From: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
To: Clemens Gruber <clemens.gruber@pqgruber.com>
Cc: linux-pwm@vger.kernel.org,
Thierry Reding <thierry.reding@gmail.com>,
Sven Van Asbroeck <TheSven73@gmail.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v6 2/7] pwm: pca9685: Support hardware readout
Date: Mon, 29 Mar 2021 19:41:54 +0200 [thread overview]
Message-ID: <20210329174154.kixw3f6r5r435a45@pengutronix.de> (raw)
In-Reply-To: <YGIKWRfT7354nkPX@workstation.tuxnet>
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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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next prev parent reply other threads:[~2021-03-29 17:43 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-29 12:57 [PATCH v6 1/7] pwm: pca9685: Switch to atomic API Clemens Gruber
2021-03-29 12:57 ` [PATCH v6 2/7] pwm: pca9685: Support hardware readout Clemens Gruber
2021-03-29 15:51 ` Uwe Kleine-König
2021-03-29 16:33 ` Clemens Gruber
2021-03-29 16:54 ` Uwe Kleine-König
2021-03-29 17:11 ` Clemens Gruber
2021-03-29 17:41 ` Uwe Kleine-König [this message]
2021-03-29 12:57 ` [PATCH v6 3/7] pwm: pca9685: Improve runtime PM behavior Clemens Gruber
2021-03-29 15:55 ` Uwe Kleine-König
2021-03-29 16:31 ` Clemens Gruber
2021-03-29 12:57 ` [PATCH v6 4/7] pwm: pca9685: Support staggered output ON times Clemens Gruber
2021-03-29 17:03 ` Uwe Kleine-König
2021-03-29 17:16 ` Clemens Gruber
2021-03-29 18:02 ` Uwe Kleine-König
2021-03-31 12:26 ` Clemens Gruber
2021-03-31 13:55 ` Clemens Gruber
2021-04-01 20:59 ` Uwe Kleine-König
2021-04-01 21:53 ` Clemens Gruber
2021-03-31 16:21 ` Thierry Reding
2021-04-01 7:50 ` Clemens Gruber
2021-04-01 13:47 ` Thierry Reding
2021-04-01 15:19 ` Clemens Gruber
2021-04-02 19:48 ` Uwe Kleine-König
2021-04-01 20:58 ` Uwe Kleine-König
2021-04-01 21:37 ` Clemens Gruber
2021-04-02 20:23 ` Uwe Kleine-König
2021-03-29 12:57 ` [PATCH v6 5/7] dt-bindings: pwm: pca9685: Add nxp,staggered-outputs property Clemens Gruber
2021-04-02 19:52 ` Uwe Kleine-König
2021-03-29 12:57 ` [PATCH v6 6/7] pwm: pca9685: Restrict period change for prescaler users Clemens Gruber
2021-03-29 17:15 ` Uwe Kleine-König
2021-03-29 17:33 ` Clemens Gruber
2021-03-29 17:49 ` Uwe Kleine-König
2021-03-29 12:57 ` [PATCH v6 7/7] pwm: pca9685: Add error messages for failed regmap calls Clemens Gruber
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