linux-kernel - Re: [PATCH v5 2/7] pwm: pca9685: Support hardware readout

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Date:   Wed, 16 Dec 2020 23:00:59 -0500
From:   Sven Van Asbroeck <thesven73@...il.com>
To:     Clemens Gruber <clemens.gruber@...ruber.com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-pwm@...r.kernel.org
Subject: Re: [PATCH v5 2/7] pwm: pca9685: Support hardware readout

On Wed, Dec 16, 2020 at 7:53 AM Clemens Gruber
<clemens.gruber@...ruber.com> wrote:
>
> Implements .get_state to read-out the current hardware state.
>

I am not convinced that we actually need this.

Looking at the pwm core, .get_state() is only called right after .request(),
to initialize the cached value of the state. The core then uses the cached
value throughout, it'll never read out the h/w again, until the next .request().

In our case, we know that the state right after request is always disabled,
because:
- we disable all pwm channels on probe (in PATCH v5 4/7)
- .free() disables the pwm channel

Conclusion: .get_state() will always return "pwm disabled", so why do we
bother reading out the h/w?

Of course, if we choose to leave the pwm enabled after .free(), then
.get_state() can even be left out! Do we want that? Genuine question, I do
not know the answer.

> 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.
>
> Also 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>
> ---
>  drivers/pwm/pwm-pca9685.c | 41 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 41 insertions(+)
>
> diff --git a/drivers/pwm/pwm-pca9685.c b/drivers/pwm/pwm-pca9685.c
> index 1b5b5fb93b43..b3398963c0ff 100644
> --- a/drivers/pwm/pwm-pca9685.c
> +++ b/drivers/pwm/pwm-pca9685.c
> @@ -331,6 +331,46 @@ static int pca9685_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>         return 0;
>  }
>
> +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 long long duty;
> +       unsigned int val;
> +
> +       /* Calculate (chip-wide) period from prescale value */
> +       regmap_read(pca->regmap, PCA9685_PRESCALE, &val);
> +       state->period = (PCA9685_COUNTER_RANGE * 1000 / PCA9685_OSC_CLOCK_MHZ) *
> +                       (val + 1);
> +
> +       /* The (per-channel) polarity is fixed */
> +       state->polarity = PWM_POLARITY_NORMAL;
> +
> +       if (pwm->hwpwm >= PCA9685_MAXCHAN) {
> +               /*
> +                * The "all LEDs" channel does not support HW readout
> +                * Return 0 and disabled for backwards compatibility
> +                */
> +               state->duty_cycle = 0;
> +               state->enabled = false;
> +               return;
> +       }
> +
> +       duty = pca9685_pwm_get_duty(pca, pwm->hwpwm);
> +
> +       state->enabled = !!duty;
> +       if (!state->enabled) {
> +               state->duty_cycle = 0;
> +               return;
> +       } else if (duty == PCA9685_COUNTER_RANGE) {
> +               state->duty_cycle = state->period;
> +               return;
> +       }
> +
> +       duty *= state->period;
> +       state->duty_cycle = duty / PCA9685_COUNTER_RANGE;
> +}
> +
>  static int pca9685_pwm_request(struct pwm_chip *chip, struct pwm_device *pwm)
>  {
>         struct pca9685 *pca = to_pca(chip);
> @@ -353,6 +393,7 @@ static void pca9685_pwm_free(struct pwm_chip *chip, struct pwm_device *pwm)
>
>  static const struct pwm_ops pca9685_pwm_ops = {
>         .apply = pca9685_pwm_apply,
> +       .get_state = pca9685_pwm_get_state,
>         .request = pca9685_pwm_request,
>         .free = pca9685_pwm_free,
>         .owner = THIS_MODULE,
> --
> 2.29.2
>