[PATCH v2 07/13] dt-bindings: riscv: Add B ISA extension description

[Guodong Xu]

Hi, Alex

On Sat, Dec 27, 2025 at 5:28 AM Alex Elder <elder at riscstar.com> wrote:
>
> On 12/23/25 12:51 AM, Guodong Xu wrote:
> > Hi, Conor
> >
> > On Tue, Dec 23, 2025 at 5:17 AM Conor Dooley <conor at kernel.org> wrote:
> >>
> >> On Mon, Dec 22, 2025 at 09:04:17PM +0800, Guodong Xu wrote:
> >>> Add description of the single-letter "B" extennsion for Bit Manipulation.
> >>> B is mandatory for RVA23U64.
> >>>
> >>> The B extension is ratified in the 20240411 version of the unprivileged
> >>> ISA specification. According to the ratified spec, "the B standard
> >>> extension comprises instructions provided by the Zba, Zbb, and Zbs
> >>> extensions.
> >>>
> >>> Hence add a schema check rule to enforce that B implies Zba, Zbb and Zbs.
> >>>
> >>> Signed-off-by: Guodong Xu <guodong at riscstar.com>
> >>> ---
> >>> v2: New patch.
> >>> ---
> >>>   .../devicetree/bindings/riscv/extensions.yaml         | 19 +++++++++++++++++++
> >>>   1 file changed, 19 insertions(+)
> >>>
> >>> diff --git a/Documentation/devicetree/bindings/riscv/extensions.yaml b/Documentation/devicetree/bindings/riscv/extensions.yaml
> >>> index 565cb2cbb49b552959392810a9b731b43346a594..385e1deb23996d294e7662693f1257f910a6e129 100644
> >>> --- a/Documentation/devicetree/bindings/riscv/extensions.yaml
> >>> +++ b/Documentation/devicetree/bindings/riscv/extensions.yaml
> >>> @@ -109,6 +109,13 @@ properties:
> >>>               The standard C extension for compressed instructions, as ratified in
> >>>               the 20191213 version of the unprivileged ISA specification.
> >>>
> >>> +        - const: b
> >>> +          description:
> >>> +            The standard B extension for bit manipulation instructions, as
> >>> +            ratified in the 20240411 version of the unprivileged ISA
> >>> +            specification. The B standard extension comprises instructions
> >>> +            provided by the Zba, Zbb, and Zbs extensions.
> >>> +
> >>>           - const: v
> >>>             description:
> >>>               The standard V extension for vector operations, as ratified
> >>> @@ -735,6 +742,18 @@ properties:
> >>>           then:
> >>>             contains:
> >>>               const: f
> >>> +      # b comprises the following extensions
> >>> +      - if:
> >>> +          contains:
> >>> +            const: b
> >>
> >> What's the value in adding b, if it depends on having all 3 of the
> >> components defined individually too? Currently all "superset" types of
> >> extensions are permitted without their component parts also being defined,
> >> this doesn't follow convention and therefore needs to be explained.
> >>
> >> You obviously need this construct because the kernel does not understand
> >> "b", and even if you added support for interpreting "b" to the kernel
> >> this is probably still needed to make sure the ABI is maintained for
> >> anything importing a devicetree from the kernel.
> >
> > Yes, exactly. Unlike other single-letter extensions, "b" was ratified
> > (Apr/2024) much later than its components zba/zbb/zbs (Jun/2021).
> > Existing software and the kernel already expect these explicit component
> > strings, so enforcing this dependency ensures cores declaring "b" will
> > also be correctly understood by older software that only looks for
> > zba/zbb/zbs.
>
> I might be misunderstanding you, but I don't think extension "b"
> should *require* the other three extensions.  Instead, the "b"
> extension should be considered *equivalent* to the other three.

You are correct in saying they are equivalent.

> That's what I understand it to mean, anyway.
>    https://github.com/riscv/riscv-b
>
> There's no point in supporting "b" in devicetree to represent
> the others if it also requires the others to be present.
>
> I think that, instead, "b", "zba", "zbb", and "zbs" should all
> be allowed.
>
> I might even go further and harden the requirement, saying that
> if you specify "b" you should *not* specify "zba", "zbb", or "zbs".

Historical reasons here. "b" came too late. The chip vendors have published
cores with "zba", "zbb", and "zbs"already.

That's a migration bridge to require "b" must be listed
together with the other three.

BR,
Guodong

> But that might not be normal practice, and it's not necessary
> because they aren't in conflict.
>
>                                         -Alex
>
> > I will update the commit message in v3 to clearly explain this reasoning.
> > Does it sound good to you?
> >
> > Thank you for the review.
> >
> > BR,
> > Guodong Xu
> >
> >>
> >>> +        then:
> >>> +          allOf:
> >>> +            - contains:
> >>> +                const: zba
> >>> +            - contains:
> >>> +                const: zbb
> >>> +            - contains:
> >>> +                const: zbs
> >>>         # Zcb depends on Zca
> >>>         - if:
> >>>             contains:
> >>>
> >>> --
> >>> 2.43.0
> >>>
> >
>