netdev - Re: [RFC net-next 2/4] ynl: add the schema for the schemas

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Date:   Tue, 16 Aug 2022 09:21:27 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
        pabeni@...hat.com, sdf@...gle.com, jacob.e.keller@...el.com,
        vadfed@...com, jiri@...nulli.us, dsahern@...nel.org,
        stephen@...workplumber.org, fw@...len.de, linux-doc@...r.kernel.org
Subject: Re: [RFC net-next 2/4] ynl: add the schema for the schemas

On Mon, 2022-08-15 at 17:47 -0700, Jakub Kicinski wrote:
> On Mon, 15 Aug 2022 22:09:11 +0200 Johannes Berg wrote:
> > On Wed, 2022-08-10 at 19:23 -0700, Jakub Kicinski wrote:
> > > 
> > > +        attributes:
> > > +          description: List of attributes in the space.
> > > +          type: array
> > > +          items:
> > > +            type: object
> > > +            required: [ name, type ]
> > > +            additionalProperties: False
> > > +            properties:
> > > +              name:
> > > +                type: string
> > > +              type: &attr-type
> > > +                enum: [ unused, flag, binary, u8, u16, u32, u64, s32, s64,
> > > +                        nul-string, multi-attr, nest, array-nest, nest-type-value ]  
> > 
> > nest-type-value?
> 
> It's the incredibly inventive nesting format used in genetlink policy
> dumps where the type of the sub-attr(s there are actually two levels)
> carry a value (index of the policy and attribute) rather than denoting
> a type :S :S :S

Hmm, OK, in the policy dump (not specific to genetlink, btw, can be used
for any policy, but is only generically hooked up for genetlink), we
have

[policy_idx] = {
  [attr_idx] = {
    [NL_POLICY_TYPE_ATTR_...] = ...
  }
}

Is that what you mean?

I guess I never really thought about this format much from a description
POV, no need to have a policy since you simply iterate (for_each_attr)
when reading it, and don't really need to care about the attribute
index, at least.

For future reference, how would you suggest to have done this instead?


> > > +              description:
> > > +                description: Documentation of the attribute.
> > > +                type: string
> > > +              type-value:
> > > +                description: Name of the value extracted from the type of a nest-type-value attribute.
> > > +                type: array
> > > +                items:
> > > +                  type: string
> > > +              len:
> > > +                oneOf: [ { type: string }, { type: integer }]
> > > +              sub-type: *attr-type
> > > +              nested-attributes:
> > > +                description: Name of the space (sub-space) used inside the attribute.
> > > +                type: string  
> > 
> > Maybe expand that description a bit, it's not really accurate for
> > "array-nest"?
> 
> Slightly guessing but I think I know what you mean -> the value of the
> array is a nest with index as the type and then inside that is the
> entry of the array with its attributes <- and that's where the space is
> applied, not at the first nest level?

Right.

> Right, I should probably put that in the docs rather than the schema,
> array-nests are expected to strip one layer of nesting and put the
> value taken from the type (:D) into an @idx member of the struct
> representing the values of the array. Or at least that's what I do in
> the C codegen.

Well mostly you're not supposed to care about the 'value'/'type', I
guess?

> Not that any of these beautiful, precious formats should be encouraged
> going forward. multi-attr all the way!

multi-attr?

> > Do you mean the "name of the enumeration" or the "name of the
> > enumeration constant"? (per C99 concepts) I'm a bit confused? I guess
> > you mean the "name of the enumeration constant" though I agree most
> > people probably don't know the names from C99 (I had to look them up too
> > for the sake of being precise here ...)
> 
> I meant the type. I think. When u32 carries values of an enum.
> Enumeration constant for the attribute type is constructed from
> it's name and the prefix/suffix kludge.
> 
Indeed, I confused myself too ...

johannes