[PATCH v2] pwm: mediatek: support inverted polarity

[Lorenz Brun <lorenz@brun.one>]

According to the MT7986 Reference Manual the Mediatek PWM controller
doesn't appear to have support for inverted polarity.

To still support inverted PWM for common use cases, this relaxes the
check for inverted polarity within the driver to allow it to work in
case usage_power is set to true, i.e. the exact waveform does not
matter. If usage_power is true and the polarity is inverted the duty
cycle is mathematically inverted before being applied to the hardware.

Signed-off-by: Lorenz Brun <lorenz@brun.one>
---
V2: Only allow mathematically inverted PWM if usage_power is true
---
 drivers/pwm/pwm-mediatek.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/drivers/pwm/pwm-mediatek.c b/drivers/pwm/pwm-mediatek.c
index 5b5eeaff35da..18791304d1ca 100644
--- a/drivers/pwm/pwm-mediatek.c
+++ b/drivers/pwm/pwm-mediatek.c
@@ -202,8 +202,16 @@ static int pwm_mediatek_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 			      const struct pwm_state *state)
 {
 	int err;
-
-	if (state->polarity != PWM_POLARITY_NORMAL)
+	u64 duty_cycle;
+
+	/* According to the MT7986 Reference Manual the peripheral does not
+	 * appear to have the capability to invert the output.
+	 * This means that inverted mode can not be fully supported as the
+	 * waveform will always start with the low period and end with the high
+	 * period. Thus reject non-normal polarity if the shape of the waveform
+	 * matters, i.e. usage_power is not set.
+	 */
+	if (state->polarity != PWM_POLARITY_NORMAL && !state->usage_power)
 		return -EINVAL;
 
 	if (!state->enabled) {
@@ -213,7 +221,11 @@ static int pwm_mediatek_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 		return 0;
 	}
 
-	err = pwm_mediatek_config(pwm->chip, pwm, state->duty_cycle, state->period);
+	duty_cycle = state->duty_cycle;
+	if (state->polarity == PWM_POLARITY_INVERSED)
+		duty_cycle = state->period - state->duty_cycle;
+
+	err = pwm_mediatek_config(pwm->chip, pwm, duty_cycle, state->period);
 	if (err)
 		return err;
 
-- 
2.39.2