Re: [PATCH RFC v3 2/9] dt-bindings: board: Introduce board-id

[Elliot Berman <quic_eberman@quicinc.com>]

Hi Conor,

Thanks for taking the time to look at the patch.

On Tue, May 21, 2024 at 08:21:45PM +0100, Conor Dooley wrote:
> On Tue, May 21, 2024 at 11:37:59AM -0700, Elliot Berman wrote:
> > Device manufcturers frequently ship multiple boards or SKUs under a
> > single softwre package. These software packages ship multiple devicetree
> > blobs and require some mechanims to pick the correct DTB for the boards
> > that use the software package.
> 
> Okay, you've got the problem statement here, nice.
> 
> > This patch introduces a common language
> > for adding board identifiers to devicetrees.
> 
> But then a completely useless remainder of the commit message.
> I open this patch, see the regexes, say "wtf", look at the commit
> message and there is absolutely no explanation of what these properties
> are for. That's quite frankly just not good enough - even for an RFC.
> 

Understood, I've been trying to walk the line of getting the idea across
to have conversation about the board-ids, while not getting into too
much of the weeds. I was hoping the example and the matching code in the
first patch would get enough of the idea across, but I totally
empathize that might not be enough. I'll reply here shortly with a
version of this patch which adds more details.

> > 
> > Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>
> > ---
> >  .../devicetree/bindings/board/board-id.yaml        | 24 ++++++++++++++++++++++
> >  1 file changed, 24 insertions(+)
> > 
> > diff --git a/Documentation/devicetree/bindings/board/board-id.yaml b/Documentation/devicetree/bindings/board/board-id.yaml
> > new file mode 100644
> > index 000000000000..99514aef9718
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/board/board-id.yaml
> > @@ -0,0 +1,24 @@
> > +# SPDX-License-Identifier: BSD-2-Clause
> > +%YAML 1.2
> > +---
> > +$id: http://devicetree.org/schemas/board/board-id.yaml#
> > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > +
> > +title: board identifiers
> > +description: Common property for board-id subnode
> 
> s/property/properties/
> 
> > +
> > +maintainers:
> > +  - Elliot Berman <quic_eberman@quicinc.com>
> > +
> > +properties:
> > +  $nodename:
> > +    const: '/'
> > +  board-id:
> > +    type: object
> > +    patternProperties:
> > +      "^.*(?!_str)$":
> 
> Does this regex even work? Take "foo_str" as an example - doesn't "^.*"
> consume all of the string, leaving the negative lookahead with nothing
> to object to? I didn't properly test this with an example and the dt
> tooling, but I lazily threw it into regex101 and both the python and
> emcascript versions agree with me. Did you test this?

Right, it should be a lookbehind, not a lookahead.

> 
> And while I am here, no underscores in property names please. And if
> "str" means string, I suggest not saving 3 characters.
> 
> > +        $ref: /schemas/types.yaml#/definitions/uint32-matrix
> > +      "^.*_str$":
> > +        $ref: /schemas/types.yaml#/definitions/string-array
> 
> Why do we even need two methods? Commit message tells me nothing and
> there's no description at all... Why do we need regexes here, rather
> than explicitly defined properties? Your commit message should explain
> the justification for that and the property descriptions (as comments if
> needs be for patternProperties) should explain why this is intended to
> be used.
> 
> How is anyone supposed to look at this binding and understand how it
> should be used?

I was thinking that firmware may only provide the data without being
able to provide the context whether the value is a number or a string.
I think this is posisble if firmware provides the device's board
identifier in the format of a DT itself. It seems natural to me in the
EBBR flow. There is example of this in example in patches 3
(fdt-select-board) and 9 (the test suite). DTB doesn't inherently
provide instruction on how to interpret a property's value, so I created
a rule that strings have to be suffixed with "-string".

One other note -- I (QCOM) don't currently have a need for board-ids to
be strings. I thought it was likely that someone might want that though.

Thanks,
Elliot