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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <praujgmc3c63j6brecp5kwn7tbdd7rcxmrxn67kxhxcr7rpyhw@pfbsgycx4aop>
Date: Wed, 6 Aug 2025 16:02:54 +0200
From: Uwe Kleine-König <ukleinek@...nel.org>
To: Mathieu Dubois-Briand <mathieu.dubois-briand@...tlin.com>
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 Wed, Aug 06, 2025 at 02:07:15PM +0200, Mathieu Dubois-Briand wrote:
> 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]

Yes, that's how I expect it.

> > 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).

Right,

	if (wf->period_length_ns < MAX7360_PWM_PERIOD_NS)
		return 1;
	else
		return 0;

So 0 = request could be matched by only rounding down, 1 = request could
be matched but rounding up was needed, negative value = error.

> >  * 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:

pwm_apply() has different rules. (.apply() fails when .period is too
small. This has the downside that finding a valid period is hard. For
that reason the waveform callbacks round up and signal that by returning
1.)

Best regards
Uwe

Download attachment "signature.asc" of type "application/pgp-signature" (489 bytes)