[PATCH] i2c: stm32f7: truncate clock period instead of rounding it

[Pierre Yves MORDRET]

Hi all,

Look good to me.

Best Regards

Reviewed-by: Pierre-Yves MORDRET <pierre-yves.mordret at foss.st.com>

On 6/17/26 10:06, Alain Volmat wrote:
> Hi Guillermo,
> 
> make sense indeed. Thanks a lot for this patch.
> 
> On Thu, Jun 11, 2026 at 12:48:56PM +0200, Guillermo Rodríguez wrote:
>> stm32f7_i2c_compute_timing() derives the I2C clock source period
>> (i2cclk) with DIV_ROUND_CLOSEST, which may round it up. When the
>> period is overestimated, all timings computed from it (SCLDEL,
>> SDADEL, SCLL, SCLH) come out shorter on the wire than calculated,
>> and the resulting bus rate can exceed the requested speed, violating
>> the I2C specification minimums for tLOW and tHIGH.
>>
>> For example, with a 104.45 MHz clock source (e.g. PCLK1, the
>> reset-default I2C clock source on STM32MP1), i2cclk is rounded from
>> 9.574 ns up to 10 ns. Requesting a 400 kHz fast mode bus with
>> 72/27 ns rise/fall times and no analog/digital filters then produces
>> an actual bus rate of 415.6 kHz with tLOW = 1254 ns, violating both
>> the 400 kHz maximum rate and the 1300 ns tLOW minimum of the
>> specification.
>>
>> Truncate the period instead, so that it can only be underestimated.
>> The error then falls on the safe side: the programmed timings come
>> out slightly longer than computed and the bus runs marginally below
>> the target rate (375.3 kHz in the example above) while meeting the
>> specification.
>>
>> i2cbus is left rounded-to-closest: it is only used as the target of
>> the clk_error comparison and is never multiplied into the programmed
>> timings, so nearest rounding remains accurate there.
>>
>> Fixes: aeb068c57214 ("i2c: i2c-stm32f7: add driver")
>> Cc: stable at vger.kernel.org
>> Signed-off-by: Guillermo Rodríguez <guille.rodriguez at gmail.com>
>> ---
>>  drivers/i2c/busses/i2c-stm32f7.c | 9 +++++++--
>>  1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/i2c/busses/i2c-stm32f7.c b/drivers/i2c/busses/i2c-stm32f7.c
>> index 53d9df70ebe4..6439620d6bed 100644
>> --- a/drivers/i2c/busses/i2c-stm32f7.c
>> +++ b/drivers/i2c/busses/i2c-stm32f7.c
>> @@ -464,8 +464,13 @@ static int stm32f7_i2c_compute_timing(struct stm32f7_i2c_dev *i2c_dev,
>>  {
>>  	struct stm32f7_i2c_spec *specs;
>>  	u32 p_prev = STM32F7_PRESC_MAX;
>> -	u32 i2cclk = DIV_ROUND_CLOSEST(NSEC_PER_SEC,
>> -				       setup->clock_src);
>> +	/*
>> +	 * Truncate instead of rounding to closest: if the clock period is
>> +	 * overestimated, the computed SCL timings will come out shorter on
>> +	 * the wire, which can push the bus above the target rate and below
>> +	 * the spec's tLOW/tHIGH minimums.
>> +	 */
>> +	u32 i2cclk = NSEC_PER_SEC / setup->clock_src;
>>  	u32 i2cbus = DIV_ROUND_CLOSEST(NSEC_PER_SEC,
>>  				       setup->speed_freq);
>>  	u32 clk_error_prev = i2cbus;
>> -- 
>> 2.25.1
>>
> 
> Acked-by: Alain Volmat <alain.volmat at foss.st.com>
> 
> Regards,
> Alain

-- 
--
~ Py MORDRET
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