pwm: stm32: Fix calculation of prescaler

A small prescaler is beneficial, as this improves the resolution of the
duty_cycle configuration. However if the prescaler is too small, the
maximal possible period becomes considerably smaller than the requested
value.

One situation where this goes wrong is the following: With a parent
clock rate of 208877930 Hz and max_arr = 0xffff = 65535, a request for
period = 941243 ns currently results in PSC = 1. The value for ARR is
then calculated to

	ARR = 941243 * 208877930 / (1000000000 * 2) - 1 = 98301

This value is bigger than 65535 however and so doesn't fit into the
respective register field. In this particular case the PWM was
configured for a period of 313733.4806027616 ns (with ARR = 98301 &
0xffff). Even if ARR was configured to its maximal value, only period =
627495.6861167669 ns would be achievable.

Fix the calculation accordingly and adapt the comment to match the new
algorithm.

With the calculation fixed the above case results in PSC = 2 and so an
actual period of 941229.1667195285 ns.

Fixes: 8002fbe ("pwm: stm32: Calculate prescaler with a division instead of a loop")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/b4d96b79917617434a540df45f20cb5de4142f88.1718979150.git.u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>

Author: ukleinek

drivers/pwm/pwm-stm32.c

@@ -321,17 +321,23 @@ static int stm32_pwm_config(struct stm32_pwm *priv, unsigned int ch,
 	 * First we need to find the minimal value for prescaler such that
 	 *
 	 *        period_ns * clkrate
-	 *   ------------------------------
+	 *   ------------------------------ < max_arr + 1
 	 *   NSEC_PER_SEC * (prescaler + 1)
 	 *
-	 * isn't bigger than max_arr.
+	 * This equation is equivalent to
+	 *
+	 *        period_ns * clkrate
+	 *   ---------------------------- < prescaler + 1
+	 *   NSEC_PER_SEC * (max_arr + 1)
+	 *
+	 * Using integer division and knowing that the right hand side is
+	 * integer, this is further equivalent to
+	 *
+	 *   (period_ns * clkrate) // (NSEC_PER_SEC * (max_arr + 1)) ≤ prescaler
 	 */
 
 	prescaler = mul_u64_u64_div_u64(period_ns, clk_get_rate(priv->clk),
-					(u64)NSEC_PER_SEC * priv->max_arr);
-	if (prescaler > 0)
-		prescaler -= 1;
-
+					(u64)NSEC_PER_SEC * ((u64)priv->max_arr + 1));
 	if (prescaler > MAX_TIM_PSC)
 		return -EINVAL;