On Mon, Mar 29, 2021 at 07:11:53PM +0200, Clemens Gruber wrote: > On Mon, Mar 29, 2021 at 06:54:29PM +0200, Uwe Kleine-König wrote: > > On Mon, Mar 29, 2021 at 02:57:02PM +0200, Clemens Gruber wrote: > > > [...] > > > + /* Calculate (chip-wide) period from prescale value */ > > > + regmap_read(pca->regmap, PCA9685_PRESCALE, &val); > > > + state->period = (PCA9685_COUNTER_RANGE * 1000 / PCA9685_OSC_CLOCK_MHZ) * > > > + (val + 1); > > > > As we have PCA9685_OSC_CLOCK_MHZ = 25 this is an integer calculation > > without loss of precision. It might be worth to point that out in a > > comment. (Otherwise doing the division at the end might be more > > sensible.) > > What comment do you have in mind? > /* 1 integer multiplication (without loss of precision) at runtime */ ?
Something like:
/*
* PCA9685_OSC_CLOCK_MHZ is 25 and so an integer divider of
* 1000. So the calculation here is only a multiplication and
* we're not loosing precision.
*/
> > > + /* The (per-channel) polarity is fixed */
> > > + state->polarity = PWM_POLARITY_NORMAL;
> > > +
> > > + if (pwm->hwpwm >= PCA9685_MAXCHAN) {
> > > + /*
> > > + * The "all LEDs" channel does not support HW readout
> > > + * Return 0 and disabled for backwards compatibility
> > > + */
> > > + state->duty_cycle = 0;
> > > + state->enabled = false;
> > > + return;
> > > + }
> > > +
> > > + duty = pca9685_pwm_get_duty(pca, pwm->hwpwm);
> > > +
> > > + state->enabled = !!duty;
> > > + if (!state->enabled) {
> > > + state->duty_cycle = 0;
> > > + return;
> > > + } else if (duty == PCA9685_COUNTER_RANGE) {
> > > + state->duty_cycle = state->period;
> > > + return;
> > > + }
> > > +
> > > + duty *= state->period;
> > > + state->duty_cycle = duty / PCA9685_COUNTER_RANGE;
> >
> > .apply uses ROUND_CLOSEST to calculate duty from state->duty_cycle,
> > still using / here (instead of ROUND_CLOSEST), but again as
> > PCA9685_OSC_CLOCK_MHZ is 25 this calculation doesn't suffer from
> > rounding errors. So if you feed the state returned here into .apply
> > again, there is (as I want it) no change.
> >
> > The only annoyance is that if PCA9685_PRESCALE holds a value smaller
> > than 3, .apply() will fail. Not sure there is any saner way to handle
> > this.
>
> According to the datasheet, "The hardware forces a minimum value that
> can be loaded into the PRE_SCALE register at '3'", so there should never
> be anything below 3 in that register.
Did you verify that the register reads back a 3 if you write a lower
value into the register?
Maybe the most defensive way would be:
+ regmap_read(pca->regmap, PCA9685_PRESCALE, &val);
+ /*
+ * According to the datasheet, the hardware forces a minimum
+ * value that can be loaded is 3, so if we read something lower
+ * assume that the hardware actually implemented a 3.
+ */
+ if (val < 3)
+ val = 3;
+ state->period = ...
Best regards
Uwe
--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | https://www.pengutronix.de/ |
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