linux-kernel - Re: (EXT) Re: [PATCH 3/4] pwm: pca9685: initialize all LED registers during probe

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Date:   Wed, 26 Feb 2020 17:13:51 +0100
From:   Matthias Schiffer <matthias.schiffer@...tq-group.com>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     thierry.reding@...il.com, linux-pwm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: (EXT) Re: [PATCH 3/4] pwm: pca9685: initialize all LED
 registers during probe

On Wed, 2020-02-26 at 16:00 +0100, Uwe Kleine-König wrote:
> On Wed, Feb 26, 2020 at 02:52:28PM +0100, Matthias Schiffer wrote:
> > Initialize all ON delays to 0 during probe, rather than doing it in
> > pca9685_pwm_enable.
> > 
> > Signed-off-by: Matthias Schiffer <matthias.schiffer@...tq-group.com
> > >
> > ---
> >  drivers/pwm/pwm-pca9685.c | 14 +++++++-------
> >  1 file changed, 7 insertions(+), 7 deletions(-)
> > 
> > diff --git a/drivers/pwm/pwm-pca9685.c b/drivers/pwm/pwm-pca9685.c
> > index 393ab92aa945..370691b21107 100644
> > --- a/drivers/pwm/pwm-pca9685.c
> > +++ b/drivers/pwm/pwm-pca9685.c
> > @@ -289,13 +289,6 @@ static int pca9685_pwm_enable(struct pwm_chip
> > *chip, struct pwm_device *pwm)
> >  {
> >  	struct pca9685 *pca = to_pca(chip);
> >  
> > -	/*
> > -	 * The PWM subsystem does not support a pre-delay.
> > -	 * So, set the ON-timeout to 0
> > -	 */
> > -	regmap_write(pca->regmap, LED_N_ON_L(pwm->hwpwm), 0);
> > -	regmap_write(pca->regmap, LED_N_ON_H(pwm->hwpwm), 0);
> > -
> >  	/*
> >  	 * Clear the full-off bit.
> >  	 * It has precedence over the others and must be off.
> > @@ -388,6 +381,13 @@ static int pca9685_pwm_probe(struct i2c_client
> > *client,
> >  	regmap_write(pca->regmap, PCA9685_ALL_LED_OFF_L, 0);
> >  	regmap_write(pca->regmap, PCA9685_ALL_LED_OFF_H, 0);
> >  
> > +	/*
> > +	 * The PWM subsystem does not support a pre-delay.
> > +	 * So, set the ON-timeout to 0
> > +	 */
> > +	regmap_write(pca->regmap, PCA9685_ALL_LED_ON_H, 0);
> > +	regmap_write(pca->regmap, PCA9685_ALL_LED_ON_L, 0);
> > +
> 
> What is a pre-delay: Something like:
>           _________                   ______
>     _____/         \_________________/
>     ^                           ^
> 
> Where ^ marks the period start and then the time between period start
> and the rising signal is the pre-delay?
> 
> If so, the IMHO more right approach is to keep the pre-delay until a
> new
> setting is applied and in .get_state ignore the pre-delay. This way
> you
> don't modify the output in .probe() which sounds right.
> 

My approach was to get the hardware into a known state by resetting
most registers (which is what the driver attempted to do so far). If
getting the hardware state via get_state is preferable, I can implement
that instead.

Matthias


> Best regards
> Uwe
>