pwm: stm32: Fix rounding issue for requests with inverted polarity

pwm: stm32: Fix rounding issue for requests with inverted polarity

The calculation of the number of pwm clk ticks from a time length in
nanoseconds involves a division and thus some rounding. That might
result in

	duty_ticks + offset_ticks < period_ticks

despite

	duty_length_ns + duty_offset_ns >= period_length_ns

. The stm32 PWM cannot configure offset_ticks freely, it can only select
0 or period_length_ns - duty_length_ns---that is the classic normal and
inverted polarity. The decision to select the hardware polarity must be
done using the ticks values and not the nanoseconds times to adhere to
the rounding rules by the pwm core.

With the pwm clk running at 208900 kHz on my test machine
(stm32mp135f-dk), a test case that was handled wrong is:

	# pwmround -P 9999962 -O 24970 -D 9974992
	period_length = 9999962
	duty_length = 9974840
	duty_offset = 25123

With this change applied the rounding is done correctly:

	# pwmround -P 9999962 -O 24970 -D 9974992
	period_length = 9999962
	duty_length = 9974840
	duty_offset = 0

Fixes: deaba9cff809 ("pwm: stm32: Implementation of the waveform callbacks")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://patch.msgid.link/c5e7767cee821b5f6e00f95bd14a5e13015646fb.1776264104.git.u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>