Re: [PATCH v4 1/6] serial: 8250: Store to lsr_save_flags after lsr read

[Andy Shevchenko]

On Wed, Jun 8, 2022 at 12:44 PM Ilpo Järvinen
<ilpo.jarvinen@xxxxxxxxxxxxxxx> wrote:
>
> Not all LSR register flags are preserved across reads. Therefore, LSR
> readers must store the non-preserved bits into lsr_save_flags.
>
> This fix was initially mixed into feature commit f6f586102add ("serial:
> 8250: Handle UART without interrupt on TEMT using em485"). However,
> that feature change had a flaw and it was reverted to make room for
> simpler approach providing the same feature. The embedded fix got
> reverted with the feature change.
>
> Re-add the lsr_save_flags fix and properly mark it's a fix.

Reviewed-by: Andy Shevchenko <andy.shevchenko@xxxxxxxxx>

> Fixes: e490c9144cfa ("tty: Add software emulated RS485 support for 8250")
> Link: https://lore.kernel.org/all/1d6c31d-d194-9e6a-ddf9-5f29af829f3@xxxxxxxxxxxxxxx/T/#m1737eef986bd20cf19593e344cebd7b0244945fc
> Co-developed-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
> ---
>  drivers/tty/serial/8250/8250_port.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index 4998799abae2..c5e0f925f4b6 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -1511,6 +1511,8 @@ static inline void __stop_tx(struct uart_8250_port *p)
>                 unsigned char lsr = serial_in(p, UART_LSR);
>                 u64 stop_delay = 0;
>
> +               p->lsr_saved_flags |= lsr & LSR_SAVE_FLAGS;
> +
>                 if (!(lsr & UART_LSR_THRE))
>                         return;
>                 /*
> --
> 2.30.2
>


-- 
With Best Regards,
Andy Shevchenko