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

[Conor Dooley <conor@kernel.org>]

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On Tue, Dec 23, 2025 at 02:51:17PM +0800, Guodong Xu wrote:
> Hi, Conor
> 
> On Tue, Dec 23, 2025 at 5:17 AM Conor Dooley <conor@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@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 will update the commit message in v3 to clearly explain this reasoning.

> Does it sound good to you?

Ye, sounds good.

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