Require FF-A compatibility with v1.2

[vstehle]

This is a start at adding requirements on FF-A.

We start by requiring compatibility with v1.2, the version recommended by @achingupta. This is only on AArch64 and only when FF-A is actually implemented.

In the future, we will add more FF-A requirements to support use cases such as TPM-over-FF-A, Virtio-over-FF-A for virtualized environments, etc.

[vstehle]

As of today, U-Boot wants (caller) FF-A v1.0.
Requiring (callee) FF-A v1.2 will work with U-Boot.

[xypron]

@vstehle
Adding an introductory sentence what FF-A is would improve the readability of the spec. E.g.

"The Arm Firmware Framework for Armv8-A (FF-A) specifies the management of secure partitions and APIs for the communication with these."

Including your remark concerning backwards compatibility would also be helpful.

[vstehle]

Adding an introductory sentence what FF-A is would improve the readability of the spec. E.g.

"The Arm Firmware Framework for Armv8-A (FF-A) specifies the management of secure partitions and APIs for the communication with these."

Sure @xypron; how about adding the following instead?

"The Firmware Framework for A-profile (FF-A) is an Arm software architecture describing interfaces that standardize communication between software images and firmware, in the Secure world and the Normal world."

Let me know if you are ok putting it in the note only, to keep it clearly separate from the requirements, or if you prefer it as the first sentence of the "AArch64 Firmware Framework" section instead.

Including your remark concerning backwards compatibility would also be helpful.

Ok, will do.

[vstehle]

Added a bit more details in the note regarding versions compatibility, to encompass the actual U-Boot case of caller version 1.0 and callee version 1.2.

[vstehle]

As of today EDK II requests FF-A (caller) v1.2.

[xypron]

LGTM

[vstehle]

LGTM

Thanks @xypron for the review!

[vstehle]

As of today Xen requests FF-A (caller) v1.1.
This will work with all (callee) versions compatible with (caller) v1.2.

Also, Xen implements "only" FF-A (callee) v1.1 but support for v1.2 is due soon (thanks @bertrand-marquis).

[vstehle]

I added the FF-A introductory sentence to the note.
@xypron let me please know how you like it, or if we should make it the section's introduction.

[vstehle]

Today TF-A supports FF-A (callee) v1.2.

[vstehle]

Today OP-TEE implements FF-A (callee) v1.2 but only v1.1 when Xen is there (this should change soon).
It can request FF-A (caller) v1.0..v1.2, which is ok with TF-A.

[vstehle]

Merging with changes discussed during the call of Mar 12.