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

From: "Heiko Stübner" <heiko@sntech.de>
To: Rob Herring <robh@kernel.org>
Cc: palmer@dabbelt.com, paul.walmsley@sifive.com,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	wefu@redhat.com, guoren@kernel.org, atishp@atishpatra.org,
	anup@brainfault.org, mick@ics.forth.gr, samuel@sholland.org,
	cmuellner@linux.com, philipp.tomsich@vrull.eu,
	krzk+dt@kernel.org, devicetree@vger.kernel.org
Subject: Re: [PATCH v2 1/3] dt-bindings: riscv: document cbom-block-size
Date: Wed, 25 May 2022 17:14:31 +0200	[thread overview]
Message-ID: <4376261.8F6SAcFxjW@diego> (raw)
In-Reply-To: <20220518002529.GA1928329-robh@kernel.org>

Am Mittwoch, 18. Mai 2022, 02:25:29 CEST schrieb Rob Herring:
> 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?

funnily enough there really seems to be _no_ constraints defined in the
spec [0] regarding the actual cache-block size.

It essentially only states
"The capacity and organization of a cache and the size of a
cache block are both implementation-specific"

and later in software-discovery:
"The initial set of CMO extensions requires the following information to
be discovered by software:
- The size of the cache block for management and prefetch instructions
- The size of the cache block for zero instructions"

[0] https://github.com/riscv/riscv-CMOs/blob/master/specifications/cmobase-v1.0.pdf

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