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

Rob

> + reg = <0x12340000 0x1000>;
> + #performance-domain-cells = <1>;
> + };
> +
> + // The node above defines a performance controller that is a performance
> + // domain provider and expects one cell as its phandle argument.
> +
> + device1: foo@56780000 {
> + compatible = "foo,bar-controller";
> + reg = <0x56780000 0x1000>;
> + performance-domains = <&performance 1>;
> + };
> +
> --
> 2.25.1
>

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