linux-kernel - Re: [PATCH v4 1/6] serial: 8250: Store to lsr_save

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Date:   Wed, 8 Jun 2022 12:46:16 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc:     "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        "Matwey V. Kornilov" <matwey@....msu.ru>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 1/6] serial: 8250: Store to lsr_save_flags after lsr read

On Wed, Jun 8, 2022 at 12:44 PM Ilpo Järvinen
<ilpo.jarvinen@...ux.intel.com> 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@...il.com>

> Fixes: e490c9144cfa ("tty: Add software emulated RS485 support for 8250")
> Link: https://lore.kernel.org/all/1d6c31d-d194-9e6a-ddf9-5f29af829f3@linux.intel.com/T/#m1737eef986bd20cf19593e344cebd7b0244945fc
> Co-developed-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
> ---
>  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