tty: serial: 8250_dma: use sgl with 2 nents to take care of buffer wrap

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The circular buffer has now been replaced by kfifo which requires a SG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>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20240716214055.102269-1-ftoth@exalondelft.nlSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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) {