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

[Michael Walle]

Am 2020-08-07 09:45, schrieb Uwe Kleine-König:
> On Fri, Aug 07, 2020 at 09:28:31AM +0200, Michael Walle wrote:
> > Hi Uwe, Hi Lee,
> >
> > Am 2020-08-06 10:40, schrieb Uwe Kleine-König:
> > > On Mon, Aug 03, 2020 at 11:35:52AM +0200, Michael Walle wrote:
> > > > diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
> > > > index 7dbcf6973d33..a0d50d70c3b9 100644
> > > > --- a/drivers/pwm/Kconfig
> > > > +++ b/drivers/pwm/Kconfig
> > > > @@ -428,6 +428,16 @@ config PWM_SIFIVE
> > > >  	  To compile this driver as a module, choose M here: the module
> > > >  	  will be called pwm-sifive.
> > > >
> > > > +config PWM_SL28CPLD
> > > > +	tristate "Kontron sl28cpld PWM support"
> > > > +	select MFD_SIMPLE_MFD_I2C
> > >
> > > Is it sensible to present this option to everyone? Maybe
> > >
> > > 	depends on SOME_SYMBOL_ONLY_TRUE_ON_SL28CPLD || COMPILE_TEST
> >
> > Because there is now no real MFD driver anymore, there is also
> > no symbol for that. The closest would be ARCH_ARM64 but I don't
> > think that is a good idea.
> >
> > Lee, what do you think about adding a symbol to the MFD, which
> > selects MFD_SIMPLE_MFD_I2C but doesn't enable any C modules?
> >
> > I.e.
> > config MFD_SL28CPLD
> >     tristate "Kontron sl28cpld"
> >     select MFD_SIMPLE_MFD_I2C
> >     help
> >       Say yes here to add support for the Kontron sl28cpld board
> >       management controller.
> >
> > Then all the other device driver could depend on the MFD_SL28CPLD
> > symbol.
> >
> > [..]
> >
> > > > +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.

I care more about returning valid values to the PWM core ;)

-michael