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

[Conor Dooley <conor@kernel.org>]

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On Fri, Dec 26, 2025 at 03:28:25PM -0600, Alex Elder 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@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 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.
> 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.

The dependency can be go both ways, to also make specifying "b" mandatory
when the three components are. That probably produces the most helpful
devicetree ultimately.

> 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".
> But that might not be normal practice, and it's not necessary
> because they aren't in conflict.

I disagree completely with this "even further", since that's potentially
actively harmful to importers of kernel devicetrees.
If "b" is to be allowed, I'm only in favour if having it requires the
component parts.

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