talari@oss.qualcomm.com should be present for upstream 
submissions. I'll fix it in the next revision

Thanks,

Praveen Talari

>
> confused,
>
> greg k-h