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

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Date:   Wed, 7 Apr 2021 07:31:35 +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>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 2/8] pwm: pca9685: Support hardware readout

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.)

> +}
> +
>  static int pca9685_pwm_request(struct pwm_chip *chip, struct pwm_device *pwm)
>  {
>  	struct pca9685 *pca = to_pca(chip);

Best regards
Uwe

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

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