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

[Andi Shyti]

Pierre-Yves,

any thought on this?

Andi

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
>