linux-kernel - Re: [PATCH v2] pwm: mediatek: support inverted polarity

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Date:   Thu, 6 Apr 2023 15:38:48 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Lorenz Brun <lorenz@...n.one>
Cc:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Matthias Brugger <matthias.bgg@...il.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        linux-pwm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH v2] pwm: mediatek: support inverted polarity

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@...n.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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