pwm: stm32: Improve capture by tuning counter prescaler

Currently, capture is based on timeout window to configure prescaler.
PWM capture framework provides 1s window at the time of writing.

There's place for improvement, after input signal has been captured once:
- Finer tune counter clock prescaler, by using 1st capture result (with
arbitrary margin).
- Do a 2nd capture, with scaled capture window.
This increases accuracy, especially at high rates.

Signed-off-by: Fabrice Gasnier <[email protected]>
Reviewed-by: Benjamin Gaignard <[email protected]>
Acked-by: Thierry Reding <[email protected]>
Signed-off-by: Lee Jones <[email protected]>

diff --git a/drivers/pwm/pwm-stm32.c b/drivers/pwm/pwm-stm32.c
index ed3961b7b938a99bcbedaff9c8dcb5acdc7b817c..9a50acde1e61d7edcf2f327215efbb913a3b246a 100644 (file)
--- a/drivers/pwm/pwm-stm32.c
+++ b/drivers/pwm/pwm-stm32.c
@@ -168,7 +168,7 @@ static int stm32_pwm_capture(struct pwm_chip *chip, struct pwm_device *pwm,
        struct stm32_pwm *priv = to_stm32_pwm_dev(chip);
        unsigned long long prd, div, dty;
        unsigned long rate;
-       unsigned int psc = 0;
+       unsigned int psc = 0, scale;
        u32 raw_prd, raw_dty;
        int ret = 0;
 
@@ -219,6 +219,28 @@ static int stm32_pwm_capture(struct pwm_chip *chip, struct pwm_device *pwm,
        if (ret)
                goto stop;
 
+       /*
+        * Got a capture. Try to improve accuracy at high rates:
+        * - decrease counter clock prescaler, scale up to max rate.
+        */
+       if (raw_prd) {
+               u32 max_arr = priv->max_arr - 0x1000; /* arbitrary margin */
+
+               scale = max_arr / min(max_arr, raw_prd);
+       } else {
+               scale = priv->max_arr; /* bellow resolution, use max scale */
+       }
+
+       if (psc && scale > 1) {
+               /* 2nd measure with new scale */
+               psc /= scale;
+               regmap_write(priv->regmap, TIM_PSC, psc);
+               ret = stm32_pwm_raw_capture(priv, pwm, tmo_ms, &raw_prd,
+                                           &raw_dty);
+               if (ret)
+                       goto stop;
+       }
+
        prd = (unsigned long long)raw_prd * (psc + 1) * NSEC_PER_SEC;
        result->period = DIV_ROUND_UP_ULL(prd, rate);
        dty = (unsigned long long)raw_dty * (psc + 1) * NSEC_PER_SEC;