[PATCH v2 2/6] dt-bindings: riscv: Add Zawrs ISA extension description

[Andrew Jones]

On Fri, Apr 19, 2024 at 03:45:46PM +0100, Conor Dooley wrote:
> On Fri, Apr 19, 2024 at 03:53:24PM +0200, Andrew Jones wrote:
> > Add description for the Zawrs (Wait-on-Reservation-Set) ISA extension
> > which was ratified in commit 98918c844281 of riscv-isa-manual.
> > 
> > Signed-off-by: Andrew Jones <ajones at ventanamicro.com>
> > ---
> >  .../devicetree/bindings/riscv/extensions.yaml        | 12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> > 
> > diff --git a/Documentation/devicetree/bindings/riscv/extensions.yaml b/Documentation/devicetree/bindings/riscv/extensions.yaml
> > index 468c646247aa..584da2f539e5 100644
> > --- a/Documentation/devicetree/bindings/riscv/extensions.yaml
> > +++ b/Documentation/devicetree/bindings/riscv/extensions.yaml
> > @@ -177,6 +177,18 @@ properties:
> >              is supported as ratified at commit 5059e0ca641c ("update to
> >              ratified") of the riscv-zacas.
> >  
> > +        - const: zawrs
> > +          description: |
> > +            The Zawrs extension for entering a low-power state or for trapping
> > +            to a hypervisor while waiting on a store to a memory location, as
> > +            ratified in commit 98918c844281 ("Merge pull request #1217 from
> > +            riscv/zawrs") of riscv-isa-manual.
> 
> This part is fine...
> 
> 
> > Linux assumes that WRS.NTO will
> > +            either always eventually terminate the stall due to the reservation
> > +            set becoming invalid, implementation-specific other reasons, or
> > +            because a higher privilege level has configured it to cause an
> > +            illegal instruction exception after an implementation-specific
> > +            bounded time limit.
> 
> ...but I don't like this bit. The binding should just describe what the
> property means for the hardware, not discuss specifics about a
> particular OS.
> 
> And with my dt-bindings hat off and my kernel hat on, I think that if we
> want to have more specific requirements than the extension provides we
> either need to a) document that zawrs means that it will always
> terminate or b) additionally document a "zawrs-always-terminates" that
> has that meaning and look for it to enable the behaviour.

IIUC, the text above mostly just needs to remove 'Linux assumes' in order
to provide what we want for (a)? I'm not sure about (b). If Zawrs is
unusable as is, then we should probably just go back to the specs and get
a new standard extension name for a new version which includes the changes
we need.

Thanks,
drew