Re: [bug report] iio: adc: ad7949: add vref selection support

[Liam Beguin <liambeguin@gmail.com>]

Hi Dan,

On Wed, Sep 22, 2021 at 09:00:26AM +0300, Dan Carpenter wrote:
> On Wed, Sep 22, 2021 at 01:10:52AM -0400, Liam Beguin wrote:
> > > 
> > >     369         if (IS_ERR(ad7949_adc->vref)) {
> > >     370                 ret = PTR_ERR(ad7949_adc->vref);
> > >     371                 if (ret != -ENODEV)
> > >     372                         return ret;
> > >     373                 /* unbuffered? */
> > >     374                 ad7949_adc->vref = devm_regulator_get_optional(dev, "vref");
> > >     375                 if (IS_ERR(ad7949_adc->vref)) {
> > >     376                         ret = PTR_ERR(ad7949_adc->vref);
> > >     377                         if (ret != -ENODEV)
> > >     378                                 return ret;
> > > 
> > > Instead of returning NULL when the regulator is disabled it returns
> > > -ENODEV.  How do you differentiate from an -ENODEV which is caused by an
> > > error vs an -ENODEV which is caused because the optional regulator is
> > > disabled?  You'll just have to hope that the errors are less common and
> > > assume it means disabled.
> > 
> > I see.. So far, I've only used fixed-regulators to provide a constant
> > voltage reference here, and I guess those are quite unlikely to fail.
> > 
> > > You might be doubting that devm_regulator_get_optional() can return
> > > -ENODEV on error?  Look at the code and prepare your heart for sadness.
> > 
> > Thanks for the warning, I see what you meant now.
> > 
> > I wasn't able to use smatch to reproduce the error with the following:
> > 
> > 	make O=builds/smatch CHECK="~/dev/smatch/smatch -p=kernel" C=1 Image.gz
> > 
> > Would you have any pointer for that?
> 
> This requires building the cross function Database:
> 
> 	~/dev/smatch/smatch_scripts/build_kernel_data.sh
> 
> The command takes 5 hours to run so here is a short cut which just
> builds the minimum two files:
> 
> ~/dev/smatch/smatch_scripts/kchecker --info drivers/regulator/core.c | tee out
> ~/dev/smatch/smatch_data/db/create_db.sh -p=kernel out
> ~/dev/smatch/smatch_scripts/kchecker --info drivers/regulator/devres.c | tee out
> ~/dev/smatch/smatch_data/db/reload_partial.sh out
> ~/dev/smatch/smatch_scripts/kchecker --spammy drivers/iio/adc/ad7949.c

Thanks, I appreciate the shortcuts! I was able to reproduce the error
with these steps.

> > Anyway, I believe the following would address the error you mentioned.
> > 
> > -- >8 --
> > diff --git a/drivers/iio/adc/ad7949.c b/drivers/iio/adc/ad7949.c
> > index 44bb5fde83de..3613f4e55e1c 100644
> > --- a/drivers/iio/adc/ad7949.c
> > +++ b/drivers/iio/adc/ad7949.c
> > @@ -368,12 +368,14 @@ static int ad7949_spi_probe(struct spi_device *spi)
> >  	ad7949_adc->vref = devm_regulator_get_optional(dev, "vrefin");
> >  	if (IS_ERR(ad7949_adc->vref)) {
> >  		ret = PTR_ERR(ad7949_adc->vref);
> > +		ad7949_adc->vref = NULL;
> 
> This is not required because it will just be reassigned in a couple
> lines.

Right, sorry about that.

> >  		if (ret != -ENODEV)
> >  			return ret;
> >  		/* unbuffered? */
> >  		ad7949_adc->vref = devm_regulator_get_optional(dev, "vref");
> >  		if (IS_ERR(ad7949_adc->vref)) {
> >  			ret = PTR_ERR(ad7949_adc->vref);
> > +			ad7949_adc->vref = NULL;
> 
> But this also won't work.  Passing a NULL to regulator_enable() will
> cause an Oops.  All the reference to ->vref need checks.  :/

I believe it still work since these conditions around
devm_regulator_get_optional() also set ad7949_adc->refsel.

ad7949_adc->refsel is then checked before calling regulator_enable() and
regulator_get_voltage().

Even without the patch, I don't think we can call regulor_enable()
without having it be defined. Am I missing something else?

Thanks,
Liam

> >  			if (ret != -ENODEV)
> >  				return ret;
> >  		} else {
> 
> regards,
> dan carpenter
>