linux-kernel - Re: [PATCH] dt-bindings: iio: st-sensors: Add IIS2MDC magnetometer

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Date: Thu, 22 Feb 2024 09:47:17 +0100
From: Marco Felsch <m.felsch@...gutronix.de>
To: Conor Dooley <conor@...nel.org>
Cc: jic23@...nel.org, lars@...afoo.de, robh+dt@...nel.org,
	krzysztof.kozlowski+dt@...aro.org, conor+dt@...nel.org,
	denis.ciocca@...com, linus.walleij@...aro.org,
	linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, kernel@...gutronix.de
Subject: Re: [PATCH] dt-bindings: iio: st-sensors: Add IIS2MDC magnetometer

On 24-02-21, Conor Dooley wrote:
> On Wed, Feb 21, 2024 at 08:45:18PM +0100, Marco Felsch wrote:
> > Hi Conor,
> > 
> > On 24-02-21, Conor Dooley wrote:
> > > On Wed, Feb 21, 2024 at 08:16:44PM +0100, Marco Felsch wrote:
> > > > On 24-02-21, Conor Dooley wrote:
> > > > > On Wed, Feb 21, 2024 at 06:58:10PM +0100, Marco Felsch wrote:
> > > > > > Add the iis2mdc magnetometer support which is equivalent to the lis2mdl.
> > > > > > 
> > > > > > Signed-off-by: Marco Felsch <m.felsch@...gutronix.de>
> > > > > > ---
> > > > > >  Documentation/devicetree/bindings/iio/st,st-sensors.yaml | 1 +
> > > > > >  1 file changed, 1 insertion(+)
> > > > > > 
> > > > > > diff --git a/Documentation/devicetree/bindings/iio/st,st-sensors.yaml b/Documentation/devicetree/bindings/iio/st,st-sensors.yaml
> > > > > > index fff7e3d83a02..ee593c8bbb65 100644
> > > > > > --- a/Documentation/devicetree/bindings/iio/st,st-sensors.yaml
> > > > > > +++ b/Documentation/devicetree/bindings/iio/st,st-sensors.yaml
> > > > > > @@ -64,6 +64,7 @@ properties:
> > > > > >            - st,lsm9ds0-gyro
> > > > > >        - description: STMicroelectronics Magnetometers
> > > > > >          enum:
> > > > > > +          - st,iis2mdc
> > > > > 
> > > > > Without a fallback compatible to the equivilent device, how does a
> > > > > driver bind to this device?
> > > > 
> > > > I skimed the datasheets and the driver already handles this binding
> > > > exactly the same as the st,lis2mdl, so my assumption is they do match.
> > > > 
> > > > Why do I you think we need a fallback compatible here?
> > > 
> > > I didn't look at the driver, there was no mention of the driver already
> > > having (undocumented) support for it. Since there was no driver change
> > > alongside this patch, I thought you'd need a fallback compatible to
> > > allow the driver to match against a compatible it recognises.
> > 
> > I explicitly did not mention the driver in the commit message else I
> > would have got a response like "dt-bindings have no dependency to
> > drivers" ;)
> 
> Putting it under the --- line is always an option. Where there are
> existing users but the compatible is just undocumented, this it's
> helpful to do.
> 
> > > Besides, having fallback compatibles is the norm when one device has the
> > > same programming model as another.
> > 
> > Not for this binding according the driver.
> 
> If they don't have the same programming model, then describing them as
> "equivalent" wouldn't be correct. That said, they seem to use the same
> sensor settings when alls said and done (see st_magn_sensors_settings),
> so I think they are actually compatible even if the driver has separate
> match data for each.

I told you that I have checked the driver and skimed the datasheets and
came to the exact same conclusion.

Regards,
  Marco


> 
> Cheers,
> Conor.