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

Hugo Villeneuve
2024-04-30T13:00:01Z

On Tue, 30 Apr 2024 07:22:54 +0200
Jiri Slaby <jirislaby@kernel.org> wrote:

> On 29. 04. 24, 15:47, Hugo Villeneuve wrote:
> > 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?
>
> Both :). What you prefer -- uint matches more IMO, given it's now a
> value and not a register...

Hi Jiri,
I will go with uint.

Thank you,
Hugo.

>
> thanks,
> --
> js
> suse labs
>
>

--
Hugo Villeneuve