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

[Guodong Xu]

On Mon, Dec 29, 2025 at 7:50 AM Alex Elder <elder at riscstar.com> wrote:
>
> On 12/27/25 8:51 PM, Guodong Xu wrote:
> >> 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.
>
> Are you saying "b" has already been included with "zba", "zbb", and
> "zbs" in an existing DTS file?


The risc-v ratification timeline is:
"b" was ratified in Apr/2024, which is about 2 years later than its
components zba/zbb/zbs (these were ratified in Jun/2021).

I can do this in linux kernel, writing a dts file which contains only "b",
but no zba/zbb/zbs. The linux kernel can correctly extend "b" to zba/zbb/zbs.
ps: after I or somebody adds the logic into cpufeature.c.

However, the problem is with the older software who reuse kernel's DTS files,
and recognizes only 'zba/zbb/zbs'. (If you search in the riscv/boot/dts,
you will notice a lot platforms supports zba/zbb/zbs.)

When there is only "b", these older software may just disable the features
related to 'zba/zbb/zbs', because when they developed their feature,
"b" doesn't exist yet.

Hopefully I explained the logic clear this time.

BR,
Guodong

>
> What I'm suggesting is that (unless someone has already done this in
> a DTS file), there is no reason to require "b" *and* the other three.
> You should allow either "b" *or* all of the other three, not both.
> That would support older platforms as well as newer ones that use
> the more concise "b" only.
>
>                                         -Alex