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

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Date:   Wed, 7 Apr 2021 23:38:02 +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 7/8] pwm: pca9685: Restrict period change for enabled
 PWMs

On Wed, Apr 07, 2021 at 10:41:27PM +0200, Clemens Gruber wrote:
> On Wed, Apr 07, 2021 at 08:12:29AM +0200, Uwe Kleine-König wrote:
> > 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@...ruber.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 if no PWM is enabled, it should return true, not false.

If no PWM is enabled we have pca->pwms_enabled = empty set which is a
subset of every set. So I'd expect this case to be handled just fine.

> > (but that's a minor issue, the suggested algorithm is correct.)
> 
> I would prefer to keep it explicit because it is a little easier to
> follow and probably not worth optimizing.

I didn't find it hard to follow, but I'm willing to accept that this
isn't representative. I'm ok with keeping the code as is.
 
> I agree that it would be nice to drop the ALL channel support.

Maybe deprecate it using a config item? But no hurry.

Best regards
Uwe

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

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