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

Ilpo Järvinen
2025-05-06T12:25:46Z

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.