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

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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>
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Stuart Yoder 2025-03-12 16:58:28 -05:00 committed by Jarkko Sakkinen
parent 9afb9eaa9f
commit da086d4e72
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Documentation/security/tpm/index.rst
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@ -10,3 +10,4 @@ Trusted Platform Module documentation
tpm_vtpm_proxy
xen-tpmfront
tpm_ftpm_tee
tpm_ffa_crb

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Documentation/security/tpm/tpm_ffa_crb.rst Normal file
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.. 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/