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

Heiko Stübner
2022-05-18T09:03:15Z

Am Mittwoch, 18. Mai 2022, 10:22:17 CEST schrieb Philipp Tomsich:
> +David Kruckemyer (who is chairing the CMO task-group within RVI).
>
> On Wed, 18 May 2022 at 02:25, Rob Herring <robh@kernel.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@sntech.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.

I agree that this should be worded better. The intent was to tell that this
is different from say the l1-cache-block-size.

I.e. these values can be the same but don't need to be. But I guess I got
too much lead on by a kernel implementation detail (L1_CACHE_BYTES constant)

> 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
> > >
> > >
>