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Message-ID: <CAGngYiXCS-FwQkA7nzizXw_5417_u7VmPAXPbZeDv4dUFs2cog@mail.gmail.com>
Date: Wed, 16 Dec 2020 23:02:57 -0500
From: Sven Van Asbroeck <thesven73@...il.com>
To: Clemens Gruber <clemens.gruber@...ruber.com>
Cc: linux-pwm@...r.kernel.org,
Thierry Reding <thierry.reding@...il.com>,
Uwe Kleine-König
<u.kleine-koenig@...gutronix.de>, Lee Jones <lee.jones@...aro.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Mika Westerberg <mika.westerberg@...ux.intel.com>,
David Jander <david@...tonic.nl>
Subject: Re: [PATCH v5 5/7] pwm: pca9685: Support staggered output ON times
Hi Clemens, see below.
On Wed, Dec 16, 2020 at 7:53 AM Clemens Gruber
<clemens.gruber@...ruber.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@...ruber.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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