Documentation: tpm: Add documentation for the CRB FF-A interface

Add documentation providing details of how the CRB driver interacts with
ARM FF-A.

[jarkko: Fine-tuned the commit message.]

Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Stuart Yoder <stuart.yoder@arm.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

Author: stuyod01

Documentation/security/tpm/index.rst

@@ -10,3 +10,4 @@ Trusted Platform Module documentation
    tpm_vtpm_proxy
    xen-tpmfront
    tpm_ftpm_tee
+   tpm_ffa_crb

Documentation/security/tpm/tpm_ffa_crb.rst

@@ -0,0 +1,65 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+========================
+TPM CRB over FF-A Driver
+========================
+
+The TPM Command Response Buffer (CRB) interface is a standard TPM interface
+defined in the TCG PC Client Platform TPM Profile (PTP) Specification [1]_.
+The CRB provides a structured set of control registers a client uses when
+interacting with a TPM as well as a data buffer for storing TPM commands and
+responses. A CRB interface can be implemented in:
+
+- hardware registers in a discrete TPM chip
+
+- in memory for a TPM running in isolated environment where shared memory
+  allows a client to interact with the TPM
+
+The Firmware Framework for Arm A-profile (FF-A) [2]_ is a specification
+that defines interfaces and protocols for the following purposes:
+
+- Compartmentalize firmware into software partitions that run in the Arm
+  Secure world environment (also know as TrustZone)
+
+- Provide a standard interface for software components in the Non-secure
+  state, for example OS and Hypervisors, to communicate with this firmware.
+
+A TPM can be implemented as an FF-A secure service.  This could be a firmware
+TPM or could potentially be a TPM service that acts as a proxy to a discrete
+TPM chip. An FF-A based TPM abstracts hardware details (e.g. bus controller
+and chip selects) away from the OS and can protect locality 4 from access
+by an OS.  The TCG-defined CRB interface is used by clients to interact
+with the TPM service.
+
+The Arm TPM Service Command Response Buffer Interface Over FF-A [3]_
+specification defines FF-A messages that can be used by a client to signal
+when updates have been made to the CRB.
+
+How the Linux CRB driver interacts with FF-A is summarized below:
+
+- The tpm_crb_ffa driver registers with the FF-A subsystem in the kernel
+  with an architected TPM service UUID defined in the CRB over FF-A spec.
+
+- If a TPM service is discovered by FF-A, the probe() function in the
+  tpm_crb_ffa driver runs, and the driver initializes.
+
+- The probing and initialization of the Linux CRB driver is triggered
+  by the discovery of a TPM advertised via ACPI.  The CRB driver can
+  detect the type of TPM through the ACPI 'start' method.  The start
+  method for Arm FF-A was defined in TCG ACPI v1.4 [4]_.
+
+- When the CRB driver performs its normal functions such as signaling 'start'
+  and locality request/relinquish it invokes the tpm_crb_ffa_start() funnction
+  in the tpm_crb_ffa driver which handles the FF-A messaging to the TPM.
+
+References
+==========
+
+.. [1] **TCG PC Client Platform TPM Profile (PTP) Specification**
+   https://trustedcomputinggroup.org/resource/pc-client-platform-tpm-profile-ptp-specification/
+.. [2] **Arm Firmware Framework for Arm A-profile (FF-A)**
+   https://developer.arm.com/documentation/den0077/latest/
+.. [3] **Arm TPM Service Command Response Buffer Interface Over FF-A**
+   https://developer.arm.com/documentation/den0138/latest/
+.. [4] **TCG ACPI Specification**
+   https://trustedcomputinggroup.org/resource/tcg-acpi-specification/