linux-kernel - Re: [PATCH v2 1/6] serial: 8250: Store to lsr_save

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Date:   Mon, 6 Jun 2022 16:41:58 +0300 (EEST)
From:   Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To:     Greg KH <gregkh@...uxfoundation.org>
cc:     linux-serial <linux-serial@...r.kernel.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        "Matwey V. Kornilov" <matwey@....msu.ru>,
        LKML <linux-kernel@...r.kernel.org>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Subject: Re: [PATCH v2 1/6] serial: 8250: Store to lsr_save_flags after lsr
 read

On Mon, 6 Jun 2022, Ilpo Järvinen 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.
> 
> 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
> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> Acked-by: Uwe Kleine-König <u.kleine-koenig@...ugtronix.de>

It seems that Uwe managed to mistype his email for the A-by which I just 
happily copy-pasted.

Greg, please let me know if you want me to resend the series (or if you 
will just change it on the fly while applying).

-- 
 i.

> 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;
>  		/*
>