Re: [PATCH v2 14/17] riscv: dts: jh7110: Add initial StarFive JH7110 device tree

[Conor Dooley <conor@kernel.org>]

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Hey Seán, David,

On Thu, Jan 19, 2023 at 01:26:52AM -0500, Sean Anderson wrote:
> On 1/19/23 01:18, David Abdurachmanov wrote:
> > On Wed, Jan 18, 2023 at 10:19 AM Yanhong Wang
> > <yanhong.wang@starfivetech.com> wrote:

> > > +               U74_4: cpu@4 {
> > > +                       compatible = "sifive,u74-mc", "riscv";
> > > +                       reg = <4>;
> > > +                       d-cache-block-size = <64>;
> > > +                       d-cache-sets = <64>;
> > > +                       d-cache-size = <32768>;
> > > +                       d-tlb-sets = <1>;
> > > +                       d-tlb-size = <40>;
> > > +                       device_type = "cpu";
> > > +                       i-cache-block-size = <64>;
> > > +                       i-cache-sets = <64>;
> > > +                       i-cache-size = <32768>;
> > > +                       i-tlb-sets = <1>;
> > > +                       i-tlb-size = <40>;
> > > +                       mmu-type = "riscv,sv39";
> > > +                       next-level-cache = <&ccache>;
> > > +                       riscv,isa = "rv64imafdcbsu";
> > 
> > Looking at SiFive U74 manuals, shouldn't this be RV64GC_Zba_Zbb_Sscofpmf?
> > U74 only supports Zba and Zbb bit manip extensions.
> > This is from the 21G3.02.00 release manual.
> > 
> > Looking more, S76 core is listed in the manual as supporting up to:
> > RV64IMAC_Zicsr_Zifencei_Zba_Zbb_Sscofpmf.
> > 
> > I almost forgot about _Zicsr_Zifencei (which are part of G). Shouldn't
> > those be listed too in riscv,isa?

AFAIU, Linux just assumes them since they weren't their own thing prior
to ISA spec 20191213. I think in- & ex- cluding them are both valid...
Yeah.

> AFAIK we don't support Z/X in U-Boot.

Does the U-Boot ISA string parsing not just ignore un-implemented
extensions? If it does ignore things you don't implement, then I think
including the Z extensions should be no harm.
IMO, it'd be nice to have this string match whatever the hardware can
support so that same Devicetree can be used for U-Boot & whatever OS it
is booting.

That said, I'm yet to be sure that this SoC supports Zba or Zbb.
I asked on the corresponding patchset for Linux and the answer I got,
not from the vendor, was that it did.
To what extent it might (or if it actually does) I have not yet
determined. It's not mentioned in any of the documentation that I have
got my hands on.
I have one of these boards, so am in the process of getting something
functional enough on it to actually test that.

Thanks!
Conor.

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