Re: [PATCH] serial: sc16is7xx: fix bug in sc16is7xx_set_baud() when using prescaler

[Jiri Slaby <jirislaby@kernel.org>]

On 26. 04. 24, 15:59, Hugo Villeneuve wrote:
> From: Hugo Villeneuve <hvilleneuve@dimonoff.com>
> 
> When using a high speed clock with a low baud rate, the 4x prescaler is
> automatically selected if required. In that case, sc16is7xx_set_baud()
> properly configures the chip registers, but returns an incorrect baud
> rate by not taking into account the prescaler value. This incorrect baud
> rate is then fed to uart_update_timeout().
> 
> For example, with an input clock of 80MHz, and a selected baud rate of 50,
> sc16is7xx_set_baud() will return 200 instead of 50.
> 
> Fix this by first changing the prescaler variable to hold the selected
> prescaler value instead of the MCR bitfield. Then properly take into
> account the selected prescaler value in the return value computation.
> 
> Also add better documentation about the divisor value computation.
> 
> Fixes: dfeae619d781 ("serial: sc16is7xx")
> Cc: stable@vger.kernel.org
> Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
> ---
>   drivers/tty/serial/sc16is7xx.c | 23 ++++++++++++++++++-----
>   1 file changed, 18 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/tty/serial/sc16is7xx.c b/drivers/tty/serial/sc16is7xx.c
> index 03cf30e20b75..dcd6c5615401 100644
> --- a/drivers/tty/serial/sc16is7xx.c
> +++ b/drivers/tty/serial/sc16is7xx.c
> @@ -555,16 +555,28 @@ static bool sc16is7xx_regmap_noinc(struct device *dev, unsigned int reg)
>   	return reg == SC16IS7XX_RHR_REG;
>   }
>   
> +/*
> + * Configure programmable baud rate generator (divisor) according to the
> + * desired baud rate.
> + *
> + * From the datasheet, the divisor is computed according to:
> + *
> + *              XTAL1 input frequency
> + *             -----------------------
> + *                    prescaler
> + * divisor = ---------------------------
> + *            baud-rate x sampling-rate
> + */
>   static int sc16is7xx_set_baud(struct uart_port *port, int baud)
>   {
>   	struct sc16is7xx_one *one = to_sc16is7xx_one(port, port);
>   	u8 lcr;
> -	u8 prescaler = 0;
> +	int prescaler = 1;

Ugh, why do you move to signed arithmetics?

regards,
-- 
js
suse labs