diff mbox series

serial: 8250: Fix ...console_fifo_write on BCM283x

Message ID 20220126141124.4086065-1-phil@raspberrypi.com
State New
Headers show
Series serial: 8250: Fix ...console_fifo_write on BCM283x | expand

Commit Message

Phil Elwell Jan. 26, 2022, 2:11 p.m. UTC
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(+)

Comments

Greg Kroah-Hartman Jan. 26, 2022, 2:21 p.m. UTC | #1
On Wed, Jan 26, 2022 at 02:11:24PM +0000, Phil Elwell wrote:
> 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")

I have just reverted this commit, it's broken for many platforms now,
not just yours.

So there shouldn't be a need for this change now, right?

thanks,

greg k-h
diff mbox series

Patch

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;
 		}
 	}
 }