Date:   Mon, 9 Nov 2020 14:15:18 -0600
From:   Rob Herring <robh@...nel.org>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        Hector Yuan <hector.yuan@...iatek.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>
Subject: Re: [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@...nel.org>
> Signed-off-by: Sudeep Holla <sudeep.holla@....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@....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@...40000 {
> +        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@...80000 {
> +        compatible = "foo,bar-controller";
> +        reg = <0x56780000 0x1000>;
> +        performance-domains = <&performance 1>;
> +    };
> +
> -- 
> 2.25.1
> 

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