Re: [PATCH v7 06/13] pwm: add support for sl28cpld PWM controller

Uwe Kleine-König
2020-08-07T10:24:52Z

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Hi Michael,

On Fri, Aug 07, 2020 at 09:55:19AM +0200, Michael Walle wrote:
> Am 2020-08-07 09:45, schrieb Uwe Kleine-König:
> > On Fri, Aug 07, 2020 at 09:28:31AM +0200, Michael Walle wrote:
> > > Am 2020-08-06 10:40, schrieb Uwe Kleine-König:
> > > > On Mon, Aug 03, 2020 at 11:35:52AM +0200, Michael Walle wrote:
> > > > > +static void sl28cpld_pwm_get_state(struct pwm_chip *chip,
> > > > > + struct pwm_device *pwm,
> > > > > + struct pwm_state *state)
> > > > > +{
> > > > > + struct sl28cpld_pwm *priv = dev_get_drvdata(chip->dev);
> > > > > + unsigned int reg;
> > > > > + int prescaler;
> > > > > +
> > > > > + sl28cpld_pwm_read(priv, SL28CPLD_PWM_CTRL, &reg);
> > > > > +
> > > > > + state->enabled = reg & SL28CPLD_PWM_CTRL_ENABLE;
> > > > > +
> > > > > + prescaler = FIELD_GET(SL28CPLD_PWM_CTRL_PRESCALER_MASK, reg);
> > > > > + state->period = SL28CPLD_PWM_PERIOD(prescaler);
> > > > > +
> > > > > + sl28cpld_pwm_read(priv, SL28CPLD_PWM_CYCLE, &reg);
> > > > > + state->duty_cycle = SL28CPLD_PWM_TO_DUTY_CYCLE(reg);
> > > >
> > > > Should reg be masked to SL28CPLD_PWM_CYCLE_MAX, or is it guaranteed that
> > > > the upper bits are zero?
> > >
> > > Mh, the hardware guarantees that bit7 is zero. So masking with
> > > SL28CPLD_PWM_CYCLE_MAX won't buy us much. But what I could think
> > > could go wrong is this: someone set the prescaler to != 0 and the
> > > duty cycle to a value greater than the max value for this particular
> > > prescaler mode. For the above calculations this would result in a
> > > duty_cycle greater than the period, if I'm not mistaken.
> > >
> > > The behavior of the hardware is undefined in that case (at the moment
> > > it will be always on, I guess). So this isn't a valid setting.
> > > Nevertheless it might happen. So what about the following:
> > >
> > > state->duty_cycle = min(state->duty_cycle, state->period);
> >
> > If you care about this: This can also happen (at least shortly) in
> > sl28cpld_pwm_apply() as you write SL28CPLD_PWM_CTRL before
> > SL28CPLD_PWM_CYCLE there.
>
> It could also happen if it was the other way around, couldn't it?
> Changing modes might glitch.

If you want to prevent this, you have to order the writes depending on
prescaler increasing or decreasing.

Best regards
Uwe

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