Re: [PATCH v7 7/8] pwm: pca9685: Restrict period change for enabled PWMs

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

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On Tue, Apr 06, 2021 at 06:41:39PM +0200, Clemens Gruber wrote:
> Previously, the last used PWM channel could change the global prescale
> setting, even if other channels are already in use.
> 
> Fix it by only allowing the first enabled PWM to change the global
> chip-wide prescale setting. If there is more than one channel in use,
> the prescale settings resulting from the chosen periods must match.
> 
> GPIOs do not count as enabled PWMs as they are not using the prescaler
> and can't change it.
> 
> Signed-off-by: Clemens Gruber <clemens.gruber@pqgruber.com>
> ---
> Changes since v6:
> - Only allow the first PWM that is enabled to change the prescaler, not
>   the first one that uses the prescaler
> 
>  drivers/pwm/pwm-pca9685.c | 66 +++++++++++++++++++++++++++++++++------
>  1 file changed, 56 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-pca9685.c b/drivers/pwm/pwm-pca9685.c
> index 24221ee7a77a..cf0c98e4ef44 100644
> --- a/drivers/pwm/pwm-pca9685.c
> +++ b/drivers/pwm/pwm-pca9685.c
> @@ -23,11 +23,11 @@
>  #include <linux/bitmap.h>
>  
>  /*
> - * Because the PCA9685 has only one prescaler per chip, changing the period of
> - * one channel affects the period of all 16 PWM outputs!
> - * However, the ratio between each configured duty cycle and the chip-wide
> - * period remains constant, because the OFF time is set in proportion to the
> - * counter range.
> + * Because the PCA9685 has only one prescaler per chip, only the first channel
> + * that is enabled is allowed to change the prescale register.
> + * PWM channels requested afterwards must use a period that results in the same
> + * prescale setting as the one set by the first requested channel.
> + * GPIOs do not count as enabled PWMs as they are not using the prescaler.
>   */
>  
>  #define PCA9685_MODE1		0x00
> @@ -78,8 +78,9 @@
>  struct pca9685 {
>  	struct pwm_chip chip;
>  	struct regmap *regmap;
> -#if IS_ENABLED(CONFIG_GPIOLIB)
>  	struct mutex lock;
> +	DECLARE_BITMAP(pwms_enabled, PCA9685_MAXCHAN + 1);
> +#if IS_ENABLED(CONFIG_GPIOLIB)
>  	struct gpio_chip gpio;
>  	DECLARE_BITMAP(pwms_inuse, PCA9685_MAXCHAN + 1);
>  #endif
> @@ -90,6 +91,22 @@ static inline struct pca9685 *to_pca(struct pwm_chip *chip)
>  	return container_of(chip, struct pca9685, chip);
>  }
>  
> +/* This function is supposed to be called with the lock mutex held */
> +static bool pca9685_prescaler_can_change(struct pca9685 *pca, int channel)
> +{
> +	/* No PWM enabled: Change allowed */
> +	if (bitmap_empty(pca->pwms_enabled, PCA9685_MAXCHAN + 1))
> +		return true;
> +	/* More than one PWM enabled: Change not allowed */
> +	if (bitmap_weight(pca->pwms_enabled, PCA9685_MAXCHAN + 1) > 1)
> +		return false;
> +	/*
> +	 * Only one PWM enabled: Change allowed if the PWM about to
> +	 * be changed is the one that is already enabled
> +	 */
> +	return test_bit(channel, pca->pwms_enabled);

Maybe this is a bit more effective?:

	DECLARE_BITMAP(blablub, PCA9685_MAXCHAN + 1);	
	bitmap_zero(blablub, PCA9685_MAXCHAN + 1);
	bitmap_set(blablub, channel);
	return bitmap_subset(pca->pwms_enabled, blablub);

(but that's a minor issue, the suggested algorithm is correct.)

So:

Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

(side-note: I wonder if the handling of the set-all channel is correct
here. But given that it is messy anyhow, (e.g. because setting some
state to this set-all channel doesn't influence pwm_get_state for the
individual channels) I don't object if there is another problem in this
corner case. IMHO just dropping this virtual channel would be nice.)

Best regards
Uwe

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

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