linux-kernel - Re: [PATCH v3] serial: 8250_dw: Preserve original value of DLF register

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Date:   Wed, 12 Jul 2023 17:26:58 +0300 (EEST)
From:   Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To:     Ruihong Luo <colorsu1922@...il.com>
cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-serial <linux-serial@...r.kernel.org>,
        luoruihong@...omi.com, weipengliang@...omi.com,
        wengjinfei@...omi.com
Subject: Re: [PATCH v3] serial: 8250_dw: Preserve original value of DLF
 register

On Fri, 30 Jun 2023, Ruihong Luo wrote:

> Preserve the original value of the Divisor Latch Fraction (DLF) register.
> When the DLF register is modified without preservation, it can disrupt
> the baudrate settings established by firmware or bootloader, leading to
> data corruption and the generation of unreadable or distorted characters.
> 
> Fixes: 701c5e73b296 ("serial: 8250_dw: add fractional divisor support")

Add also:

Cc: stable@...r.kernel.org

> Signed-off-by: Ruihong Luo <colorsu1922@...il.com>
> ---
> v3:
> - modify the commit message
> - use personal email to sign
>  drivers/tty/serial/8250/8250_dwlib.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_dwlib.c b/drivers/tty/serial/8250/8250_dwlib.c
> index 75f32f054ebb..d30957722da8 100644
> --- a/drivers/tty/serial/8250/8250_dwlib.c
> +++ b/drivers/tty/serial/8250/8250_dwlib.c
> @@ -244,7 +244,7 @@ void dw8250_setup_port(struct uart_port *p)
>  	struct dw8250_port_data *pd = p->private_data;
>  	struct dw8250_data *data = to_dw8250_data(pd);
>  	struct uart_8250_port *up = up_to_u8250p(p);
> -	u32 reg;
> +	u32 reg, orig;
>  
>  	pd->hw_rs485_support = dw8250_detect_rs485_hw(p);
>  	if (pd->hw_rs485_support) {
> @@ -270,9 +270,11 @@ void dw8250_setup_port(struct uart_port *p)
>  	dev_dbg(p->dev, "Designware UART version %c.%c%c\n",
>  		(reg >> 24) & 0xff, (reg >> 16) & 0xff, (reg >> 8) & 0xff);
>  
> +	/* Preserve value written by firmware or bootloader  */
> +	orig = dw8250_readl_ext(p, DW_UART_DLF);
>  	dw8250_writel_ext(p, DW_UART_DLF, ~0U);
>  	reg = dw8250_readl_ext(p, DW_UART_DLF);
> -	dw8250_writel_ext(p, DW_UART_DLF, 0);
> +	dw8250_writel_ext(p, DW_UART_DLF, orig);
>  
>  	if (reg) {
>  		pd->dlf_size = fls(reg);

The change looks good but I'd prefer the variable to be called old_dlf 
which is inline how 8250_port names its variables does when it preserves a 
registers across some auto-detection write.

-- 
 i.