Re: [PATCH] serial: dw8250: Make DLF feature independent of ADDITIONAL_FEATURE

["Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>]

On Mon, 4 Dec 2023, Dawei Li wrote:

> DW apb uart databook defines couples of configuration parameters of
> dw8250 IP, among which there are 2 of them:
> - ADDTIONAL_FEATURE
> " Configure the peripheral to have the option to include FIFO status
>   registers, shadow registers and encoded parameter register. Also
>   configures the peripheral to have UART component version and the
>   peripheral ID registers. "
> 
> - FRACTIONAL_BAUD_DIVISOR_EN
> " Configures the peripheral to have Fractional Baud Rate Divisor.
>   .... "
> 
> These two parameters are completely irrelevant, and supposed to be
> independent of each other. However, in current dw8250 driver
> implementation, they are hooked together.
> 
> The bug was hit when we are bringing up dw8250 IP on our hardware
> platform, in which parameters are configured in such combination:
> - ADDTIONAL_FEATURE disabled;
> - FRACTIONAL_BAUD_DIVISOR_EN enabled;
> 
> Fixes: 701c5e73b296 ("serial: 8250_dw: add fractional divisor support")
> Cc: stable@kernel.org
> Signed-off-by: Dawei Li <dawei.li@shingroup.cn>
> ---
>  drivers/tty/serial/8250/8250_dwlib.c | 22 +++++++++++-----------
>  1 file changed, 11 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_dwlib.c b/drivers/tty/serial/8250/8250_dwlib.c
> index 84843e204a5e..136ad093c5b6 100644
> --- a/drivers/tty/serial/8250/8250_dwlib.c
> +++ b/drivers/tty/serial/8250/8250_dwlib.c
> @@ -259,17 +259,6 @@ void dw8250_setup_port(struct uart_port *p)
>  	}
>  	up->capabilities |= UART_CAP_NOTEMT;
>  
> -	/*
> -	 * If the Component Version Register returns zero, we know that
> -	 * ADDITIONAL_FEATURES are not enabled. No need to go any further.
> -	 */
> -	reg = dw8250_readl_ext(p, DW_UART_UCV);
> -	if (!reg)
> -		return;
> -
> -	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  */
>  	old_dlf = dw8250_readl_ext(p, DW_UART_DLF);
>  	dw8250_writel_ext(p, DW_UART_DLF, ~0U);
> @@ -282,6 +271,17 @@ void dw8250_setup_port(struct uart_port *p)
>  		p->set_divisor = dw8250_set_divisor;
>  	}
>  
> +	/*
> +	 * If the Component Version Register returns zero, we know that
> +	 * ADDITIONAL_FEATURES are not enabled. No need to go any further.
> +	 */
> +	reg = dw8250_readl_ext(p, DW_UART_UCV);
> +	if (!reg)
> +		return;
> +
> +	dev_dbg(p->dev, "Designware UART version %c.%c%c\n",
> +		(reg >> 24) & 0xff, (reg >> 16) & 0xff, (reg >> 8) & 0xff);
> +
>  	reg = dw8250_readl_ext(p, DW_UART_CPR);
>  	if (!reg) {
>  		reg = data->pdata->cpr_val;
> 

The very same code change is already in tty-next (from another author).

-- 
 i.