Re: [PATCH] tty: serial: 8250_omap: fix TX with DMA for am33xx

Jiri Slaby
2025-05-14T07:22:27Z

On 14. 05. 25, 9:20, Jiri Slaby (SUSE) wrote:
> Commit 1788cf6a91d9 ("tty: serial: switch from circ_buf to kfifo")
> introduced an error in the TX DMA handling for 8250_omap.
>
> When the OMAP_DMA_TX_KICK flag is set, the "skip_byte" is pulled from
> the kfifo and emitted directly in order to start the DMA. While the
> kfifo is updated, dma->tx_size is not decreased. This leads to
> uart_xmit_advance() called in omap_8250_dma_tx_complete() advancing the
> kfifo by one too much.
>
> In practice, transmitting N bytes has been seen to result in the last
> N-1 bytes being sent repeatedly.
>
> This change fixes the problem by moving all of the dma setup after the
> OMAP_DMA_TX_KICK handling and using kfifo_len() instead of the DMA size
> for the 4-byte cutoff check. This slightly changes the behaviour at
> buffer wraparound, but it still transmits the correct bytes somehow.
>
> Now, the "skip_byte" would no longer be accounted to the stats. As
> previously, dma->tx_size included also this skip byte, up->icount.tx was
> updated by aforementioned uart_xmit_advance() in
> omap_8250_dma_tx_complete(). Fix this by using the uart_fifo_out()
> helper instead of bare kfifo_get().
>
> Based on patch by Mans Rullgard <mans@mansr.com>
>
> Fixes: 1788cf6a91d9 ("tty: serial: switch from circ_buf to kfifo")
> Reported-by: Mans Rullgard <mans@mansr.com>
> Cc: stable@vger.kernel.org

I should have added this too:
Link: https://lore.kernel.org/all/20250506150748.3162-1-mans@mansr.com/

> ---
> The same as for the original patch, I would appreaciate if someone
> actually tests this one on a real HW too.
>
> A patch to optimize the driver to use 2 sgls is still welcome. I will
> not add it without actually having the HW.
> ---
> drivers/tty/serial/8250/8250_omap.c | 25 ++++++++++---------------
> 1 file changed, 10 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c
> index c9b1c689a045..bb23afdd63f2 100644
> --- a/drivers/tty/serial/8250/8250_omap.c
> +++ b/drivers/tty/serial/8250/8250_omap.c
> @@ -1151,16 +1151,6 @@ static int omap_8250_tx_dma(struct uart_8250_port *p)
> return 0;
> }
>
> - sg_init_table(&sg, 1);
> - ret = kfifo_dma_out_prepare_mapped(&tport->xmit_fifo, &sg, 1,
> - UART_XMIT_SIZE, dma->tx_addr);
> - if (ret != 1) {
> - serial8250_clear_THRI(p);
> - return 0;
> - }
> -
> - dma->tx_size = sg_dma_len(&sg);
> -
> if (priv->habit & OMAP_DMA_TX_KICK) {
> unsigned char c;
> u8 tx_lvl;
> @@ -1185,18 +1175,22 @@ static int omap_8250_tx_dma(struct uart_8250_port *p)
> ret = -EBUSY;
> goto err;
> }
> - if (dma->tx_size < 4) {
> + if (kfifo_len(&tport->xmit_fifo) < 4) {
> ret = -EINVAL;
> goto err;
> }
> - if (!kfifo_get(&tport->xmit_fifo, &c)) {
> + if (!uart_fifo_out(&p->port, &c, 1)) {
> ret = -EINVAL;
> goto err;
> }
> skip_byte = c;
> - /* now we need to recompute due to kfifo_get */
> - kfifo_dma_out_prepare_mapped(&tport->xmit_fifo, &sg, 1,
> - UART_XMIT_SIZE, dma->tx_addr);
> + }
> +
> + sg_init_table(&sg, 1);
> + ret = kfifo_dma_out_prepare_mapped(&tport->xmit_fifo, &sg, 1, UART_XMIT_SIZE, dma->tx_addr);
> + if (ret != 1) {
> + ret = -EINVAL;
> + goto err;
> }
>
> desc = dmaengine_prep_slave_sg(dma->txchan, &sg, 1, DMA_MEM_TO_DEV,
> @@ -1206,6 +1200,7 @@ static int omap_8250_tx_dma(struct uart_8250_port *p)
> goto err;
> }
>
> + dma->tx_size = sg_dma_len(&sg);
> dma->tx_running = 1;
>
> desc->callback = omap_8250_dma_tx_complete;

--
js
suse labs