UART on MPC83xx in irq loop

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

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Hello,

I have a system based on MPC8313ERDB here that in some situations gets
stuck in an irq loop. I have a reproducer here that works reliably. A
workaround is:

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 45b8a59d937c..5ab32b6ba701 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2009,6 +2009,14 @@ int serial8250_handle_irq(struct uart_port *port, unsigned int iir)
 
 	status = serial_port_in(port, UART_LSR);
 
+	/*
+	 * Sometimes a "Character time-out" (IID3:0 == 0xc) happens on MPC8313,
+	 * but LSR doesn't report "Data ready". To clear the former the receive
+	 * buffer must be read. It's unclear if the char read is valid or not.
+	 */
+	if ((iir & UART_IIR_ID) == UART_IIR_RX_TIMEOUT)
+		status |= UART_LSR_DR;
+
 	/*
 	 * If port is stopped and there are no error conditions in the
 	 * FIFO, then don't drain the FIFO, as this may lead to TTY buffer

I havn't debugged that further than written in the comment but I wonder
if this is a known issue (didn't find it in the errata though) and/or if
someone with hardware knowledge could confirm this to be a hardware
fault.

Without feedback from NXP I'd look in more detail into that to for
example find out the timing and so maybe more hints about the hardware
and a better SW workaround/fix.

Any input is very welcome.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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