Re: [PATCH v2] Documentation: dt: Add bindings for Secure-only devices

From: Rob Herring
Date: Thu Nov 12 2015 - 16:33:53 EST

On Thu, Nov 12, 2015 at 04:24:50PM +0000, Peter Maydell wrote:
> The existing device tree bindings assume that we are only trying to
> describe a single address space with a device tree (for ARM, either
> the Normal or the Secure world). Some uses for device tree need to
> describe both Normal and Secure worlds in a single device tree. Add
> documentation of how to do this, by adding extra properties which
> describe when a device appears differently in the two worlds or when
> it only appears in one of them.
> The binding describes the general principles for adding new
> properties describing the secure world, but for now we only need a
> single new property, "secure-status", which can be used to annotate
> devices to indicate that they are only visible in one of the two
> worlds.
> The primary expected use of this binding is for a virtual machine
> like QEMU to describe the VM layout to a TrustZone aware firmware
> (which would then use the secure-only devices itself, and pass the DT
> on to a kernel running in the non-secure world, which ignores the
> secure-only devices and uses the rest).
> Signed-off-by: Peter Maydell <peter.maydell@xxxxxxxxxx>

I'd specifically like Mark's ack on this one.

> ---
> This binding doesn't affect the kernel itself, but the kernel
> Documentation/ tree is the de-facto current place where all DT
> bindings are documented, so Grant suggested this was the right
> place to send a doc patch.
> Changes v1->v2:
> * list all the status/secure-status combinations explicitly
> * use /* */ comment syntax, not //
> Documentation/devicetree/bindings/arm/secure.txt | 51 ++++++++++++++++++++++++
> 1 file changed, 51 insertions(+)
> create mode 100644 Documentation/devicetree/bindings/arm/secure.txt
> diff --git a/Documentation/devicetree/bindings/arm/secure.txt b/Documentation/devicetree/bindings/arm/secure.txt
> new file mode 100644
> index 0000000..7ed5ed6
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/arm/secure.txt
> @@ -0,0 +1,51 @@
> +* ARM Secure world bindings
> +
> +ARM CPUs with TrustZone support have two distinct address spaces,
> +"Normal" and "Secure". Most devicetree consumers (including the Linux
> +kernel) are not TrustZone aware and run entirely in either the Normal
> +world or the Secure world. However some devicetree consumers are
> +TrustZone aware and need to be able to determine whether devices are
> +visible only in the Secure address space, only in the Normal address
> +space, or visible in both. (One example of that situation would be a
> +virtual machine which boots Secure firmware and wants to tell the
> +firmware about the layout of the machine via devicetree.)
> +
> +The general principle of the naming scheme for Secure world bindings
> +is that any property that needs a different value in the Secure world
> +can be supported by prefixing the property name with "secure-". So for
> +instance "secure-reg" would override "reg". If there is no "secure-"

I'd prefer this be "secure-foo" and "foo", rather than reg given I
specifically have a differing opinion on how to support reg.

Also, would it be secure-vendor,foo or vendor,secure-foo for properties
with vendor prefix? The latter looks more correct to me, but the former
would be easier to search for both variants of the property. I'd lean
towards the latter.

> +property then the Secure world value is the same as specified for the
> +Normal world by the non-prefixed property. However, only the
> +properties listed below may validly have "secure-" versions; this list
> +will be enlarged on a case-by-case basis.
> +
> +Defining the bindings in this way means that a device tree which has
> +been annotated to indicate the presence of Secure-only devices can
> +still be processed unmodified by existing Non-secure software (and in
> +particular by the kernel).
> +
> +Note that it is still valid for bindings intended for purely Secure
> +world consumers (like kernels that run entirely in Secure) to simply
> +describe the view of Secure world using the standard bindings. These
> +secure- bindings only need to be used where both the Secure and Normal
> +world views need to be described in a single device tree.
> +
> +Valid Secure world properties:
> +
> +- secure-status : specifies whether the device is present and usable
> + in the secure world. The combination of this with "status" allows
> + the various possible combinations of device visibility to be
> + specified. If "secure-status" is not specified it defaults to the
> + same value as "status"; if "status" is not specified either then
> + both default to "okay". This means the following combinations are
> + possible:
> +
> + /* Neither specified: default to visible in both S and NS */
> + secure-status = "okay"; /* visible in both */
> + status = "okay"; /* visible in both */
> + status = "okay"; secure-status = "okay"; /* visible in both */
> + secure-status = "disabled"; /* NS-only */
> + status = "okay"; secure-status = "disabled"; /* NS-only */
> + status = "disabled"; secure-status = "okay"; /* S-only */
> + status = "disabled"; /* disabled in both */
> + status = "disabled"; secure-status = "disabled"; /* disabled in both */
> --
> 1.9.1
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