tty/serial: pl011: restrict RX burst FIFO threshold

If the associated DMA controller has lower burst length support than the
level the FIFO is set to, then bytes will be left in the RX FIFO at the
end of a DMA block - requiring a round-trip through the timeout interrupt
handler rather than an end-of-block DMA interrupt.

Signed-off-by: Jonathan Bell <jonathan@raspberrypi.com>

Author: P33M

drivers/tty/serial/amba-pl011.c

@@ -487,6 +487,12 @@ static void pl011_dma_probe(struct uart_amba_port *uap)
 					"RX DMA disabled - no residue processing\n");
 				return;
 			}
+			/*
+			 * DMA controllers with smaller burst capabilities than 1/4
+			 * the FIFO depth will leave more bytes than expected in the
+			 * RX FIFO if mismatched.
+			 */
+			rx_conf.src_maxburst = min(caps.max_burst, rx_conf.src_maxburst);
 		}
 		dmaengine_slave_config(chan, &rx_conf);
 		uap->dmarx.chan = chan;