Messages in this thread | | | Date | Tue, 30 Apr 2024 07:22:54 +0200 | Subject | Re: [PATCH] serial: sc16is7xx: fix bug in sc16is7xx_set_baud() when using prescaler | From | Jiri Slaby <> |
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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...
thanks, -- js suse labs
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