[PATCH] serial: 8250: Fix ...console_fifo_write on BCM283x

[Phil Elwell <phil@raspberrypi.com>]

The mini-UART on BCM283x is doubly crippled - it has 8-byte FIFOs and
the THRE bit indicates that the TX FIFO is not-full rather than empty.

The optimisation to enable the use of the FIFO assumes that it is safe
to write fifosize bytes whenever THRE is set, but the BCM283x quirk
(indicated by the presence of UART_CAP_MINI) makes it necessary to
check the FIFO state after each byte.

See: https://github.com/raspberrypi/linux/issues/4849

Fixes: 5021d709b31b ("tty: serial: Use fifo in 8250 console driver")
Signed-off-by: Phil Elwell <phil@raspberrypi.com>
---
 drivers/tty/serial/8250/8250_port.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 2abb3de11a48..8a2b462d363c 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -3357,6 +3357,13 @@ static void serial8250_console_fifo_write(struct uart_8250_port *up,
 				serial_out(up, UART_TX, *s++);
 				cr_sent = false;
 			}
+			/*
+			 * The BCM2835 MINI UART THRE bit is really a not-full
+			 * bit, so be prepared to bail out early.
+			 */
+			if ((up->capabilities & UART_CAP_MINI) &&
+			    !(serial_in(up, UART_LSR) & UART_LSR_THRE))
+				break;
 		}
 	}
 }
-- 
2.25.1