linux-kernel - Re: [PATCH v12 04/10] pwm: max7360: Add MAX7360 PWM support

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Message-Id: <DBVBZ48R7DNR.850O5X7MLMEF@bootlin.com>
Date: Wed, 06 Aug 2025 14:07:15 +0200
From: "Mathieu Dubois-Briand" <mathieu.dubois-briand@...tlin.com>
To: Uwe Kleine-König <ukleinek@...nel.org>
Cc: "Lee Jones" <lee@...nel.org>, "Rob Herring" <robh@...nel.org>,
 "Krzysztof Kozlowski" <krzk+dt@...nel.org>, "Conor Dooley"
 <conor+dt@...nel.org>, "Kamel Bouhara" <kamel.bouhara@...tlin.com>, "Linus
 Walleij" <linus.walleij@...aro.org>, "Bartosz Golaszewski" <brgl@...ev.pl>,
 "Dmitry Torokhov" <dmitry.torokhov@...il.com>, "Michael Walle"
 <mwalle@...nel.org>, "Mark Brown" <broonie@...nel.org>, "Greg
 Kroah-Hartman" <gregkh@...uxfoundation.org>, "Rafael J. Wysocki"
 <rafael@...nel.org>, "Danilo Krummrich" <dakr@...nel.org>,
 <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
 <linux-gpio@...r.kernel.org>, <linux-input@...r.kernel.org>,
 <linux-pwm@...r.kernel.org>, <andriy.shevchenko@...el.com>,
 Grégory Clement <gregory.clement@...tlin.com>, "Thomas
 Petazzoni" <thomas.petazzoni@...tlin.com>, "Andy Shevchenko"
 <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH v12 04/10] pwm: max7360: Add MAX7360 PWM support

On Fri Aug 1, 2025 at 12:11 PM CEST, Uwe Kleine-König wrote:
> On Tue, Jul 22, 2025 at 06:23:48PM +0200, Mathieu Dubois-Briand wrote:
>> +static int max7360_pwm_round_waveform_tohw(struct pwm_chip *chip,
>> +					   struct pwm_device *pwm,
>> +					   const struct pwm_waveform *wf,
>> +					   void *_wfhw)
>> +{
>> +	struct max7360_pwm_waveform *wfhw = _wfhw;
>> +	u64 duty_steps;
>> +
>> +	/*
>> +	 * Ignore user provided values for period_length_ns and duty_offset_ns:
>> +	 * we only support fixed period of MAX7360_PWM_PERIOD_NS and offset of 0.
>> +	 * Values from 0 to 254 as duty_steps will provide duty cycles of 0/256
>> +	 * to 254/256, while value 255 will provide a duty cycle of 100%.
>> +	 */
>> +	if (wf->duty_length_ns >= MAX7360_PWM_PERIOD_NS) {
>> +		duty_steps = MAX7360_PWM_MAX;
>> +	} else {
>> +		duty_steps = (u32)wf->duty_length_ns * MAX7360_PWM_STEPS / MAX7360_PWM_PERIOD_NS;
>> +		if (duty_steps == MAX7360_PWM_MAX)
>> +			duty_steps = MAX7360_PWM_MAX - 1;
>> +	}
>> +
>> +	wfhw->duty_steps = min(MAX7360_PWM_MAX, duty_steps);
>> +	wfhw->enabled = !!wf->period_length_ns;
>> +
>> +	return 0;
>
> The unconditional return 0 is wrong and testing with PWM_DEBUG enabled
> should tell you that.
>

When you say should, does that mean the current version of PWM core will
tell me that with PWM_DEBUG enabled, or does that mean we should modify
the code so it does show a warning? As I did not see any warning when
specifying a wf->period_length_ns > MAX7360_PWM_PERIOD_NS, even with
PWM_DEBUG enabled.

On the other hand, if I specify a wf->period_length_ns value below
MAX7360_PWM_PERIOD_NS, I indeed get an error:
pwm pwmchip0: Wrong rounding: requested 1000000/1000000 [+0], result 1000000/2000000 [+0]

> I think the right thing to do here is:
>
> 	if (wf->period_length_ns > MAX7360_PWM_PERIOD_NS)
> 		return 1;
> 	else
> 		return 0;

I can definitely do that, but now I'm a bit confused by the meaning of
this return value: is it 0 on success, 1 if some rounding was made,
-errno on error? So I believe I should only return 0 if
wf->period_length_ns == MAX7360_PWM_PERIOD_NS, no?

Or reading this comment on pwm_round_waveform_might_sleep(), maybe we
only have to return 1 if some value is rounded UP. So I believe the test
should be (wf->period_length_ns < MAX7360_PWM_PERIOD_NS).

>  * Returns: 0 on success, 1 if at least one value had to be rounded up or a
>  * negative errno.

This is kinda confirmed by this other comment, in the code checking the
above returned value in __pwm_apply(), even its just typical examples:

> if (err > 0)
> 	/*
> 	 * This signals an invalid request, typically
> 	 * the requested period (or duty_offset) is
> 	 * smaller than possible with the hardware.
> 	 */
> 	return -EINVAL;

So, yeah, sorry, but I'm really confused about what is the correct
return value here.

>
> Otherwise looks fine.
>
> Best regards
> Uwe

Thanks again for your time.

Best regards,
Mathieu

-- 
Mathieu Dubois-Briand, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com