Re: [PATCH] dt-bindings: dvfs: Add support for generic performance domains

[Sudeep Holla <sudeep.holla@arm.com>]

On Mon, Nov 09, 2020 at 02:15:18PM -0600, Rob Herring wrote:
> On Thu, Nov 05, 2020 at 05:35:39PM +0000, Sudeep Holla wrote:
> > The CLKSCREW attack [0] exposed security vulnerabilities in energy management
> > implementations where untrusted software had direct access to clock and
> > voltage hardware controls. In this attack, the malicious software was able to
> > place the platform into unsafe overclocked or undervolted configurations. Such
> > configurations then enabled the injection of predictable faults to reveal
> > secrets.
> > 
> > Many Arm-based systems used to or still use voltage regulator and clock
> > frameworks in the kernel. These frameworks allow callers to independently
> > manipulate frequency and voltage settings. Such implementations can render
> > systems susceptible to this form of attack.
> > 
> > Attacks such as CLKSCREW are now being mitigated by not having direct and
> > independent control of clock and voltage in the kernel and moving that
> > control to a trusted entity, such as the SCP firmware or secure world
> > firmware/software which are to perform sanity checking on the requested
> > performance levels, thereby preventing any attempted malicious programming.
> > 
> > With the advent of such an abstraction, there is a need to replace the
> > generic clock and regulator bindings used by such devices with a generic
> > performance domains bindings.
> > 
> > [0] https://www.usenix.org/conference/usenixsecurity17/technical-sessions/presentation/tang
> > 
> > Cc: Rob Herring <robh+dt@kernel.org>
> > Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
> > ---
> >  .../bindings/dvfs/performance-domain.yaml     | 67 +++++++++++++++++++
> >  1 file changed, 67 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/dvfs/performance-domain.yaml
> > 
> > diff --git a/Documentation/devicetree/bindings/dvfs/performance-domain.yaml b/Documentation/devicetree/bindings/dvfs/performance-domain.yaml
> > new file mode 100644
> > index 000000000000..fa0151f63ac9
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/dvfs/performance-domain.yaml
> > @@ -0,0 +1,67 @@
> > +# SPDX-License-Identifier: GPL-2.0
> 
> Dual license new bindings.
> 
> > +%YAML 1.2
> > +---
> > +$id: http://devicetree.org/schemas/dvfs/performance-domain.yaml#
> > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > +
> > +title: Generic performance domains
> > +
> > +maintainers:
> > +  - Sudeep Holla <sudeep.holla@arm.com>
> > +
> > +description: |+
> > +  This binding is intended for performance management of groups of devices or
> > +  CPUs that run in the same performance domain. Performance domains must not
> > +  be confused with power domains. A performance domain is defined by a set
> > +  of devices that always have to run at the same performance level. For a given
> > +  performance domain, there is a single point of control that affects all the
> > +  devices in the domain, making it impossible to set the performance level of
> > +  an individual device in the domain independently from other devices in
> > +  that domain. For example, a set of CPUs that share a voltage domain, and
> > +  have a common frequency control, is said to be in the same performance
> > +  domain.
> > +
> > +  This device tree binding can be used to bind performance domain consumer
> > +  devices with their performance domains provided by performance domain
> > +  providers. A performance domain provider can be represented by any node in
> > +  the device tree and can provide one or more performance domains. A consumer
> > +  node can refer to the provider by a phandle and a set of phandle arguments
> > +  (so called performance domain specifiers) of length specified by the
> > +  \#performance-domain-cells property in the performance domain provider node.
> 
> select: true
> 
> Otherwise, this schema is never used.
> 
> > +
> > +properties:
> > +  "#performance-domain-cells":
> > +    description:
> > +      Number of cells in a performance domain specifier. Typically 0 for nodes
> > +      representing a single performance domain and 1 for nodes providing
> > +      multiple performance domains (e.g. performance controllers), but can be
> > +      any value as specified by device tree binding documentation of particular
> > +      provider.
> 
> enum: [ 0, 1 ]
> 
> If we need more, it can be extended.
> 
> > +
> > +  performance-domains:
> 
> Needs a type ref (phandle-array).
> 
> > +    description:
> > +      A phandle and performance domain specifier as defined by bindings of the
> > +      performance controller/provider specified by phandle.
> > +
> > +required:
> > +  - "#performance-domain-cells"
> > +
> > +additionalProperties: true
> > +
> > +examples:
> > +  - |
> > +    performance: performance-controller@12340000 {
> > +        compatible = "foo,performance-controller";
>
> At some point in the future, this is going to generate warnings as an
> undocumented binding. So we'll have to remove it, add a schema for it,
> or replace with a real example. This is a standard DT design pattern, so
> I'd lean toward removing the example.
>

Thanks for the review, I have fixed all other things pointed out.
I have retained the example in v2 with real compatibles as I believe
it is good to have an example for the new binding. I am happy to drop
it if it still looks wrong.

--
Regards,
Sudeep