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

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

[Phil Elwell]

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

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

[Greg Kroah-Hartman]

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

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

[Phil Elwell]

Hi Greg,

On 26/01/2022 14:21, Greg Kroah-Hartman wrote:
> 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?

This patch is sufficient on the BCM283x family of devices, but a Fix
for a non-existent patch is clearly superfluous. The author of the
original patch can consider this as a hint about the kind of issues that
need to be considered before resubmitting.

Thanks,

Phil