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Date: Tue, 30 Apr 2024 07:22:54 +0200
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Subject: Re: [PATCH] serial: sc16is7xx: fix bug in sc16is7xx_set_baud() when
 using prescaler
To: Hugo Villeneuve <hugo@hugovil.com>
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
References: <20240426135937.3810959-1-hugo@hugovil.com>
 <17d2cc58-cf68-430d-9248-25abe4c5b0f0@kernel.org>
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From: Jiri Slaby <jirislaby@kernel.org>
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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