Documentation/security/tpm/tpm_ffa_crb.rst
.. SPDX-License-Identifier: GPL-2.0
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.
.. [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/