linux-kernel - Re: [PATCH v8 6/8] pwm: pca9685: Support new PWM_USAGE

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Message-ID: <20210413103412.ngvtk5cw2ftyjvob@pengutronix.de>
Date:   Tue, 13 Apr 2021 12:34:12 +0200
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Clemens Gruber <clemens.gruber@...ruber.com>
Cc:     linux-pwm@...r.kernel.org,
        Thierry Reding <thierry.reding@...il.com>,
        Sven Van Asbroeck <TheSven73@...il.com>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 6/8] pwm: pca9685: Support new PWM_USAGE_POWER flag

Hi Clemens,

On Mon, Apr 12, 2021 at 07:11:58PM +0200, Clemens Gruber wrote:
> On Mon, Apr 12, 2021 at 06:30:45PM +0200, Uwe Kleine-König wrote:
> > On Mon, Apr 12, 2021 at 03:27:43PM +0200, Clemens Gruber wrote:
> > >  static unsigned int pca9685_pwm_get_duty(struct pca9685 *pca, int channel)
> > >  {
> > > -	unsigned int off_h = 0, val = 0;
> > > +	struct pwm_device *pwm = &pca->chip.pwms[channel];
> > > +	unsigned int off = 0, on = 0, val = 0;
> > >  
> > >  	if (WARN_ON(channel >= PCA9685_MAXCHAN)) {
> > >  		/* HW does not support reading state of "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;
> > >  	}
> > >  
> > > -	val = 0;
> > >  	regmap_read(pca->regmap, LED_N_OFF_L(channel), &val);
> > > -	return ((off_h & 0xf) << 8) | (val & 0xff);
> > > +	off = ((off & 0xf) << 8) | (val & 0xff);
> > > +	if (!pwm->args.usage_power)
> > > +		return off;
> > > +
> > > +	/* Read ON register to calculate duty cycle of staggered output */
> > > +	val = 0;
> > > +	regmap_read(pca->regmap, LED_N_ON_L(channel), &val);
> > > +	on = ((on & 0xf) << 8) | (val & 0xff);
> > > +	return (off - on) & (PCA9685_COUNTER_RANGE - 1);
> > 
> > If LED_N_ON is != 0 but usage_power is false, the returned state is
> > bogus.
> 
> If usage_power is false, LED_N_ON is guaranteed to be 0. It is reset to
> 0 in probe and never changed. Or did I miss something?

Ah right, so my concern is only a challenge once the reset in probe goes
away.

> > >  }
> > >  
> > >  #if IS_ENABLED(CONFIG_GPIOLIB)
> > > @@ -439,9 +469,11 @@ static int pca9685_pwm_probe(struct i2c_client *client,
> > >  	reg &= ~(MODE1_ALLCALL | MODE1_SUB1 | MODE1_SUB2 | MODE1_SUB3);
> > >  	regmap_write(pca->regmap, PCA9685_MODE1, reg);
> > >  
> > > -	/* Reset OFF registers to POR default */
> > > +	/* Reset OFF/ON registers to POR default */
> > >  	regmap_write(pca->regmap, PCA9685_ALL_LED_OFF_L, LED_FULL);
> > >  	regmap_write(pca->regmap, PCA9685_ALL_LED_OFF_H, LED_FULL);
> > > +	regmap_write(pca->regmap, PCA9685_ALL_LED_ON_L, 0);
> > > +	regmap_write(pca->regmap, PCA9685_ALL_LED_ON_H, 0);
> > >  
> > >  	pca->chip.ops = &pca9685_pwm_ops;
> > >  	/* Add an extra channel for ALL_LED */
> > > @@ -450,6 +482,9 @@ static int pca9685_pwm_probe(struct i2c_client *client,
> > >  	pca->chip.dev = &client->dev;
> > >  	pca->chip.base = -1;
> > >  
> > > +	pca->chip.of_xlate = of_pwm_xlate_with_flags;
> > > +	pca->chip.of_pwm_n_cells = 3;
> > > +
> > 
> > Huh, you're incompatibly changing the device tree binding here.
> 
> No, I don't think so:
> 
> The third cell is optional with of_pwm_xlate_with_flags.
> So previous DTs with pwm-cells = <2> will still work.

I thought that .of_pwm_n_cells was enforced, let me check the code ... I
had in mind that of_pwm_get() enforced that, but I cannot find it, so I
guess you're right and my concern is unjustified.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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