Re: [PATCH v3 1/2] dt-bindings: i2c-mux: Add property for settle time

[Horatiu Vultur <horatiu.vultur@microchip.com>]

The 11/01/2021 15:32, Peter Rosin wrote:

Hi Peter,

> 
> On 2021-11-01 13:25, Horatiu Vultur wrote:
> > Some HW requires some time for the signals to settle after the muxing is
> > changed. Allow this time to be specified in device tree.
> >
> > Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
> > ---
> >  Documentation/devicetree/bindings/i2c/i2c-mux.yaml | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/Documentation/devicetree/bindings/i2c/i2c-mux.yaml b/Documentation/devicetree/bindings/i2c/i2c-mux.yaml
> > index 24cac36037f5..4628ff6340c1 100644
> > --- a/Documentation/devicetree/bindings/i2c/i2c-mux.yaml
> > +++ b/Documentation/devicetree/bindings/i2c/i2c-mux.yaml
> > @@ -29,6 +29,12 @@ properties:
> >    '#size-cells':
> >      const: 0
> >
> > +  settle-time-us:
> > +    default: 0
> > +    description:
> > +      The time required for the signals to settle. Currently only the
> > +      i2c-mux-gpmux driver supports this optional binding.
> 
> The information about how i2c-mux-gpmux is special is bound to go stale,
> and I don't think we should mention such specific details in the binding.
> What I meant was a generic warnings about optional bindings perhaps not
> being supported by all drivers, along the lines of this from i2c.txt:
> 
> "These properties may not be supported by all drivers. However, if a driver
>  wants to support one of the below features, it should adapt these bindings."
> 
> However, I now notice that this sentence makes no sense. It looks like it
> should be s/adapt/adopt/.
> 
> And, in the i2c-mux.yaml case it can simply say "Optional properties"
> instead of "These properites" (which refers to a subset of properties
> immediately below the text) since with a yaml binding it is always
> clear which properties are optional and which are required. Lastly, I
> guess this warning belongs in the description.
> 
> > +
> >  patternProperties:
> >    '^i2c@[0-9a-f]+$':
> >      $ref: /schemas/i2c/i2c-controller.yaml
> >
> 
> Since this is the first optional property, you now need to specify what
> properties are required, which is everything but settle-time-us. If you
> don't, all properties are required. Which is not what we want...
> 
> Something like this should do it, I think:
> 
> required:
>   - compatible
>   - '#address-cells'
>   - '#size-cells'

Thanks for a detail explanation but I am still struggling with these
bindings. Were you thinking to have something like this?

---
diff --git a/Documentation/devicetree/bindings/i2c/i2c-mux.yaml b/Documentation/devicetree/bindings/i2c/i2c-mux.yaml
index 24cac36037f5..c9fde1bb0fea 100644
--- a/Documentation/devicetree/bindings/i2c/i2c-mux.yaml
+++ b/Documentation/devicetree/bindings/i2c/i2c-mux.yaml
@@ -19,6 +19,9 @@ description: |+
   populating the i2c child busses.  If an 'i2c-mux' subnode is present, only
   subnodes of this will be considered as i2c child busses.

+  Optional properties may not be supported by all drivers. However, if a driver
+  wants to support one of the below features, it should adopt these bindings.
+
 properties:
   $nodename:
     pattern: '^(i2c-?)?mux'
@@ -29,6 +32,11 @@ properties:
   '#size-cells':
     const: 0

+  settle-time-us:
+    default: 0
+    description:
+      The time required for the signals to settle.
+
 patternProperties:
   '^i2c@[0-9a-f]+$':
     $ref: /schemas/i2c/i2c-controller.yaml
@@ -41,6 +49,11 @@ patternProperties:

 additionalProperties: true

+required:
+  - compatible
+  - '#address-cells'
+  - '#size-cells'
+
 examples:
   - |
     /*
---

If I have this then my problem is with the required properties because then I
start to get new warnings once I run:

make ARCH=arm CROSS_COMPILE=arm-linux- dtbs_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/i2c/i2c-mux.yaml

For example, one of new the warnings is this:

/home/hvultur/linux/arch/arm/boot/dts/am335x-icev2.dt.yaml: mux-mii-hog: 'compatible' is a required property
	From schema: /home/hvultur/linux/Documentation/devicetree/bindings/i2c/i2c-mux.yaml
/home/hvultur/linux/arch/arm/boot/dts/am335x-icev2.dt.yaml: mux-mii-hog: '#address-cells' is a required property
	From schema: /home/hvultur/linux/Documentation/devicetree/bindings/i2c/i2c-mux.yaml
/home/hvultur/linux/arch/arm/boot/dts/am335x-icev2.dt.yaml: mux-mii-hog: '#size-cells' is a required property
	From schema: /home/hvultur/linux/Documentation/devicetree/bindings/i2c/i2c-mux.yaml

If I don't have the required properties then I don't see these new warnings.

Does it mean that actually the properties are optional by default?

> 
> Cheers,
> Peter

-- 
/Horatiu