Re: [PATCH v2] pwm: mediatek: support inverted polarity

Thierry Reding
2023-04-06T13:39:03Z

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On Thu, Mar 09, 2023 at 02:04:10AM +0100, Lorenz Brun wrote:
> 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

Block comments should have no text on the first line:

/*
* According
* ...
*/

> + * 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;

That's not really what state->usage_power was meant to address. What's
wrong with just reversing the duty cycle in the pwm-fan? If you use DT
it's quite trivial to do that by just reversing the entries in your
cooling-levels property. Does that not work for you?

Thierry

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