Re: [PATCH v2 2/4] dt-bindings: iio: ti,tmp117: add binding for the TMP116

[Jonathan Cameron <jic23@kernel.org>]

On Fri, 23 Dec 2022 16:03:38 +0100
Marco Felsch <m.felsch@pengutronix.de> wrote:

> On 22-12-23, Jonathan Cameron wrote:
> > On Wed, 21 Dec 2022 10:27:59 +0100
> > Marco Felsch <m.felsch@pengutronix.de> wrote:
> >   
> > > The TMP116 is the predecessor of the TMP117.
> > > 
> > > Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>  
> > I'm not sure this is introducing a valid fallback. The driver changes
> > imply some things the tmp117 driver supports, that this device
> > does not. A fallback compatible would mean that a new DT
> > with an old kernel would load the tmp117 against a tmp116 and
> > expect it to fully work.  
> 
> Since driver does all the detection an update of the bindings isn't
> really necessary. It is just to have a compatible already in place in
> case there a things we can't detected during runtime. This flow is
> common for a lot of SoC drivers. The fallback will be used as long as
> possible and once a specific feature can't be detected only via the
> binding, the driver adds the new binding to it of_compatible.

That's true going forwards and for drivers that introduce a shared
generic compatible alongside the initial binding. It can't be easily
retrofit.

Fallback compatible is also to allow this to work with old kernels
- which it doesn't because the kernel driver
a) rejects non tmp117 ids (we should fix that by just warning instead 
if the ID isn't what we expect. It would be good to factor that out
as a separate patch that we can backport)

b) assumes the tmp116 (after above fixed) supports things it doesn't.

So it's not a valid use of a fallback compatible.  A driver can't
rely on matching device IDs it didn't previously know about. It sees
tmp116 compatible and thinks it knows how to handle the device, which
it doesn't. This might lead to some very grumpy people not understanding
why their old kernel doesn't work.

Jonathan

> 
> Regards,
>   Marco
> 
> > An example is calibbias which you've dropped from the channels
> > array entry.
> > 
> > Jonathan
> > 
> >   
> > > ---
> > > v2:
> > > - drop items from single enum
> > > 
> > >  .../bindings/iio/temperature/ti,tmp117.yaml        | 14 ++++++++++----
> > >  1 file changed, 10 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/Documentation/devicetree/bindings/iio/temperature/ti,tmp117.yaml b/Documentation/devicetree/bindings/iio/temperature/ti,tmp117.yaml
> > > index 8d1ec4d39b28..9b78357d6a79 100644
> > > --- a/Documentation/devicetree/bindings/iio/temperature/ti,tmp117.yaml
> > > +++ b/Documentation/devicetree/bindings/iio/temperature/ti,tmp117.yaml
> > > @@ -7,8 +7,9 @@ $schema: "http://devicetree.org/meta-schemas/core.yaml#"
> > >  title: "TI TMP117 - Digital temperature sensor with integrated NV memory"
> > >  
> > >  description: |
> > > -    TI TMP117 - Digital temperature sensor with integrated NV memory that supports
> > > -    I2C interface.
> > > +    TI TMP116/117 - Digital temperature sensor with integrated NV memory that
> > > +    supports I2C interface.
> > > +      https://www.ti.com/lit/gpn/tmp116
> > >        https://www.ti.com/lit/gpn/tmp117
> > >  
> > >  maintainers:
> > > @@ -16,8 +17,13 @@ maintainers:
> > >  
> > >  properties:
> > >    compatible:
> > > -    enum:
> > > -      - ti,tmp117
> > > +    oneOf:
> > > +      - enum:
> > > +          - ti,tmp117
> > > +      - items:
> > > +          - enum:
> > > +              - ti,tmp116
> > > +          - const: ti,tmp117
> > >  
> > >    reg:
> > >      maxItems: 1  
> > 
> >