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Date: Wed, 25 Nov 2020 09:35:08 +0100
From: Clemens Gruber <clemens.gruber@...ruber.com>
To: Sven Van Asbroeck <thesven73@...il.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 v3 1/4] pwm: pca9685: Switch to atomic API
On Tue, Nov 24, 2020 at 10:49:27PM -0500, Sven Van Asbroeck wrote:
> Hi Clemens, I checked the patch against the datasheet register definitions.
> More constructive feedback below.
>
> On Tue, Nov 24, 2020 at 1:10 PM Clemens Gruber
> <clemens.gruber@...ruber.com> wrote:
> >
> > The switch to the atomic API goes hand in hand with a few fixes to
> > previously experienced issues:
> > - The duty cycle is no longer lost after disable/enable (previously the
> > OFF registers were cleared in disable and the user was required to
> > call config to restore the duty cycle settings)
> > - If one sets a period resulting in the same prescale register value,
> > the sleep and write to the register is now skipped
> > - The prescale register is now set to the default value in probe. On
> > systems without CONFIG_PM, the chip is woken up at probe time.
> >
> > The hardware readout may return slightly different values than those
> > that were set in apply due to the limited range of possible prescale and
> > counter register values. If one channel is reconfigured with new duty
> > cycle and period, the others will keep the same relative duty cycle to
> > period ratio as they had before, even though the per-chip / global
> > frequency changed. (The PCA9685 has only one prescaler!)
> >
> > Note that although the datasheet mentions 200 Hz as default frequency
> > when using the internal 25 MHz oscillator, the calculated period from
> > the default prescaler register setting of 30 is 5079040ns.
> >
> > Signed-off-by: Clemens Gruber <clemens.gruber@...ruber.com>
> > ---
> > Changes since v2:
> > - Always set default prescale value in probe
> > - Simplified probe code
> > - Inlined functions with one callsite
> >
> > Changes since v1:
> > - Fixed a logic error
> > - Impoved PM runtime handling and fixed !CONFIG_PM
> > - Write default prescale reg value if invalid in probe
> > - Reuse full_off/_on functions throughout driver
> > - Use cached prescale value whenever possible
> >
> > drivers/pwm/pwm-pca9685.c | 251 +++++++++++++++++++-------------------
> > 1 file changed, 128 insertions(+), 123 deletions(-)
> >
> > diff --git a/drivers/pwm/pwm-pca9685.c b/drivers/pwm/pwm-pca9685.c
> > index 4a55dc18656c..4cfbf1467f15 100644
> > --- a/drivers/pwm/pwm-pca9685.c
> > +++ b/drivers/pwm/pwm-pca9685.c
> > @@ -47,11 +47,11 @@
> > #define PCA9685_ALL_LED_OFF_H 0xFD
> > #define PCA9685_PRESCALE 0xFE
> >
> > +#define PCA9685_PRESCALE_DEF 0x1E /* => default frequency of ~200 Hz */
> > #define PCA9685_PRESCALE_MIN 0x03 /* => max. frequency of 1526 Hz */
> > #define PCA9685_PRESCALE_MAX 0xFF /* => min. frequency of 24 Hz */
> >
> > #define PCA9685_COUNTER_RANGE 4096
> > -#define PCA9685_DEFAULT_PERIOD 5000000 /* Default period_ns = 1/200 Hz */
> > #define PCA9685_OSC_CLOCK_MHZ 25 /* Internal oscillator with 25 MHz */
> >
> > #define PCA9685_NUMREGS 0xFF
> > @@ -74,7 +74,7 @@
> > struct pca9685 {
> > struct pwm_chip chip;
> > struct regmap *regmap;
> > - int period_ns;
> > + int prescale;
> > #if IS_ENABLED(CONFIG_GPIOLIB)
> > struct mutex lock;
> > struct gpio_chip gpio;
> > @@ -87,6 +87,54 @@ static inline struct pca9685 *to_pca(struct pwm_chip *chip)
> > return container_of(chip, struct pca9685, chip);
> > }
> >
> > +static void pca9685_pwm_full_off(struct pca9685 *pca, int index)
> > +{
> > + int reg;
> > +
> > + /*
> > + * Set the full OFF bit to cause the PWM channel to be always off.
> > + * The full OFF bit has precedence over the other register values.
> > + */
> > +
> > + if (index >= PCA9685_MAXCHAN)
> > + reg = PCA9685_ALL_LED_OFF_H;
> > + else
> > + reg = LED_N_OFF_H(index);
> > +
> > + regmap_write(pca->regmap, reg, LED_FULL);
> > +}
> > +
> > +static void pca9685_pwm_full_on(struct pca9685 *pca, int index)
> > +{
> > + int reg;
> > +
> > + /*
> > + * Clear the OFF registers (including the full OFF bit) and set
> > + * the full ON bit to cause the PWM channel to be always on.
> > + */
> > +
> > + if (index >= PCA9685_MAXCHAN)
> > + reg = PCA9685_ALL_LED_OFF_L;
> > + else
> > + reg = LED_N_OFF_L(index);
> > +
> > + regmap_write(pca->regmap, reg, 0);
> > +
> > + if (index >= PCA9685_MAXCHAN)
> > + reg = PCA9685_ALL_LED_OFF_H;
> > + else
> > + reg = LED_N_OFF_H(index);
> > +
> > + regmap_write(pca->regmap, reg, 0);
> > +
> > + if (index >= PCA9685_MAXCHAN)
> > + reg = PCA9685_ALL_LED_ON_H;
> > + else
> > + reg = LED_N_ON_H(index);
> > +
> > + regmap_write(pca->regmap, reg, LED_FULL);
> > +}
> > +
> > #if IS_ENABLED(CONFIG_GPIOLIB)
> > static bool pca9685_pwm_test_and_set_inuse(struct pca9685 *pca, int pwm_idx)
> > {
> > @@ -141,31 +189,27 @@ static int pca9685_pwm_gpio_get(struct gpio_chip *gpio, unsigned int offset)
> > struct pwm_device *pwm = &pca->chip.pwms[offset];
> > unsigned int value;
> >
> > - regmap_read(pca->regmap, LED_N_ON_H(pwm->hwpwm), &value);
> > + regmap_read(pca->regmap, LED_N_OFF_H(pwm->hwpwm), &value);
> >
> > - return value & LED_FULL;
> > + return !(value & LED_FULL);
> > }
> >
> > static void pca9685_pwm_gpio_set(struct gpio_chip *gpio, unsigned int offset,
> > int value)
> > {
> > struct pca9685 *pca = gpiochip_get_data(gpio);
> > - struct pwm_device *pwm = &pca->chip.pwms[offset];
> > - unsigned int on = value ? LED_FULL : 0;
> > -
> > - /* Clear both OFF registers */
> > - regmap_write(pca->regmap, LED_N_OFF_L(pwm->hwpwm), 0);
> > - regmap_write(pca->regmap, LED_N_OFF_H(pwm->hwpwm), 0);
> >
> > - /* Set the full ON bit */
> > - regmap_write(pca->regmap, LED_N_ON_H(pwm->hwpwm), on);
> > + if (value)
> > + pca9685_pwm_full_on(pca, offset);
> > + else
> > + pca9685_pwm_full_off(pca, offset);
> > }
> >
> > static void pca9685_pwm_gpio_free(struct gpio_chip *gpio, unsigned int offset)
> > {
> > struct pca9685 *pca = gpiochip_get_data(gpio);
> >
> > - pca9685_pwm_gpio_set(gpio, offset, 0);
> > + pca9685_pwm_full_off(pca, offset);
> > pm_runtime_put(pca->chip.dev);
> > pca9685_pwm_clear_inuse(pca, offset);
> > }
> > @@ -246,18 +290,53 @@ static void pca9685_set_sleep_mode(struct pca9685 *pca, bool enable)
> > }
> > }
> >
> > -static int pca9685_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
> > - int duty_ns, int period_ns)
> > +static void pca9685_pwm_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
> > + struct pwm_state *state)
> > +{
> > + struct pca9685 *pca = to_pca(chip);
> > + unsigned int val, duty;
> > + int reg;
> > +
> > + /* Calculate (chip-wide) period from cached prescale value */
> > + state->period = (PCA9685_COUNTER_RANGE * 1000 / PCA9685_OSC_CLOCK_MHZ) *
> > + (pca->prescale + 1);
> > +
> > + /* The (per-channel) polarity is fixed */
> > + state->polarity = PWM_POLARITY_NORMAL;
> > +
> > + /* Read out current duty cycle and enabled state */
> > + reg = pwm->hwpwm >= PCA9685_MAXCHAN ? PCA9685_ALL_LED_OFF_H :
> > + LED_N_OFF_H(pwm->hwpwm);
> > + regmap_read(pca->regmap, reg, &val);
>
> The datasheet I found for this chip indicates that every ALL_LED_XXX register
> is write-only. Those registers cannot be read back from the chip.
>
> Datasheet "Rev. 4 - 16 April 2015"
Thanks, good catch! Would you agree that we should just return 0 duty
cycle and disabled state if the "all LEDs" channel is used?
>
> > + duty = (val & 0xf) << 8;
> > +
> > + state->enabled = !(val & LED_FULL);
> > +
> > + reg = pwm->hwpwm >= PCA9685_MAXCHAN ? PCA9685_ALL_LED_OFF_L :
> > + LED_N_OFF_L(pwm->hwpwm);
> > + regmap_read(pca->regmap, reg, &val);
> > + duty |= (val & 0xff);
> > +
> > + if (duty < PCA9685_COUNTER_RANGE) {
>
> How can duty >= 4096 ?
>
> > + duty *= state->period;
> > + state->duty_cycle = duty / (PCA9685_COUNTER_RANGE - 1);
>
> is this calculation correct?
> imagine led_on = 0 (default), and led_off = 4095
> then the led is off for one single tick per cycle
> but the above formula would calculate it as full on (period == duty_cycle)?
>
> > + } else
> > + state->duty_cycle = 0;
>
> what if the full on bit is set. would this function return the correct
> duty cycle?
No.. Sorry for not noticing this myself earlier!
>
>
> > +}
> > +
> > +static int pca9685_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> > + const struct pwm_state *state)
> > {
> > struct pca9685 *pca = to_pca(chip);
> > - unsigned long long duty;
> > + unsigned long long duty, prescale;
> > unsigned int reg;
> > - int prescale;
> >
> > - if (period_ns != pca->period_ns) {
> > - prescale = DIV_ROUND_CLOSEST(PCA9685_OSC_CLOCK_MHZ * period_ns,
> > - PCA9685_COUNTER_RANGE * 1000) - 1;
> > + if (state->polarity != PWM_POLARITY_NORMAL)
> > + return -EOPNOTSUPP;
> >
> > + prescale = DIV_ROUND_CLOSEST_ULL(PCA9685_OSC_CLOCK_MHZ * state->period,
> > + PCA9685_COUNTER_RANGE * 1000) - 1;
> > + if (prescale != pca->prescale) {
> > if (prescale >= PCA9685_PRESCALE_MIN &&
> > prescale <= PCA9685_PRESCALE_MAX) {
> > /*
> > @@ -270,12 +349,12 @@ static int pca9685_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
> > pca9685_set_sleep_mode(pca, true);
> >
> > /* Change the chip-wide output frequency */
> > - regmap_write(pca->regmap, PCA9685_PRESCALE, prescale);
> > + regmap_write(pca->regmap, PCA9685_PRESCALE, (int)prescale);
> >
> > /* Wake the chip up */
> > pca9685_set_sleep_mode(pca, false);
> >
> > - pca->period_ns = period_ns;
> > + pca->prescale = (int)prescale;
> > } else {
> > dev_err(chip->dev,
> > "prescaler not set: period out of bounds!\n");
> > @@ -283,46 +362,18 @@ static int pca9685_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
> > }
> > }
> >
> > - if (duty_ns < 1) {
> > - if (pwm->hwpwm >= PCA9685_MAXCHAN)
> > - reg = PCA9685_ALL_LED_OFF_H;
> > - else
> > - reg = LED_N_OFF_H(pwm->hwpwm);
> > -
> > - regmap_write(pca->regmap, reg, LED_FULL);
> > -
> > + if (!state->enabled || state->duty_cycle < 1) {
> > + pca9685_pwm_full_off(pca, pwm->hwpwm);
> > return 0;
> > }
> >
> > - if (duty_ns == period_ns) {
> > - /* Clear both OFF registers */
> > - if (pwm->hwpwm >= PCA9685_MAXCHAN)
> > - reg = PCA9685_ALL_LED_OFF_L;
> > - else
> > - reg = LED_N_OFF_L(pwm->hwpwm);
> > -
> > - regmap_write(pca->regmap, reg, 0x0);
> > -
> > - if (pwm->hwpwm >= PCA9685_MAXCHAN)
> > - reg = PCA9685_ALL_LED_OFF_H;
> > - else
> > - reg = LED_N_OFF_H(pwm->hwpwm);
> > -
> > - regmap_write(pca->regmap, reg, 0x0);
> > -
> > - /* Set the full ON bit */
> > - if (pwm->hwpwm >= PCA9685_MAXCHAN)
> > - reg = PCA9685_ALL_LED_ON_H;
> > - else
> > - reg = LED_N_ON_H(pwm->hwpwm);
> > -
> > - regmap_write(pca->regmap, reg, LED_FULL);
> > -
> > + if (state->duty_cycle == state->period) {
> > + pca9685_pwm_full_on(pca, pwm->hwpwm);
> > return 0;
> > }
> >
> > - duty = PCA9685_COUNTER_RANGE * (unsigned long long)duty_ns;
> > - duty = DIV_ROUND_UP_ULL(duty, period_ns);
> > + duty = (PCA9685_COUNTER_RANGE - 1) * state->duty_cycle;
> > + duty = DIV_ROUND_UP_ULL(duty, state->period);
>
> is there a rounding issue here?
> imagine period = 10000, duty_cycle = 9999
> then the above formula says duty = 4095/4096
> but in reality the requested duty is much closer to full on!
> same issue in reverse when period = 10000, duty_cycle = 1
> suggestion: do a DIV_ROUND_CLOSEST to calculate the closest duty,
> then check against 0 and 4096 to see if full off / full on.
> this also prevents led_off and led_on to be programmed with the same
> value because of subtle rounding errors (not allowed as per datasheet)
Yes that would be more accurate, good suggestion!
>
> >
> > if (pwm->hwpwm >= PCA9685_MAXCHAN)
> > reg = PCA9685_ALL_LED_OFF_L;
> > @@ -349,64 +400,6 @@ static int pca9685_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
> > return 0;
> > }
> >
> > -static int pca9685_pwm_enable(struct pwm_chip *chip, struct pwm_device *pwm)
> > -{
> > - struct pca9685 *pca = to_pca(chip);
> > - unsigned int reg;
> > -
> > - /*
> > - * The PWM subsystem does not support a pre-delay.
> > - * So, set the ON-timeout to 0
> > - */
> > - if (pwm->hwpwm >= PCA9685_MAXCHAN)
> > - reg = PCA9685_ALL_LED_ON_L;
> > - else
> > - reg = LED_N_ON_L(pwm->hwpwm);
> > -
> > - regmap_write(pca->regmap, reg, 0);
> > -
> > - if (pwm->hwpwm >= PCA9685_MAXCHAN)
> > - reg = PCA9685_ALL_LED_ON_H;
> > - else
> > - reg = LED_N_ON_H(pwm->hwpwm);
> > -
> > - regmap_write(pca->regmap, reg, 0);
> > -
> > - /*
> > - * Clear the full-off bit.
> > - * It has precedence over the others and must be off.
> > - */
> > - if (pwm->hwpwm >= PCA9685_MAXCHAN)
> > - reg = PCA9685_ALL_LED_OFF_H;
> > - else
> > - reg = LED_N_OFF_H(pwm->hwpwm);
> > -
> > - regmap_update_bits(pca->regmap, reg, LED_FULL, 0x0);
> > -
> > - return 0;
> > -}
> > -
> > -static void pca9685_pwm_disable(struct pwm_chip *chip, struct pwm_device *pwm)
> > -{
> > - struct pca9685 *pca = to_pca(chip);
> > - unsigned int reg;
> > -
> > - if (pwm->hwpwm >= PCA9685_MAXCHAN)
> > - reg = PCA9685_ALL_LED_OFF_H;
> > - else
> > - reg = LED_N_OFF_H(pwm->hwpwm);
> > -
> > - regmap_write(pca->regmap, reg, LED_FULL);
> > -
> > - /* Clear the LED_OFF counter. */
> > - if (pwm->hwpwm >= PCA9685_MAXCHAN)
> > - reg = PCA9685_ALL_LED_OFF_L;
> > - else
> > - reg = LED_N_OFF_L(pwm->hwpwm);
> > -
> > - regmap_write(pca->regmap, reg, 0x0);
> > -}
> > -
> > static int pca9685_pwm_request(struct pwm_chip *chip, struct pwm_device *pwm)
> > {
> > struct pca9685 *pca = to_pca(chip);
> > @@ -422,15 +415,14 @@ static void pca9685_pwm_free(struct pwm_chip *chip, struct pwm_device *pwm)
> > {
> > struct pca9685 *pca = to_pca(chip);
> >
> > - pca9685_pwm_disable(chip, pwm);
> > + pca9685_pwm_full_off(pca, pwm->hwpwm);
> > pm_runtime_put(chip->dev);
> > pca9685_pwm_clear_inuse(pca, pwm->hwpwm);
> > }
> >
> > static const struct pwm_ops pca9685_pwm_ops = {
> > - .enable = pca9685_pwm_enable,
> > - .disable = pca9685_pwm_disable,
> > - .config = pca9685_pwm_config,
> > + .get_state = pca9685_pwm_get_state,
> > + .apply = pca9685_pwm_apply,
> > .request = pca9685_pwm_request,
> > .free = pca9685_pwm_free,
> > .owner = THIS_MODULE,
> > @@ -461,7 +453,6 @@ static int pca9685_pwm_probe(struct i2c_client *client,
> > ret);
> > return ret;
> > }
> > - pca->period_ns = PCA9685_DEFAULT_PERIOD;
> >
> > i2c_set_clientdata(client, pca);
> >
> > @@ -505,14 +496,21 @@ static int pca9685_pwm_probe(struct i2c_client *client,
> > return ret;
> > }
> >
> > - /* The chip comes out of power-up in the active state */
> > - pm_runtime_set_active(&client->dev);
> > /*
> > - * Enable will put the chip into suspend, which is what we
> > - * want as all outputs are disabled at this point
> > + * Always initialize with default prescale, but chip must be
> > + * in sleep mode while changing prescaler.
> > */
> > + pca9685_set_sleep_mode(pca, true);
> > + pca->prescale = PCA9685_PRESCALE_DEF;
> > + regmap_write(pca->regmap, PCA9685_PRESCALE, pca->prescale);
> > + pm_runtime_set_suspended(&client->dev);
> > pm_runtime_enable(&client->dev);
> >
> > + if (!IS_ENABLED(CONFIG_PM)) {
> > + /* Wake the chip up on non-PM environments */
> > + pca9685_set_sleep_mode(pca, false);
> > + }
> > +
> > return 0;
> > }
> >
> > @@ -524,7 +522,14 @@ static int pca9685_pwm_remove(struct i2c_client *client)
> > ret = pwmchip_remove(&pca->chip);
> > if (ret)
> > return ret;
> > +
> > pm_runtime_disable(&client->dev);
> > +
> > + if (!IS_ENABLED(CONFIG_PM)) {
> > + /* Put chip in sleep state on non-PM environments */
> > + pca9685_set_sleep_mode(pca, true);
> > + }
> > +
> > return 0;
> > }
> >
> > --
> > 2.29.2
> >
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