Re: [PATCH 1/2] dt-bindings: interrupt-controller: arm,gic-v3: Add dma-noncoherent property

[Lorenzo Pieralisi <lpieralisi@kernel.org>]

On Tue, Sep 05, 2023 at 12:17:51PM +0100, Robin Murphy wrote:
> On 05/09/2023 11:47 am, Lorenzo Pieralisi wrote:
> > The GIC v3 specifications allow redistributors and ITSes interconnect
> > ports used to access memory to be wired up in a way that makes the
> > respective initiators/memory observers non-coherent.
> > 
> > Add the standard dma-noncoherent property to the GICv3 bindings to
> > allow firmware to describe the redistributors/ITSes components and
> > interconnect ports behaviour in system designs where the redistributors
> > and ITSes are not coherent with the CPU.
> > 
> > Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
> > Cc: Rob Herring <robh@kernel.org>
> > ---
> >   .../bindings/interrupt-controller/arm,gic-v3.yaml         | 8 ++++++++
> >   1 file changed, 8 insertions(+)
> > 
> > diff --git a/Documentation/devicetree/bindings/interrupt-controller/arm,gic-v3.yaml b/Documentation/devicetree/bindings/interrupt-controller/arm,gic-v3.yaml
> > index 39e64c7f6360..0a81ae4519a6 100644
> > --- a/Documentation/devicetree/bindings/interrupt-controller/arm,gic-v3.yaml
> > +++ b/Documentation/devicetree/bindings/interrupt-controller/arm,gic-v3.yaml
> > @@ -106,6 +106,10 @@ properties:
> >       $ref: /schemas/types.yaml#/definitions/uint32
> >       maximum: 4096
> > +  dma-noncoherent:
> > +    description: |
> > +      Present if the GIC redistributors are not cache coherent with the CPU.
> 
> I wonder if it's worth being a bit more specific here, e.g. "if the GIC
> {redistributors,ITS} permit programming cacheable inner-shareable memory
> attributes, but are connected to a non-coherent downstream interconnect."

In my opinion it is and I wanted to elaborate on what I wrote but then I
thought that this is a standard DT property, I wasn't sure whether we
really need to explain what it is there for.

We are using the property to plug a hole so I agree with you, we should
be as clear as possible in the property definition but I will rely on
Rob/Marc's opinion, I don't know what's the DT policy for this.

> That might help clarify why the negative property, which could seem a bit
> backwards at first glance, and that it's not so important in the cases where
> the GIC itself is fundamentally non-coherent anyway (which *is*
> software-discoverable).

Is it ? Again, see above, are we defining "dma-noncoherent" to fix a bug
or to fix the specs ? The shareability bits are writeable and even a
fundamentally non-coherent GIC design could allow writing them, AFAIU.

I would avoid putting ourselves into a corner where we can't use
this property because the binding itself is too strict on what it is
solving.
 
> Otherwise, this is the same approach that I like and have previously lobbied
> for, so obviously I approve :)
> 
> (plus I do think it's the right shape to be able to slot an equivalent field
> into ACPI MADT entries without *too* much bother)

We are in agreement, let's see what others think.

Thanks,
Lorenzo

> 
> Thanks,
> Robin.
> 
> > +
> >     msi-controller:
> >       description:
> >         Only present if the Message Based Interrupt functionality is
> > @@ -193,6 +197,10 @@ patternProperties:
> >         compatible:
> >           const: arm,gic-v3-its
> > +      dma-noncoherent:
> > +        description: |
> > +          Present if the GIC ITS is not cache coherent with the CPU.
> > +
> >         msi-controller: true
> >         "#msi-cells":
> 
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