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: 8002fbeef1e4 ("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>

diff --git a/drivers/pwm/pwm-stm32.c b/drivers/pwm/pwm-stm32.c
index 3e7b2a8..97d3de2 100644 (file)
--- a/drivers/pwm/pwm-stm32.c
+++ b/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;