Re: Next steps for schema language

[Rob Herring <robh-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

On Fri, Nov 3, 2017 at 9:41 AM, Pantelis Antoniou
<pantelis.antoniou-OWPKS81ov/FWk0Htik3J/w@public.gmane.org> wrote:
> Hi Rob,
>
>> On Nov 3, 2017, at 16:31 , Rob Herring <robh-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org> wrote:
>>
>> On Fri, Nov 3, 2017 at 9:11 AM, Pantelis Antoniou
>> <pantelis.antoniou-OWPKS81ov/FWk0Htik3J/w@public.gmane.org> wrote:
>>> Hi Rob,
>>>
>>>> On Nov 3, 2017, at 15:59 , Rob Herring <robh-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org> wrote:
>>>>
>>>> On Thu, Nov 2, 2017 at 11:44 AM, Grant Likely <grant.likely-s3s/WqlpOiPyB63q8FvJNQ@public.gmane.org> wrote:
>>>>> Hi Pantelis and Rob,
>>>>>
>>>>> After the workshop next week, I'm trying to capture the direction
>>>>> we're going for the schema format. Roughly I think we're aiming
>>>>> towards:
>>>>>
>>>>> - Schema files to be written in YAML
>>>>> - DT files shall remain written in DTS for the foreseeable future.
>>>>> YAML will be treated as an intermediary format
>>>>> - That said, we'll try to make design decisions that allow YAML to
>>>>> be used as a source format.
>>>>> - All schema files and yaml-encoded-dt files must be parsable by stock
>>>>> YAML parsers
>>>>> - Schema files to use the jsonschema vocabulary
>>>>> - (jsonschema assumes json files, but YAML is a superset so this will be okay)
>>>>> - Extended to add vocabulary for DT concepts (ignored by stock validators)
>>>>>   - C-like expressions as used in Pantelis' yamldt could be added in this way
>>>>> - Need to write a jsonschema "metaschema" do define DT specific extensions
>>>>>   - metaschema will be used to validate format of schema files
>>>>>   - Existing tools can confirm is schema files are in the right format.
>>>>>   - will make review a lot easier.
>>>>
>>>> I want to start small here with defining top-level board/soc bindings.
>>>> This is essentially just defining the root node compatible strings.
>>>> Seems easy enough, right? However, I quickly run into the problem of
>>>> how to match for when to apply the schema. "compatible" is the obvious
>>>> choice, but that's also what I'm checking. We can't key off of what we
>>>> are validating. So we really need 2 schema. The first is for matching
>>>> on any valid compatible for board, then 2nd is checking for valid
>>>> combinations (e.g. 1 board compatible followed by 1 SoC compatible). I
>>>> don't like that as we'd be listing compatibles twice. An alternative
>>>> would be we apply every board schema and exactly 1 must pass. Perhaps
>>>> we generate a schema that's a "oneOf" of all the boards? Then we just
>>>> need to tag board schemas in some way.
>>>>
>>>
>>> I’ve run into this as the first problem with validation using compatible properties.
>>>
>>> The way I’ve solved it is by having a ‘selected’ property that is generating
>>> a test for when to check a binding against a node.
>>
>> Okay, but what's the "jsonschema way" to do this is my question really.
>>
>
> No idea :)
>
> DT is weird enough that there might not be a way to describe this in
> a regular jsonschema form. I would wait until Grant pitches in.

I've played around with things a bit and the more I do the less happy
I am with jsonschema. Maybe this is not what Grant has in mind, but
here's the snippet of the compatible check I have:

properties:
  compatible:
    description: |
      Compatible strings for the board example.

    type: array
    items:
      type: string
      oneOf:
        - enum:
          - "example,board"
          - "example,board2"
        - enum:
          - "example,soc"


First, it is more verbose than I'd like and not a language immediately
intuitive to low-level C developers (at least for me). My first
mistake was that *Of values have to be schema objects when I really
want logic ops for values. Second, the constraints are not complete
and and I've not come up with how you would express them. Essentially,
we need to express at least one of each set is required and
"example,soc" must be last. I suppose we can come up with custom
expressions, but it seems to me that if we can't even express a simple
example like this with standard jsonschema then it is not a good
choice.

Don't take this as we should use eBPF either. Given the reasoning so
far for picking it, I'm not sold on it. Seems like a nice shiny hammer
looking for a problem.

Rob