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

[PATCH] 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: aa63d786cea2 ("serial: 8250: dw: Add support for DMA flow controlling devices")
Signed-off-by: James Hilliard <james.hilliard1@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] serial: 8250: calculate TX length before preparing DMA flow control

[Miquel Raynal]

Hi James,

On 22/10/2026 at 11:14:41 -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: aa63d786cea2 ("serial: 8250: dw: Add support for DMA flow
> controlling devices")

This is the wrong fixes. Watch out 1788cf6a91d9 ("tty: serial: switch
from circ_buf to kfifo"). This is the one moving the size assignment
after the do_prepare_tx_dma() call.

Also requires to be backported, so Cc: stable.

Thanks,
Miquèl