Re: [PATCH v5 5/7] pwm: pca9685: Support staggered output ON times

[Sven Van Asbroeck]

Hi Clemens, see below.

On Wed, Dec 16, 2020 at 7:53 AM Clemens Gruber
<clemens.gruber@xxxxxxxxxxxx> wrote:
>
> The PCA9685 supports staggered LED output ON times to minimize current
> surges and reduce EMI.
> When this new option is enabled, the ON times of each channel are
> delayed by channel number x counter range / 16, which avoids asserting
> all enabled outputs at the same counter value while still maintaining
> the configured duty cycle of each output.
>
> Signed-off-by: Clemens Gruber <clemens.gruber@xxxxxxxxxxxx>
> ---
>  drivers/pwm/pwm-pca9685.c | 62 +++++++++++++++++++++++++++++++--------
>  1 file changed, 50 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/pwm/pwm-pca9685.c b/drivers/pwm/pwm-pca9685.c
> index 38aadaf50996..ff916980de49 100644
> --- a/drivers/pwm/pwm-pca9685.c
> +++ b/drivers/pwm/pwm-pca9685.c
> @@ -79,6 +79,7 @@
>  struct pca9685 {
>         struct pwm_chip chip;
>         struct regmap *regmap;
> +       bool staggered_outputs;
>  #if IS_ENABLED(CONFIG_GPIOLIB)
>         struct mutex lock;
>         struct gpio_chip gpio;
> @@ -93,45 +94,79 @@ static inline struct pca9685 *to_pca(struct pwm_chip *chip)
>
>  static void pca9685_pwm_set_duty(struct pca9685 *pca, int channel, unsigned int duty)
>  {
> +       unsigned int on, off;
> +
>         if (duty == 0) {
>                 /* Set the full OFF bit, which has the highest precedence */
>                 regmap_write(pca->regmap, REG_OFF_H(channel), LED_FULL);
> +               return;
>         } else if (duty >= PCA9685_COUNTER_RANGE) {
>                 /* Set the full ON bit and clear the full OFF bit */
>                 regmap_write(pca->regmap, REG_ON_H(channel), LED_FULL);
>                 regmap_write(pca->regmap, REG_OFF_H(channel), 0);
> -       } else {
> -               /* Set OFF time (clears the full OFF bit) */
> -               regmap_write(pca->regmap, REG_OFF_L(channel), duty & 0xff);
> -               regmap_write(pca->regmap, REG_OFF_H(channel), (duty >> 8) & 0xf);
> -               /* Clear the full ON bit */
> -               regmap_write(pca->regmap, REG_ON_H(channel), 0);
> +               return;
> +       }
> +
> +       if (pca->staggered_outputs) {
> +               if (channel < PCA9685_MAXCHAN) {
> +                       /*
> +                        * To reduce EMI, the ON times of each channel are
> +                        * spread out evenly within the counter range, while
> +                        * still maintaining the configured duty cycle
> +                        */
> +                       on = channel * PCA9685_COUNTER_RANGE / PCA9685_MAXCHAN;
> +                       off = (on + duty) % PCA9685_COUNTER_RANGE;
> +                       regmap_write(pca->regmap, REG_ON_L(channel), on & 0xff);
> +                       regmap_write(pca->regmap, REG_ON_H(channel), (on >> 8) & 0xf);
> +                       regmap_write(pca->regmap, REG_OFF_L(channel), off & 0xff);
> +                       regmap_write(pca->regmap, REG_OFF_H(channel), (off >> 8) & 0xf);
> +                       return;
> +               }
> +               /* No staggering possible if "all LEDs" channel is used */
> +               regmap_write(pca->regmap, PCA9685_ALL_LED_ON_L, 0);
>         }
> +       /* Set OFF time (clears the full OFF bit) */
> +       regmap_write(pca->regmap, REG_OFF_L(channel), duty & 0xff);
> +       regmap_write(pca->regmap, REG_OFF_H(channel), (duty >> 8) & 0xf);
> +       /* Clear the full ON bit */
> +       regmap_write(pca->regmap, REG_ON_H(channel), 0);
>  }

I find the set_duty() function quite hard to follow.
Can we simplify this by eliminating the !staggered_outputs special case?
E.g. always calculate and write 'on' and 'off'.
But if !staggered_outputs => on = 0 and off = duty.

Yes this adds one extra/unnecessary register write in the !staggered case,
but simplicity/maintainability >>> efficiency here. And this "issue" will
disappear when we switch on regmap caching.

>
>  static unsigned int pca9685_pwm_get_duty(struct pca9685 *pca, int channel)
>  {
> -       unsigned int off_h, val;
> +       unsigned int off, on, val;
>
>         if (WARN_ON(channel >= PCA9685_MAXCHAN)) {
>                 /* Hardware readout not supported for "all LEDs" channel */
>                 return 0;
>         }
>
> -       regmap_read(pca->regmap, LED_N_OFF_H(channel), &off_h);
> -       if (off_h & LED_FULL) {
> +       regmap_read(pca->regmap, LED_N_OFF_H(channel), &off);
> +       if (off & LED_FULL) {
>                 /* Full OFF bit is set */
>                 return 0;
>         }
>
> -       regmap_read(pca->regmap, LED_N_ON_H(channel), &val);
> -       if (val & LED_FULL) {
> +       regmap_read(pca->regmap, LED_N_ON_H(channel), &on);
> +       if (on & LED_FULL) {
>                 /* Full ON bit is set */
>                 return PCA9685_COUNTER_RANGE;
>         }
>
>         regmap_read(pca->regmap, LED_N_OFF_L(channel), &val);
> -       return ((off_h & 0xf) << 8) | (val & 0xff);
> +       off = ((off & 0xf) << 8) | (val & 0xff);
> +
> +       if (pca->staggered_outputs) {
> +               regmap_read(pca->regmap, LED_N_ON_L(channel), &val);
> +               on = ((on & 0xf) << 8) | (val & 0xff);
> +
> +               if (off >= on)
> +                       return off - on;
> +               else
> +                       return off + PCA9685_COUNTER_RANGE - on;

I think the if/else is unnecessary. unsigned int is twos-complement,
so we can just write:

        return (off - on) & 0xfff; (or PCA9685_COUNTER_RANGE - 1)

and it will "magically" work even if off < on.


> +       }
> +
> +       return off;
>  }

As in set_duty(), consider removing the !staggered_outputs special case,
to make this function clearer and simpler.

>
>  #if IS_ENABLED(CONFIG_GPIOLIB)
> @@ -442,6 +477,9 @@ static int pca9685_pwm_probe(struct i2c_client *client,
>
>         regmap_write(pca->regmap, PCA9685_MODE2, reg);
>
> +       pca->staggered_outputs = device_property_read_bool(
> +               &client->dev, "nxp,staggered-outputs");
> +
>         /* Disable all LED ALLCALL and SUBx addresses to avoid bus collisions */
>         regmap_read(pca->regmap, PCA9685_MODE1, &reg);
>         reg &= ~(MODE1_ALLCALL | MODE1_SUB1 | MODE1_SUB2 | MODE1_SUB3);
> --
> 2.29.2
>