linux-kernel - Re: [PATCH v7 2/8] pwm: pca9685: Support hardware readout

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Message-ID: <YG2BEGsPU8jWzvPq@workstation.tuxnet>
Date:   Wed, 7 Apr 2021 11:53:20 +0200
From:   Clemens Gruber <clemens.gruber@...ruber.com>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     linux-pwm@...r.kernel.org,
        Thierry Reding <thierry.reding@...il.com>,
        Sven Van Asbroeck <TheSven73@...il.com>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 2/8] pwm: pca9685: Support hardware readout

On Wed, Apr 07, 2021 at 11:09:43AM +0200, Uwe Kleine-König wrote:
> On Wed, Apr 07, 2021 at 09:33:20AM +0200, Clemens Gruber wrote:
> > On Wed, Apr 07, 2021 at 07:31:35AM +0200, Uwe Kleine-König wrote:
> > > On Tue, Apr 06, 2021 at 06:41:34PM +0200, Clemens Gruber wrote:
> > > > Implements .get_state to read-out the current hardware state.
> > > > 
> > > > The hardware readout may return slightly different values than those
> > > > that were set in apply due to the limited range of possible prescale and
> > > > counter register values.
> > > > 
> > > > Also note that although the datasheet mentions 200 Hz as default
> > > > frequency when using the internal 25 MHz oscillator, the calculated
> > > > period from the default prescaler register setting of 30 is 5079040ns.
> > > > 
> > > > Signed-off-by: Clemens Gruber <clemens.gruber@...ruber.com>
> > > > ---
> > > > Changes since v6:
> > > > - Added a comment regarding the division (Suggested by Uwe)
> > > > - Rebased
> > > > 
> > > >  drivers/pwm/pwm-pca9685.c | 46 +++++++++++++++++++++++++++++++++++++++
> > > >  1 file changed, 46 insertions(+)
> > > > 
> > > > diff --git a/drivers/pwm/pwm-pca9685.c b/drivers/pwm/pwm-pca9685.c
> > > > index 5a2ce97e71fd..d4474c5ff96f 100644
> > > > --- a/drivers/pwm/pwm-pca9685.c
> > > > +++ b/drivers/pwm/pwm-pca9685.c
> > > > @@ -333,6 +333,51 @@ static int pca9685_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> > > >  	return 0;
> > > >  }
> > > >  
> > > > +static void pca9685_pwm_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
> > > > +				  struct pwm_state *state)
> > > > +{
> > > > +	struct pca9685 *pca = to_pca(chip);
> > > > +	unsigned long long duty;
> > > > +	unsigned int val = 0;
> > > > +
> > > > +	/* Calculate (chip-wide) period from prescale value */
> > > > +	regmap_read(pca->regmap, PCA9685_PRESCALE, &val);
> > > > +	/*
> > > > +	 * PCA9685_OSC_CLOCK_MHZ is 25, i.e. an integer divider of 1000.
> > > > +	 * The following calculation is therefore only a multiplication
> > > > +	 * and we are not losing precision.
> > > > +	 */
> > > > +	state->period = (PCA9685_COUNTER_RANGE * 1000 / PCA9685_OSC_CLOCK_MHZ) *
> > > > +			(val + 1);
> > > > +
> > > > +	/* 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;
> > > 
> > > Given that with duty = 0 the chip is still "on" and changing the duty
> > > will first complete the currently running period, I'd model duty=0 as
> > > enabled. This also simplifies the code a bit, to something like:
> > > 
> > > 
> > > 	state->enabled = true;
> > > 	duty = pca9685_pwm_get_duty(pca, pwm->hwpwm);
> > > 	state->duty_cycle = div_round_up(duty * state->period, PCA9685_COUNTER_RANGE);
> > > 
> > > (I'm using round-up here assuming apply uses round-down to get
> > > idempotency. In the current patch set state this is wrong however.)
> > 
> > So, in your opinion, every requested PWM of the pca9685 should always be
> > enabled by default (from the PWM core viewpoint) ?
> > 
> > And this wouldn't break the following because pwm_get_state does not
> > actually read out the hw state:
> > pwm_get_state -> enabled=true duty=0
> > pwm_apply_state -> enabled =false duty=0
> > pwm_get_state -> enabled=false duty=0
> 
> I don't see any breakage here. Either there is none or I failed to grasp
> where you see a problem.

Me neither, I was just thinking out loud.

Clemens