Re: [PATCH v1 1/1] tty: serial: 8250_dma: use sgl with 2 nents to take care of buffer wrap

["Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>]

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On Tue, 16 Jul 2024, Ferry Toth wrote:

> Previously 8250_dma used a circular xmit->buf as DMA output buffer. This
> causes messages that wrap around in the circular buffer to be
> transmitted using 2 DMA transfers. Depending on baud rate and processor
> load this can cause an interchar gap in the middle of the message. On
> the receiving end the gap may cause a short receive timeout, possibly
> long enough to terminate a DMA transfer, but too short to restart a
> receive DMA transfer in time thus causing a receive buffer overrun.
> 
> This is especially a problem for devices with high speed UARTs (HSU)
> where even deep 64 byte FIFO's are not sufficient to handle interrupt
> latency.
> 
> The circular buffer has now been replaced by kfifo which requires a SG
> list with a single entry, which still causes 2 dma transfers when a wrap
> around occurs. Fix this by allowing up to 2 entries in the sgl.
> 
> Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
> Signed-off-by: Ferry Toth <ftoth@exalondelft.nl>
> ---
>  drivers/tty/serial/8250/8250_dma.c | 19 ++++++++++---------
>  1 file changed, 10 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_dma.c b/drivers/tty/serial/8250/8250_dma.c
> index 8a353e3cc3dd..d215c494ee24 100644
> --- a/drivers/tty/serial/8250/8250_dma.c
> +++ b/drivers/tty/serial/8250/8250_dma.c
> @@ -89,7 +89,9 @@ int serial8250_tx_dma(struct uart_8250_port *p)
>  	struct tty_port			*tport = &p->port.state->port;
>  	struct dma_async_tx_descriptor	*desc;
>  	struct uart_port		*up = &p->port;
> -	struct scatterlist sg;
> +	struct scatterlist		*sg;
> +	struct scatterlist		sgl[2];
> +	int i;
>  	int ret;
>  
>  	if (dma->tx_running) {
> @@ -110,18 +112,17 @@ int serial8250_tx_dma(struct uart_8250_port *p)
>  
>  	serial8250_do_prepare_tx_dma(p);
>  
> -	sg_init_table(&sg, 1);
> -	/* kfifo can do more than one sg, we don't (quite yet) */
> -	ret = kfifo_dma_out_prepare_mapped(&tport->xmit_fifo, &sg, 1,
> +	sg_init_table(sgl, ARRAY_SIZE(sgl));
> +
> +	ret = kfifo_dma_out_prepare_mapped(&tport->xmit_fifo, sgl, ARRAY_SIZE(sgl),
>  					   UART_XMIT_SIZE, dma->tx_addr);
>  
> -	/* we already checked empty fifo above, so there should be something */
> -	if (WARN_ON_ONCE(ret != 1))
> -		return 0;
> +	dma->tx_size = 0;
>  
> -	dma->tx_size = sg_dma_len(&sg);
> +	for_each_sg(sgl, sg, ret, i)
> +		dma->tx_size += sg_dma_len(sg);
>  
> -	desc = dmaengine_prep_slave_sg(dma->txchan, &sg, 1,
> +	desc = dmaengine_prep_slave_sg(dma->txchan, sgl, ret,
>  				       DMA_MEM_TO_DEV,
>  				       DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
>  	if (!desc) {

Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>

I envision though that this would be part of serial core so that it's not 
limited to 8250 DMA but that can be changed on top of this at a later 
time.

-- 
 i.