[PATCH] serial: 8250: calculate TX length before preparing DMA flow control
From: James Hilliard
Date: Thu Oct 22 2026 - 13:20:36 EST
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@xxxxxxxxx>
---
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@xxxxxxxxx>