Re: [PATCH v3 3/8] dt-bindings: arm: Add MPAM MSC binding

[Yin Li]

On 10/16/2026 3:36 AM, Rob Herring wrote:
> On Mon, Oct 12, 2026 at 06:24:06PM +0800, Yin Li wrote:
> > On 10/12/2026 3:05 PM, Yin Li wrote:
> > > On 10/9/2026 5:27 AM, Rob Herring wrote:
> > > > On Thu, Oct 08, 2026 at 02:41:12PM +0800, Yin Li wrote:
> > > > > From: Rob Herring <robh@xxxxxxxxxx>
> > > > >
> > > > > The binding is designed around the assumption that an MSC will be a
> > > > > sub-block of something else such as a memory controller, cache
> > > > > controller,
> > > > > or IOMMU. However, it's certainly possible a design does not have that
> > > > > association or has a mixture of both, so the binding illustrates
> > > > > how we can
> > > > > support that with RIS child nodes.
> > > > >
> > > > > A key part of MPAM is we need to know about all of the MSCs in
> > > > > the system
> > > > > before it can be enabled. This drives the need for the genericish
> > > > > 'arm,mpam-msc' compatible. Though we can't assume an MSC is accessible
> > > > > until a h/w specific driver potentially enables the h/w.
> > > > >
> > > > > Cc: James Morse <james.morse@xxxxxxx>
> > > > > Signed-off-by: Rob Herring <robh@xxxxxxxxxx>
> > > > > Signed-off-by: James Morse <james.morse@xxxxxxx>
> > > > > [ Yin Li and Huang Yiwei: use oneOf for the compatible to allow both
> > > > >     standalone arm,mpam-msc and the two-item combination; fix
> > > > > schema field
> > > > >     order, type constraints and yamllint spacing; fix example reg
> > > > > addresses,
> > > > >     add arm,mpam-device phandle and remove TODO comment ]
> > > > > Co-developed-by: Huang Yiwei <huang.yiwei@xxxxxxxxxxxxxxxx>
> > > > > Signed-off-by: Huang Yiwei <huang.yiwei@xxxxxxxxxxxxxxxx>
> > > > > Signed-off-by: Yin Li <yin.li@xxxxxxxxxxxxxxxx>
> > > > > ---
> > > > >    .../devicetree/bindings/arm/arm,mpam-msc.yaml      | 202
> > > > > ++++++++++ +++++++++++
> > > > >    1 file changed, 202 insertions(+)
> > > > >
> > > > > diff --git
> > > > > a/Documentation/devicetree/bindings/arm/arm,mpam-msc.yaml
> > > > > b/Documentation/devicetree/bindings/arm/arm,mpam-msc.yaml
> > > > > new file mode 100644
> > > > > index 000000000000..3c7b13a23ff2
> > > > > --- /dev/null
> > > > > +++ b/Documentation/devicetree/bindings/arm/arm,mpam-msc.yaml
> > > > > @@ -0,0 +1,202 @@
> > > > > +# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
> > > > > +%YAML 1.2
> > > > > +---
> > > > > +$id: http://devicetree.org/schemas/arm/arm,mpam-msc.yaml#
> > > > > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > > > > +
> > > > > +title: Arm Memory System Resource Partitioning and Monitoring (MPAM)
> > > > > +
> > > > > +maintainers:
> > > > > +  - Rob Herring <robh@xxxxxxxxxx>
> > > > > +
> > > > > +description: |
> > > > > +  The Arm MPAM specification can be found here:
> > > > > +
> > > > > +  https://developer.arm.com/documentation/ddi0598/latest
> > > > > +
> > > > > +properties:
> > > > > +  compatible:
> > > > > +    oneOf:
> > > > > +      - const: arm,mpam-msc                   # Further details
> > > > > are discoverable
> > > > > +      - items:
> > > > > +          - const: arm,mpam-memory-controller-msc
> > > >
> > > > Why do we need this?
> > >
> > > Hi Rob,
> > >
> > > Thanks for the review.
> > >
> > > The 'arm,mpam-memory-controller-msc' compatible is needed because,
> > > unlike cache MSCs, memory controller MSCs cannot rely on the parent node
> > > to identify the resource type.
> > >
> > > Cache controllers universally carry a 'cache' compatible, so the driver
> > > can recognise any cache parent and derive the cache ID from standard
> > > properties. No equivalent convention exists for memory controllers, and
> > > the component ID comes from the MSC's own 'numa-node-id' rather than the
> > > parent, so there is no parent attribute to rely on.
> >
> > Hi Rob,
> >
> > As a follow-up to my previous reply:
> > Whatever the outcome of the discussion on "arm_mpam: Add support for
> > memory controller MSC on DT platforms", we will keep the binding and
> > driver support consistent. If it's dropped, the
> > 'arm,mpam-memory-controller-msc' compatible, 'numa-node-id' property, and
> > the memory controller example will be removed together.
>
> Which parts are you actually using/needing and not using? If you don't
> need RIS support, for example, we should drop it.

Hi Rob,

The Kaanapali DTS only uses 'arm,mpam-msc' nested under a cache node,
deriving type and affinity from the parent — following your earlier
suggestion. No RIS child nodes are used. However, this is not 
universally applicable. On most platforms, L2 cache nodes sit under CPU 
nodes, which do not provide an MMIO address space. Nesting an MSC there 
causes address translation failures. The Kaanapali DNM patch moves the 
cache nodes out of the CPU to work around this, which also has the 
benefit of making the MSC-cache parent relationship explicit in the 
device tree — though this is still an artificial arrangement for 
demonstration purposes.

In fact, if Kaanapali supported direct MMIO access to the MPAM
registers, it would also need to place the MSC independently and use
RIS with 'arm,mpam-device' to point to the L2 cache node under the
CPU — the same approach required on any platform where the cache
topology cannot be modified.

Therefore, apart from the memory controller MSC support, we believe
the remaining parts — including RIS — are needed on the Kaanapali
platform.

Thanks,
Yin


> Rob

--
Thx and BRs,
Yin