Re: [PATCH 3/5] riscv: dts: add initial T-HEAD light SoC device tree

Conor Dooley
2023-05-08T08:36:06Z

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On Mon, May 08, 2023 at 10:23:02AM +0200, Heiko Stübner wrote:
> Am Montag, 8. Mai 2023, 05:32:17 CEST schrieb Icenowy Zheng:
> > 在 2023-05-07星期日的 22:35 +0100，Conor Dooley写道：
> > > Hey Jisheng,
> > >
> > > On Mon, May 08, 2023 at 02:23:02AM +0800, Jisheng Zhang wrote:
> > >
> > > > + c910_0: cpu@0 {
> > > > + compatible = "thead,c910", "riscv";
> > > > + device_type = "cpu";
> > > > + riscv,isa = "rv64imafdc";
> > >
> > > Does this support more than "rv64imafdc"?
> > > I assume there's some _xtheadfoo extensions that it does support,
> > > although I am not sure how we are proceeding with those - Heiko might
> > > have a more nuanced take.
>
> I guess the interesting question still is, are these part of the isa
> string or more of an errata?

Yeah, I dunno. That's possible a policy decision more than anything
else. I don't remember if it was one of your patchsets or elsewhere, but
I do recall a split between xtheadba etc and vector, where xtheadba was
defined as a vendor extension, whereas vector is not. Their extension
spec repo <https://github.com/T-head-Semi/thead-extension-spec> appears
to be aligned with that view, apart from the CMOs that we have already
called an erratum.

> The binding currently says
> Identifies the specific RISC-V instruction set architecture
> supported by the hart. These are documented in the RISC-V
> User-Level ISA document, available from
> https://riscv.org/specifications/
>
>
> I guess if we decide to make them part of the isa-string the binding
> then should get a paragraph mention _xfoo vendor-extensions too.

I have an idea in the works that may allow dealing with this kind of
thing, but it's a bit of a departure from the existing binding.
I will hopefully post an early RFC of it later today.
That said, the binding does currently allow you to put in _xfoo vendor
extensions as-is.

> Personally, making these part of the ISA string definitly sounds like
> the best solution though :-) .

You would say that wouldn't you! In general, I'd rather we filled in as
much information as possible here, even if it is not currently in use,
to avoid having to retrofit as support becomes available.

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