Defining schemas for Device Tree

[swarren@wwwdotorg.org (Stephen Warren)]

On 07/28/2013 06:21 PM, Tomasz Figa wrote:
...
> b) What information should be specified in schemas? What level of 
>    granularity is required?
> 
> For each property we need to have at least following data specified:
>  - property name (or property name format, e.g. regex),
>  - whether the property is mandatory or optional,
>  - data type of value.

We might want general restrictions on node/property names, e.g. it seems
we generally prefer - rather than _ in them, although there are a few
entrenched exceptions.

> As for now, I can think of following data types used in device trees:
>  - boolean (i.e. without value),
>  - array of strings (including single string),
>  - array of u32 (including single u32),
>  - specifier (aka phandle with args, including cases with 0 args),
>  - variable-length cells (e.g. #address-cells of u32s).
> 
> Some properties might require a combination of data types to be specified 

I believe phandles are just cells in terms of the storage type at least.
Hence, I'd argue we don't need mixing of types (boolean, cell, string),
but I see your point about semantically needing to represent different
cells meaning different things.

> or even an array of combinations, like interrupt-map property, which is an 
> array of entries consisting of:
>  - #address-cells u32s,
>  - #interrupt-cells u32s,
>  - specifier (phandle of interrupt controller and u32 of count defined by 
>    #interrupt-cells of the controller).
> 
> We probably want to define allowed range of values for given property, be 
> it contiguous or enumerated.

Where defining the length of a property (or part of it) in terms of
another #xxx-cells property, we need to specify where that other
property exists. Sometimes it's the nearest parent node containing the
property (typical for #address-cells), sometimes it's the node
containing the property itself (typical for one of the #address-cells
referenced by a ranges property), and sometimes it's in the node
referenced by the phandle associated with a specifier.

Other content restrictions might be:

* List must contain (at least) these entries/values.

* List can't contain any other entries/values not specified here.

* List must be in this order vs. any order.

* List logical length (in entries, not bytes/cells) must match the
logical length of another cell (consider clocks/clock-names).

* Ordering of one property must match ordering of another property
(again consider clocks/clock-names), although it'd be difficult to tell
whether this condition was met, perhaps we can at least document it.

I'm sure there will be many more criteria to validate that we're
forgetting. This is possible the most complex area?

> As for subnodes, I think we need to define following constraints:
>  - node name (or node name format, e.g. regex),

I don't think node names are supposed to convey any semantic meaning, so
they probably shouldn't be restricted (much?) by schema.

> d) When should the validation happen and what should handle it?
...
> well. Nothing stops us from running validation on already compiled dtbs, 
> though, using an extra tool.

In a DTB, I'm not sure whether all the type information is still present.

For example, in DTS, the following are very obviously different:

prop = "abc", "def";
prop = <0x61626300> <0x64656600>;

... whereas in the DTB, I think they are both just two 32-bit values
that just happen to encode string data or not. Presumably we want the
validator to force string properties to be specified using string syntax?

Equally, I'm not sure there's any difference between:

prop = <&node>;
prop = <0x23648689>;

... in the DTB, except that the value just *happens* to match another
node's phandle value, which could end up leading to false positive
matches if only applied on the raw values not the syntax?