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

[Andi Shyti]

Hi Guillermo,

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>

Merged to i2c/i2c-host.

Thanks to Alain and Pierre-Yves for their review!

Andi