linux-kernel - Re: [PATCH v6 2/7] pwm: pca9685: Support hardware readout

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Date:   Mon, 29 Mar 2021 18:54:29 +0200
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Clemens Gruber <clemens.gruber@...ruber.com>
Cc:     linux-pwm@...r.kernel.org,
        Thierry Reding <thierry.reding@...il.com>,
        Sven Van Asbroeck <TheSven73@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 2/7] pwm: pca9685: Support hardware readout

On Mon, Mar 29, 2021 at 02:57:02PM +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>
> ---
>  drivers/pwm/pwm-pca9685.c | 41 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 41 insertions(+)
> 
> diff --git a/drivers/pwm/pwm-pca9685.c b/drivers/pwm/pwm-pca9685.c
> index 0ed1013737e3..fb026a25fb61 100644
> --- a/drivers/pwm/pwm-pca9685.c
> +++ b/drivers/pwm/pwm-pca9685.c
> @@ -333,6 +333,46 @@ 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);
> +	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.)

> +	/* 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.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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