Re: UART on MPC83xx in irq loop

["Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>]

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On Mon, 17 Oct 2022, Uwe Kleine-König wrote:

> 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.

I find it bit odd you seem to assume w/o any justification that the data 
would be valid (that workaround will read one byte and consider it 
valid, no?). According to the comments & workarounds to the same problem
(just look for IIR_RX_TIMEOUT and you'll find a few), they all do dummy 
reads rather than assume the data is valid.

-- 
 i.