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

[Hugo Villeneuve <hugo@hugovil.com>]

On Mon, 29 Apr 2024 08:39:22 +0200
Jiri Slaby <jirislaby@kernel.org> wrote:

> 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?

Hi Jiri,
before this patch, the variable prescaler was used to store an 8 bit
bitfield. Now the variable meaning is changed to be used as the
prescaler value, which can be 1 or 4 in this case. Leaving
it as u8 would still be ok, or making it "unsigned int" maybe?

Hugo.

-- 
Hugo Villeneuve