Re: [PATCH v5 7/7] pwm: pca9685: Restrict period change for prescaler users

[Clemens Gruber <clemens.gruber@pqgruber.com>]

Hi Sven,

On Wed, Dec 16, 2020 at 11:03:39PM -0500, Sven Van Asbroeck wrote:
> Hi Clemens, see below.
> 
> On Wed, Dec 16, 2020 at 7:53 AM Clemens Gruber
> <clemens.gruber@pqgruber.com> wrote:
> >
> > Previously, the last used PWM channel could change the global prescale
> > setting, even if other channels were already in use.
> >
> > Fix it by only allowing the first user of the prescaler 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.
> >
> > PWMs that are disabled or have a duty cycle of 0% or 100% are not
> > considered to be using the prescaler as they have the full OFF or full
> > ON bits set. This also applies to channels used as GPIOs.
> >
> > Signed-off-by: Clemens Gruber <clemens.gruber@pqgruber.com>
> > ---
> >  drivers/pwm/pwm-pca9685.c | 51 +++++++++++++++++++++++++++++++++------
> >  1 file changed, 44 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/pwm/pwm-pca9685.c b/drivers/pwm/pwm-pca9685.c
> > index ff916980de49..438492d4aed4 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 uses the prescaler 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, unless they
> > + * use duty cycles of 0% or 100% (prescaler not used for full OFF/ON).
> >   */
> >
> >  #define PCA9685_MODE1          0x00
> > @@ -80,6 +80,8 @@ struct pca9685 {
> >         struct pwm_chip chip;
> >         struct regmap *regmap;
> >         bool staggered_outputs;
> > +       struct mutex prescaler_users_lock;
> 
> Keep things simple by re-using the "struct mutex lock" below?
> This code isn't performance-intensive, so having a single lock for
> pwm/gpio requests + pwm_apply() is probably ok.

Yes, I think this could work. Good idea.

> 
> > +       DECLARE_BITMAP(prescaler_users, PCA9685_MAXCHAN + 1);
> 
> Rename to pwms_use_prescale ?

Yes, fine with me.

> 
> >  #if IS_ENABLED(CONFIG_GPIOLIB)
> >         struct mutex lock;
> >         struct gpio_chip gpio;
> > @@ -92,6 +94,18 @@ 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 prescaler_users_lock held */
> > +static inline bool pca9685_may_change_prescaler(struct pca9685 *pca, int channel)
> 
> Drop the inline? Only the compiler knows if inlining this function makes sense
> on a platform (armv7, x86, etc). Compilers are usually better at this then
> humans...

You're probably right. I will drop the inline.

> 
> Rename to pca9685_prescaler_can_change() ?

Sounds good!

> 
> > +{
> > +       /*
> > +        * A PWM channel may only change the prescaler if there are no users of
> > +        * the prescaler yet or that same channel is the only one in use.
> > +        */
> > +       return bitmap_empty(pca->prescaler_users, PCA9685_MAXCHAN + 1) ||
> > +               (bitmap_weight(pca->prescaler_users, PCA9685_MAXCHAN + 1) == 1 &&
> > +                test_bit(channel, pca->prescaler_users));
> > +}
> 
> I found this logic expression quite complex to read. Perhaps simplify by using
> a few steps? For example:
> 
> /* if prescaler not in use, we can always change it */
> if (empty) return true;
> /* if more than one pwm is using the prescaler, we can never change it */
> if (weight > 1) return false;
> /* one pwm is using the prescaler, we can only change it if it's us */
> return test_bit(us);

Good point, I will simplify it!

> 
> > +
> >  static void pca9685_pwm_set_duty(struct pca9685 *pca, int channel, unsigned int duty)
> >  {
> >         unsigned int on, off;
> > @@ -337,16 +351,25 @@ static int pca9685_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> >         duty = PCA9685_COUNTER_RANGE * state->duty_cycle;
> >         duty = DIV_ROUND_CLOSEST_ULL(duty, state->period);
> >
> > +       mutex_lock(&pca->prescaler_users_lock);
> > +
> >         if (!state->enabled || duty < 1) {
> >                 pca9685_pwm_set_duty(pca, pwm->hwpwm, 0);
> > -               return 0;
> > +               goto prescaler_unused;
> >         } else if (duty == PCA9685_COUNTER_RANGE) {
> >                 pca9685_pwm_set_duty(pca, pwm->hwpwm, duty);
> > -               return 0;
> > +               goto prescaler_unused;
> >         }
> >
> >         regmap_read(pca->regmap, PCA9685_PRESCALE, &val);
> >         if (prescale != val) {
> > +               if (!pca9685_may_change_prescaler(pca, pwm->hwpwm)) {
> > +                       mutex_unlock(&pca->prescaler_users_lock);
> > +                       dev_err(chip->dev,
> > +                               "prescaler not set: already in use with different setting!\n");
> > +                       return -EBUSY;
> > +               }
> > +
> >                 /*
> >                  * Putting the chip briefly into SLEEP mode
> >                  * at this point won't interfere with the
> > @@ -364,6 +387,14 @@ static int pca9685_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> >         }
> >
> >         pca9685_pwm_set_duty(pca, pwm->hwpwm, duty);
> > +
> > +       set_bit(pwm->hwpwm, pca->prescaler_users);
> > +       mutex_unlock(&pca->prescaler_users_lock);
> > +       return 0;
> > +
> > +prescaler_unused:
> > +       clear_bit(pwm->hwpwm, pca->prescaler_users);
> > +       mutex_unlock(&pca->prescaler_users_lock);
> >         return 0;
> >  }
> 
> The need for the mutex makes this function quite "messy": we have to guard all
> the exits, and that's easy to forget.

I agree.

> 
> Maybe simplify the function by moving the mutex to a helper?
> Example:
> 
> static int __pca9685_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>      const struct pwm_state *state)
> {
>  ... just do stuff and don't worry about the mutex
> }
> 
> static int pca9685_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>      const struct pwm_state *state)
> {
>     /* document why we serialize pwm_apply */
>     mutex_lock();
>     __pca9685_pwm_apply(chip, pwm, state);
>     mutex_unlock();
> }

Also a good idea!

As always, great review! Thank you!

> 
> >
> > @@ -422,7 +453,11 @@ static void pca9685_pwm_free(struct pwm_chip *chip, struct pwm_device *pwm)
> >  {
> >         struct pca9685 *pca = to_pca(chip);
> >
> > +       mutex_lock(&pca->prescaler_users_lock);
> > +       clear_bit(pwm->hwpwm, pca->prescaler_users);
> >         pca9685_pwm_set_duty(pca, pwm->hwpwm, 0);
> > +       mutex_unlock(&pca->prescaler_users_lock);
> > +
> >         pm_runtime_put(chip->dev);
> >         pca9685_pwm_clear_inuse(pca, pwm->hwpwm);
> >  }
> > @@ -463,6 +498,8 @@ static int pca9685_pwm_probe(struct i2c_client *client,
> >
> >         i2c_set_clientdata(client, pca);
> >
> > +       mutex_init(&pca->prescaler_users_lock);
> > +
> >         regmap_read(pca->regmap, PCA9685_MODE2, &reg);
> >
> >         if (device_property_read_bool(&client->dev, "invert"))
> > --
> > 2.29.2
> >