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Date: Tue, 30 Apr 2024 08:59:54 -0400
From: Hugo Villeneuve <hugo@hugovil.com>
To: Jiri Slaby <jirislaby@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Jon Ringle
 <jringle@gridpoint.com>, ria.freelander@gmail.com, Hugo Villeneuve
 <hvilleneuve@dimonoff.com>, stable@vger.kernel.org,
 linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org
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Subject: Re: [PATCH] serial: sc16is7xx: fix bug in sc16is7xx_set_baud() when
 using prescaler

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