Re: [PATCH] iio: light: vcnl4000: Don't power on/off chip in config

[Mårten Lindahl]

Hi Jonathan!

Thanks! I'll rephrase it and add the fixes tags.

Kind regards

Mårten

On 9/10/23 15:32, Jonathan Cameron wrote:
> On Thu, 7 Sep 2023 12:53:14 +0200
> Mårten Lindahl <marten.lindahl@xxxxxxxx> wrote:
>
> Hi Mårten,
>
> Agree with your reasoning etc (and I guess you've triggered this for real)
> so just a few patch description etc comments.
>
> > After enabling/disabling interrupts on the vcnl4040 chip the als and/or
> > ps sensor is powered on or off depending on the interrupt enable bits.
> > This is made as a last step in write_event_config.
> >
> > But there is no reason to do this as the runtime PM handles the power
> > state of the sensors. Interfering with this may impact sensor readings.
> I think the following example could be clearer. I haven't checked
> the naming of states in runtime pm, but a few things seem backwards to me.
>
> > Consider the following:
> >   1. Userspace makes sensor data reading which triggers 2000ms RPM resume
> >      (sensor powered on) timeout
> It triggers a timeout to do runtime suspend if no access in 2000ms, not resume.
>
> >   2. Userspace disables interrupts => powers sensor off
> >   3. Userspace reads sensor data = 0 because sensor is off and RPM didn't
> >      power on the sensor as resume timeout is still active
> suspend timeout hasn't yet run out, so the runtime pm subsystem thinks the
> device is still powered up and doesn't need resuming.
>
> >   4. RPM resume timeout passed
> suspend timeout.
>
> > Powering sensor off in (2) risks a time window of close to 2000ms where
> > sensor data readings are disabled as in (3).
> >
> > Skip setting power state when writing new event config.
> >
> > Signed-off-by: Mårten Lindahl <marten.lindahl@xxxxxxxx>
> Fixes tag - probably 2 of them. One for the recent change that added
> the || als_int part and one for wherever the bug originally came from
> with comments to say why there are two fixes tags.
>
> > ---
> >   drivers/iio/light/vcnl4000.c | 1 -
> >   1 file changed, 1 deletion(-)
> >
> > diff --git a/drivers/iio/light/vcnl4000.c b/drivers/iio/light/vcnl4000.c
> > index 3a52b09c2823..fdf763a04b0b 100644
> > --- a/drivers/iio/light/vcnl4000.c
> > +++ b/drivers/iio/light/vcnl4000.c
> > @@ -1513,7 +1513,6 @@ static int vcnl4040_write_event_config(struct iio_dev *indio_dev,
> >   
> >   out:
> >   	mutex_unlock(&data->vcnl4000_lock);
> > -	data->chip_spec->set_power_state(data, data->ps_int || data->als_int);
> This will need manual backporting as this line changed recently and I'm fairly
> sure the argument is equally valid for the older code.
>
> >   
> >   	return ret;
> >   }
> >
> > ---
> > base-commit: 7ba2090ca64ea1aa435744884124387db1fac70f
> > change-id: 20230907-vcnl4000-pm-fix-b58dc0dffb5c
> >
> > Best regards,