Re: [PATCH] pwm: pca9685: Fix misuse of regmap_update_bits

[Thierry Reding <thierry.reding@gmail.com>]

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On Mon, Dec 05, 2016 at 05:34:02PM +0100, Clemens Gruber wrote:
> On Tue, Nov 29, 2016 at 06:02:50PM +0100, Florian Vaussard wrote:
> > Using regmap_update_bits(..., mask, 1) with 'mask' following (1 << k)
> > and k greater than 0 is wrong. Indeed, _regmap_update_bits will perform
> > (mask & 1), which results in 0 if LSB of mask is 0. Thus the call
> > regmap_update_bits(..., mask, 1) is in reality equivalent to
> > regmap_update_bits(..., mask, 0).
> > 
> > In such a case, the correct use is regmap_update_bits(..., mask, mask).
> > 
> > This driver is performing such a mistake with the MODE1_RESTART mask,
> > which equals (1 << 6). Fix the driver to make it consistent with the
> > API. Please note that this change is untested, as I do not have this
> > piece of hardware. Testers are welcome!
> > 
> > Signed-off-by: Florian Vaussard <florian.vaussard@heig-vd.ch>
> > ---
> >  drivers/pwm/pwm-pca9685.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/pwm/pwm-pca9685.c b/drivers/pwm/pwm-pca9685.c
> > index 117fccf..6b9ff6c 100644
> > --- a/drivers/pwm/pwm-pca9685.c
> > +++ b/drivers/pwm/pwm-pca9685.c
> > @@ -124,7 +124,8 @@ static int pca9685_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
> >  			 */
> >  			if (duty_ns == pca->duty_ns) {
> >  				regmap_update_bits(pca->regmap, PCA9685_MODE1,
> > -						   MODE1_RESTART, 0x1);
> > +						   MODE1_RESTART,
> > +						   MODE1_RESTART);
> >  				return 0;
> >  			}
> >  		} else {
> > -- 
> > 2.5.5
> 
> Good catch!
> During testing your change however, I noticed that this whole
> conditional for duty_ns == pca->duty_ns (which I added) is bogus:
> Restarting the chip means using the same ON and OFF times as before, so
> the duty cycle "ratio" stays the same, relative to the period.
> Here we are checking for an equal duty cycle in nanoseconds though..
> 
> Instead we would have to check if the ratio changed and only if it did
> not, set the RESTART bit.
> 
> Or we could just remove that conditional. This is only an optimization
> for the special case of changing both period_ns and duty_ns at the same
> time but with the same ratio as before.

So what's the status on this?

Thierry

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