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
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