[PATCH v2] serial: 8250: calculate TX length before preparing DMA flow control

[PATCH v2] serial: 8250: calculate TX length before preparing DMA flow control

[James Hilliard]

The Renesas RZ/N1 preparation hook programs the UART DMA block size from
dma->tx_size. The generic TX path calls that hook before calculating the
length of the new transfer, so it programs zero on the first transfer and
the previous length on subsequent transfers.

Calculate the scatterlist length before calling the preparation hook so
the UART flow controller and DMA descriptor use the same transfer size.

Fixes: 1788cf6a91d9 ("tty: serial: switch from circ_buf to kfifo")
Cc: stable@vger.kernel.org
Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
---
Changes in v2:
- Correct the Fixes tag to the kfifo conversion, as pointed out by
  Miquel Raynal.
- Add Cc: stable@vger.kernel.org.
- Link to v1: https://patch.msgid.link/20261022-submit-serial-8250-dma-tx-length-v1-v1-1-18cecc89a099@gmail.com
---
 drivers/tty/serial/8250/8250_dma.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_dma.c b/drivers/tty/serial/8250/8250_dma.c
index 1da605cc8601..2dc47adafec0 100644
--- a/drivers/tty/serial/8250/8250_dma.c
+++ b/drivers/tty/serial/8250/8250_dma.c
@@ -110,8 +110,6 @@ int serial8250_tx_dma(struct uart_8250_port *p)
 		return 0;
 	}
 
-	serial8250_do_prepare_tx_dma(p);
-
 	sg_init_table(sgl, ARRAY_SIZE(sgl));
 
 	ret = kfifo_dma_out_prepare_mapped(&tport->xmit_fifo, sgl, ARRAY_SIZE(sgl),
@@ -122,6 +120,8 @@ int serial8250_tx_dma(struct uart_8250_port *p)
 	for_each_sg(sgl, sg, ret, i)
 		dma->tx_size += sg_dma_len(sg);
 
+	serial8250_do_prepare_tx_dma(p);
+
 	desc = dmaengine_prep_slave_sg(dma->txchan, sgl, ret,
 				       DMA_MEM_TO_DEV,
 				       DMA_PREP_INTERRUPT | DMA_CTRL_ACK);

---
base-commit: 1184f87e2790e8af9be3116f608d2e86b4792d7d
change-id: 20261022-submit-serial-8250-dma-tx-length-v1-bf627278129a

Best regards,
--  
James Hilliard <james.hilliard1@gmail.com>

Re: [PATCH v2] serial: 8250: calculate TX length before preparing DMA flow control

[Miquel Raynal]

On 23/10/2026 at 09:40:50 -06, James Hilliard <james.hilliard1@gmail.com> wrote:

> The Renesas RZ/N1 preparation hook programs the UART DMA block size from
> dma->tx_size. The generic TX path calls that hook before calculating the
> length of the new transfer, so it programs zero on the first transfer and
> the previous length on subsequent transfers.
>
> Calculate the scatterlist length before calling the preparation hook so
> the UART flow controller and DMA descriptor use the same transfer size.
>
> Fixes: 1788cf6a91d9 ("tty: serial: switch from circ_buf to kfifo")
> Cc: stable@vger.kernel.org
> Signed-off-by: James Hilliard <james.hilliard1@gmail.com>

Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>