| Message ID | 20240703212613.56024-2-ftoth@exalondelft.nl |
|---|---|
| State | Superseded |
| Headers | show |
| Series | [PATCH/RFC,v1,1/1] tty: serial: 8250_dma: use sgl with 2 nents to take care of buffer wrap | expand |
On 03. 07. 24, 22:56, 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 The circular buffer has now been replaced by kfifo which requires a SG 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. As I stated earlier, from the DMA and TTY perspective, this looks all good™. So I welcome this. From the devices perspective, obviously testers needed ;). I believe we can merge this in 6.12-rc1 (or even 6.11-rc1?) and see. So please post a non-RFC patch. 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) {
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) {
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. Signed-off-by: Ferry Toth <ftoth@exalondelft.nl> --- drivers/tty/serial/8250/8250_dma.c | 19 ++++++++++--------- 1 file changed, 10 insertions(+), 9 deletions(-)