Re: Next steps for schema language

[David Gibson <david-xT8FGy+AXnRB3Ne2BGzF6laj5H9X9Tb+@public.gmane.org>]

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On Fri, Nov 03, 2017 at 04:11:46PM +0200, Pantelis Antoniou 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@secretlab.ca> 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.
> 
> For a concrete example using the spi-slave binding
> 
> —- spi-slave.yaml ---
> ...
> inherings: *device-compatible
> …
> 
> Where device compatible is:
> 
> %YAML 1.1
> ---
> device-compatible: &device-compatible
>   title: Contraint for devices with compatible properties
>   # select node for checking when the compatible constraint and
>   # the device status enable constraint are met.
>   selected: [ "compatible", *device-status-enabled ]
> 
>   class: constraint
>   virtual: true
> 
> The selected property here declares that any “compatible” property constraint
> for the node under test must match and that the device-status-enabled
> rule matches too.

I think the basic idea of having a "selector" that says when the
binding applies is correct.  However, I think the details of this
verison (AFAICT) are a bit too ad hoc.

I think of it in terms of "patterns" which a node might or might not
match.  Those can be simple {compatible includes "blah"} or complex -
a full set of binding requirements.

Bindings will generally be of the form:
	if (PatternA) matches
		then (PatternB) must match

e.g.
	if (compatible includes "blah")
		then ("blah" binding details) must match

or
	if ("interrupts" exists)
		then (irq binding must apply)

But in order to keep things general, both the "selector" and the
"requirements" patterns should be specified in the same way.

I had the feeling that jsonschema worked that way from Grant's talk,
although I'm not 100% certain.

> 
> The device-status-enabled is declared as follows:
> 
> %YAML 1.1
> ---
> device-enabled:
>   title: Contraint for enabled devices
>  
>   class: constraint
>   virtual: true
>   
>   properties:
>     status: &device-status-enabled
>       category: optional
>       type: str
>       description: Marks device state as enabled
>       constraint: |
>         !exists || streq(v, "okay") || streq(v, "ok”)
> 
> This declares that the constraint matches when either the optional
> status property is not present or if it is, it’s either “okay” or “ok”.
> 
> Taken together when the spi-slave binding is inherited (for instance
> by the jedec,spi-nor binding:
> 
> — jedec,spi-nor.yaml —
> 
> …
> inherits: *spi-slave
> 
> …
>   compatible: &jedec-spi-nor-compatible
>   category: required
>   type: strseq
>   constraint: | 
>     anystreq(v,  "at25df321a") ||
>      ….
> 
> 
> > I'm thinking of this very specific case, but no doubt it is going to
> > apply in other places. This has been one of the harder problems with
> > writing dtc checks. What to key off of to trigger checks and how to
> > check errors in the key itself.
> > 
> > Rob
> 
> Regards
> 
> — Pantelis
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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