[1/1] dt-bindings: iio: adc: add missing vref-supply

Although this property is used right now for IIO_CHAN_INFO_SCALE,
this ADC has two internal reference voltages, which the driver currently
doesn't make use of.

Fixes: db73419d8c06 ("dt-bindings: iio: adc: Add binding documentation for NXP IMX8QXP ADC")
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
---
 .../devicetree/bindings/iio/adc/nxp,imx8qxp-adc.yaml          | 4 ++++
 1 file changed, 4 insertions(+)

On Wed, 1 Feb 2023 13:13:43 +0100
Marco Felsch <m.felsch@pengutronix.de> wrote:

> Hi Alexander,
> 
> On 23-01-31, Alexander Stein wrote:
> > Hi Marco,
> > 
> > thanks for the feedback.
> > 
> > Am Dienstag, 31. Januar 2023, 11:48:16 CET schrieb Marco Felsch:  
> > > Hi Alexander,
> > > 
> > > On 23-01-31, Alexander Stein wrote:  
> > > > Although this property is used right now for IIO_CHAN_INFO_SCALE,
> > > > this ADC has two internal reference voltages, which the driver currently
> > > > doesn't make use of.
> > > > 
> > > > Fixes: db73419d8c06 ("dt-bindings: iio: adc: Add binding documentation for
> > > > NXP IMX8QXP ADC") Signed-off-by: Alexander Stein
> > > > <alexander.stein@ew.tq-group.com>
> > > > ---
> > > > 
> > > >  .../devicetree/bindings/iio/adc/nxp,imx8qxp-adc.yaml          | 4 ++++
> > > >  1 file changed, 4 insertions(+)
> > > > 
> > > > diff --git
> > > > a/Documentation/devicetree/bindings/iio/adc/nxp,imx8qxp-adc.yaml
> > > > b/Documentation/devicetree/bindings/iio/adc/nxp,imx8qxp-adc.yaml index
> > > > 63369ba388e4..879768af0303 100644
> > > > --- a/Documentation/devicetree/bindings/iio/adc/nxp,imx8qxp-adc.yaml
> > > > +++ b/Documentation/devicetree/bindings/iio/adc/nxp,imx8qxp-adc.yaml
> > > > 
> > > > @@ -39,6 +39,9 @@ properties:
> > > >    power-domains:
> > > >      maxItems: 1
> > > > 
> > > > +  vref-supply:
> > > > +    description: External ADC reference voltage supply on VREFH pad.  
> > > 
> > > Please add it to the list of required properties, we can remove it as
> > > soon as the driver has support for the internal reference voltages.  
> > 
> > I was thinking in doing so before as well. But DT describes the hardware, and 
> > this ADC apparently would be functioning without a reference voltage on that 
> > pad, using a different one. What the driver actual does is a different matter.  
> 
> I have also thought about it first but than I checked the RM which says
> that "multi-reference selection" is chip dependent.

Oh goody. So is it detectable?

If we are going to stick to a single compatible rather than adding them for
the variants with and without this feature, should probably add a note at least
to say it is required for some parts.

Also, link if public would be good for purposes of discussion as my google
fu didn't find relevant doc. (assuming it's public)


Jonathan


> 
> Regards,
>   Marco
> 
> 
> > 
> > Best regards,
> > Alexander
> >   
> > > Regards,
> > >   Marco
> > >   
> > > > +
> > > > 
> > > >    "#io-channel-cells":
> > > >      const: 1
> > > > 
> > > > @@ -72,6 +75,7 @@ examples:
> > > >              assigned-clocks = <&clk IMX_SC_R_ADC_0>;
> > > >              assigned-clock-rates = <24000000>;
> > > >              power-domains = <&pd IMX_SC_R_ADC_0>;
> > > > 
> > > > +            vref-supply = <&reg_1v8>;
> > > > 
> > > >              #io-channel-cells = <1>;
> > > >          
> > > >          };
> > > >      
> > > >      };  
> > 
> > 
> > 
> > 
> >

Hi,

On 23-02-03, Alexander Stein wrote:

...

> > > > > > +  vref-supply:
> > > > > > +    description: External ADC reference voltage supply on VREFH
> > > > > > pad.
> > > > > 
> > > > > Please add it to the list of required properties, we can remove it as
> > > > > soon as the driver has support for the internal reference voltages.
> > > > 
> > > > I was thinking in doing so before as well. But DT describes the
> > > > hardware, and this ADC apparently would be functioning without a
> > > > reference voltage on that pad, using a different one. What the driver
> > > > actual does is a different matter.> 
> > > I have also thought about it first but than I checked the RM which says
> > > that "multi-reference selection" is chip dependent.
> 
> Nice for pointing this out. I wasn't aware that there are differences.
> 
> > Oh goody. So is it detectable?
> 
> That's my problem. I didn't find any source of information which chips do 
> support multiple references and which don't.
> Marco, do you have some information on this?

You can download the RM from the NXP website but you need an account for
it:
https://www.nxp.com/products/processors-and-microcontrollers/arm-processors/i-mx-applications-processors/i-mx-8-applications-processors/i-mx-8-family-arm-cortex-a53-cortex-a72-virtualization-vision-3d-graphics-4k-video:i.MX8

Or is this the wrong model? The naming scheme is quite confusing to me.

> > If we are going to stick to a single compatible rather than adding them for
> > the variants with and without this feature, should probably add a note at
> > least to say it is required for some parts.
> 
> That's a good idea. I'm okay with that, until there is more information 
> available.

According the RM there is a bit which can be read: Multi Vref
Implemented (MVI).

Regards,
  Marco