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

Jiri Slaby
2024-04-29T06:39:26Z

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