Re: [PATCH v5 4/4] serial: 8250_dw: fix PSLVERR on RX_TIMEOUT

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

I think shortlog should include mention to "dummy read" to make it a 
bit more specific.

On Tue, 6 May 2025, Yunhui Cui wrote:

> In the case of RX_TIMEOUT, to avoid PSLVERR, disable the FIFO

As with patch 2, please don't assume it is know to the reader how PSLVERR 
is triggered.

> before reading UART_RX when UART_LSR_DR is not set.

IMO, it would be better to explain the problem better first, something 
along these lines:

DW UART can fire RX_TIMEOUT interrupt without data and remain in that 
state forever. dw8250_handle_irq() detects this condition by checking if 
UART_LSR_DR is not asserted when RX_TIMEOUT occurred, and if detected, 
performs a dummy read to kick DW UART out of this state.

Performing dummy read from UART_RX is problematic because with ... it lead 
to ...

And only then explain the solution (disable FIFO for while performing of 
the dummy read).

> 
> Fixes: 424d79183af0 ("serial: 8250_dw: Avoid "too much work" from bogus rx timeout interrupt")
> Signed-off-by: Yunhui Cui <cuiyunhui@bytedance.com>
> ---
>  drivers/tty/serial/8250/8250_dw.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
> index f41c4a9ed58b..ffa8cb10b39c 100644
> --- a/drivers/tty/serial/8250/8250_dw.c
> +++ b/drivers/tty/serial/8250/8250_dw.c
> @@ -288,9 +288,17 @@ static int dw8250_handle_irq(struct uart_port *p)
>  		uart_port_lock_irqsave(p, &flags);
>  		status = serial_lsr_in(up);
>  
> -		if (!(status & (UART_LSR_DR | UART_LSR_BI)))
> +		if (!(status & (UART_LSR_DR | UART_LSR_BI))) {
> +			/* To avoid PSLVERR, disable the FIFO first. */
> +			if (up->fcr & UART_FCR_ENABLE_FIFO)
> +				serial_out(up, UART_FCR, 0);
> +
>  			(void) p->serial_in(p, UART_RX);
>  
> +			if (up->fcr & UART_FCR_ENABLE_FIFO)
> +				serial_out(up, UART_FCR, up->fcr);
> +		}
> +
>  		uart_port_unlock_irqrestore(p, flags);
>  	}
>  
> 

-- 
 i.