Messages in this thread | | | From | Sven Van Asbroeck <> | Date | Wed, 16 Dec 2020 23:02:57 -0500 | Subject | Re: [PATCH v5 5/7] pwm: pca9685: Support staggered output ON times |
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Hi Clemens, see below.
On Wed, Dec 16, 2020 at 7:53 AM Clemens Gruber <clemens.gruber@pqgruber.com> 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@pqgruber.com> > --- > 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 &= ~(MODE1_ALLCALL | MODE1_SUB1 | MODE1_SUB2 | MODE1_SUB3); > -- > 2.29.2 >
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