linux-kernel - Re: [PATCH v2 1/3] dt-bindings: riscv: document cbom-block-size

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Message-ID: <CAAeLtUBLpDHeJHfPtaUb_OLZb_6cUQa1Z_F+06pkdJMSHtWrfQ@mail.gmail.com>
Date:   Wed, 18 May 2022 10:22:17 +0200
From:   Philipp Tomsich <philipp.tomsich@...ll.eu>
To:     Rob Herring <robh@...nel.org>
Cc:     Heiko Stuebner <heiko@...ech.de>, palmer@...belt.com,
        paul.walmsley@...ive.com, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, wefu@...hat.com, guoren@...nel.org,
        atishp@...shpatra.org, anup@...infault.org, mick@....forth.gr,
        samuel@...lland.org, cmuellner@...ux.com, krzk+dt@...nel.org,
        devicetree@...r.kernel.org
Subject: Re: [PATCH v2 1/3] dt-bindings: riscv: document cbom-block-size

+David Kruckemyer (who is chairing the CMO task-group within RVI).

On Wed, 18 May 2022 at 02:25, Rob Herring <robh@...nel.org> wrote:
>
> On Wed, May 11, 2022 at 11:41:30PM +0200, Heiko Stuebner wrote:
> > The Zicbom operates on a block-size defined for the cpu-core,
> > which does not necessarily match other cache-sizes used.
> >
> > So add the necessary property for the system to know the core's
> > block-size.
> >
> > Signed-off-by: Heiko Stuebner <heiko@...ech.de>
> > ---
> >  Documentation/devicetree/bindings/riscv/cpus.yaml | 7 +++++++
> >  1 file changed, 7 insertions(+)
> >
> > diff --git a/Documentation/devicetree/bindings/riscv/cpus.yaml b/Documentation/devicetree/bindings/riscv/cpus.yaml
> > index d632ac76532e..b179bfd155a3 100644
> > --- a/Documentation/devicetree/bindings/riscv/cpus.yaml
> > +++ b/Documentation/devicetree/bindings/riscv/cpus.yaml
> > @@ -63,6 +63,13 @@ properties:
> >        - riscv,sv48
> >        - riscv,none
> >
> > +  riscv,cbom-block-size:
> > +    $ref: /schemas/types.yaml#/definitions/uint32
>
> Any value 0-2^32 is valid?
>
> > +    description:
> > +      Blocksize in bytes for the Zicbom cache operations. The block
> > +      size is a property of the core itself and does not necessarily
> > +      match other software defined cache sizes.
>
> What about hardware defined cache sizes? I'm scratching my head as to
> what a 'software defined cache size' is.

This seems to be a misnomer, as the specification doesn't use the term
and rather talks about the "size of a cache block for [operation
name]".

There are currently two such 'operation sizes' discoverable by software:
- size of the cache block for management and prefetch instructions
- size of the cache block for zero instructions

For whatever it's worth, cache operations in RISC-V attempt to
disassociate the underlying hardware cache geometry from software.
See https://github.com/riscv/riscv-CMOs/blob/master/specifications/cmobase-v1.0.1.pdf
for the CMO specification, and the discoverable parameters are listed
in section 2.7.

Philipp.

> > +
> >    riscv,isa:
> >      description:
> >        Identifies the specific RISC-V instruction set architecture
> > --
> > 2.35.1
> >
> >