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

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Date:   Fri, 03 Mar 2023 23:23:07 +0100
From:   Lorenz Brun <lorenz@...n.one>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        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] pwm: mediatek: support inverted polarity


On Fri, Mar 3 2023 at 22:17:25 +01:00:00, Uwe Kleine-König 
<u.kleine-koenig@...gutronix.de> wrote:
> On Fri, Mar 03, 2023 at 09:58:21PM +0100, Lorenz Brun wrote:
>>  According to the MT7986 Reference Manual the Mediatek  PWM 
>> controller
>>  doesn't appear to have support for inverted polarity.
>> 
>>  This implements the same solution as in pwm-meson and just inverts 
>> the
>>  duty cycle instead, which results in the same outcome.
> 
> This idea is broken. This was recently discussed on the linux-pwm list
> and I hope will be fixed soon. See
> https://lore.kernel.org/linux-pwm/20230228093911.bh2sbp4tyfir2z5g@pengutronix.de/T/#meda75ffbd4ef2048991ea2cd091c0c14b1bb09c2
> 
Is the issue here emulating PWM_POLARITY_INVERSED by inverting the 
period or the overflow issues?
This driver currently rejects PWM_POLARITY_INVERSED, but the problem is 
that I have a board which inverts the output of the PWM peripheral 
(low-side MOSFET for higher-voltage open-drain output), thus I need to 
set the PWM node to output an inverted signal so that the final 
open-drain output behaves correctly as the signal has been inverted 
twice now.

In my specific case this logic could also be added to pwm-fan, but this 
would lead to more complexity there as this type of circuit is 
generally handled by the PWM driver.

> So this patch won't be accepted, still pointing out a style problem
> below.
> 
>>  Signed-off-by: Lorenz Brun <lorenz@...n.one>
>>  ---
>>   drivers/pwm/pwm-mediatek.c | 13 +++++++++----
>>   1 file changed, 9 insertions(+), 4 deletions(-)
>> 
>>  diff --git a/drivers/pwm/pwm-mediatek.c b/drivers/pwm/pwm-mediatek.c
>>  index 5b5eeaff35da..6f4a54c8299f 100644
>>  --- a/drivers/pwm/pwm-mediatek.c
>>  +++ b/drivers/pwm/pwm-mediatek.c
>>  @@ -202,9 +202,7 @@ 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)
>>  -		return -EINVAL;
>>  +	u64 duty_cycle;
>> 
>>   	if (!state->enabled) {
>>   		if (pwm->state.enabled)
>>  @@ -213,7 +211,14 @@ 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);
>>  +	// According to the MT7986 Reference Manual the peripheral does 
>> not
>>  +	// appear to have the capability to invert the output. Instead 
>> just
>>  +	// invert the duty cycle.
> 
> Wrong commenting style, please stick to C-style comments (/* ... */)
I can fix that if I end up submitting a V2 of this patch, but this 
didn't get picked up by checkpatch.

> 
>>  +	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;
> 
> Best regards
> Uwe
> 
> --
> Pengutronix e.K.                           | Uwe Kleine-König        
>     |
> Industrial Linux Solutions                 | 
> https://www.pengutronix.de/ |

Regards,
Lorenz