From: Clemens Gruber <clemens.gruber@pqgruber.com>
To: Sven Van Asbroeck <thesven73@gmail.com>
Cc: Thierry Reding <thierry.reding@gmail.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-pwm@vger.kernel.org, u.kleine-koenig@pengutronix.de
Subject: Re: [PATCH v5 2/7] pwm: pca9685: Support hardware readout
Date: Thu, 17 Dec 2020 18:43:04 +0100 [thread overview]
Message-ID: <X9uYqGboZg5DuEtf@workstation.tuxnet> (raw)
In-Reply-To: <CAGngYiWkKZGkQ4TTTy8bQYvnGBK45V0A0JCe_+M5V+vuVU+zkQ@mail.gmail.com>
On Wed, Dec 16, 2020 at 11:00:59PM -0500, Sven Van Asbroeck wrote:
> On Wed, Dec 16, 2020 at 7:53 AM Clemens Gruber
> <clemens.gruber@pqgruber.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?
If there are no plans for the PWM core to call .get_state more often in
the future, we could just read out the period and return 0 duty and
disabled.
Thierry, Uwe, what's your take on this?
> 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.
I do not think we should leave it enabled after free. It is less
complicated if we know that unrequested channels are not in use.
>
> > 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@pqgruber.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
> >
next prev parent reply other threads:[~2020-12-17 17:43 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20201216125320.5277-1-clemens.gruber@pqgruber.com>
[not found] ` <20201216125320.5277-2-clemens.gruber@pqgruber.com>
2020-12-17 4:00 ` [PATCH v5 2/7] pwm: pca9685: Support hardware readout Sven Van Asbroeck
2020-12-17 17:43 ` Clemens Gruber [this message]
2020-12-17 17:52 ` Sven Van Asbroeck
2021-01-03 17:04 ` Clemens Gruber
2021-01-07 14:18 ` Sven Van Asbroeck
2021-01-11 20:43 ` Uwe Kleine-König
2021-03-22 8:34 ` Thierry Reding
2021-03-31 10:25 ` Uwe Kleine-König
2021-03-31 15:52 ` Thierry Reding
2021-04-06 6:33 ` Uwe Kleine-König
2021-04-06 13:47 ` Thierry Reding
2021-04-06 20:44 ` Uwe Kleine-König
2021-03-22 8:15 ` Thierry Reding
2021-01-11 20:35 ` Uwe Kleine-König
2021-01-14 17:16 ` Clemens Gruber
2021-01-14 18:05 ` Uwe Kleine-König
2021-03-22 8:53 ` Thierry Reding
[not found] ` <CAGngYiW=KhCOZX3tPMFykXzpWLpj3qusN2OXVPSfHLRcyts+wA@mail.gmail.com>
2021-01-29 16:31 ` Clemens Gruber
2021-01-29 18:05 ` Sven Van Asbroeck
2021-01-29 20:37 ` Clemens Gruber
2021-01-29 21:24 ` Sven Van Asbroeck
2021-01-29 22:16 ` Sven Van Asbroeck
2021-02-01 17:24 ` Clemens Gruber
2021-03-01 21:52 ` Uwe Kleine-König
2021-03-04 13:22 ` Clemens Gruber
2021-02-14 14:46 ` Clemens Gruber
2021-03-22 9:19 ` Thierry Reding
[not found] ` <CAHp75Ve2FFEMsAv8S18bUDFsH2UkiQ5UvgcRtZ=j30syQtEirw@mail.gmail.com>
2021-03-22 11:22 ` Uwe Kleine-König
2021-03-22 11:40 ` Andy Shevchenko
2021-03-22 11:48 ` Uwe Kleine-König
2021-03-22 12:15 ` Andy Shevchenko
2021-03-22 13:25 ` Uwe Kleine-König
2021-03-27 16:05 ` Clemens Gruber
2021-03-22 9:14 ` Thierry Reding
2021-03-22 8:47 ` Thierry Reding
2020-12-15 21:22 [PATCH v5 1/7] pwm: pca9685: Switch to atomic API Clemens Gruber
2020-12-15 21:22 ` [PATCH v5 2/7] pwm: pca9685: Support hardware readout Clemens Gruber
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