Re: [PATCH v5 5/5] Documentation: tpm: add documentation for the CRB FF-A interface

[Jarkko Sakkinen]

On Wed, Feb 19, 2025 at 02:10:14PM -0600, Stuart Yoder wrote:
> Add documentation providing details of how the CRB driver interacts
> with FF-A.
> 
> Signed-off-by: Stuart Yoder <stuart.yoder@xxxxxxx>
> ---
>  Documentation/security/tpm/tpm_ffa_crb.rst | 65 ++++++++++++++++++++++
>  1 file changed, 65 insertions(+)
>  create mode 100644 Documentation/security/tpm/tpm_ffa_crb.rst
> 
> diff --git a/Documentation/security/tpm/tpm_ffa_crb.rst b/Documentation/security/tpm/tpm_ffa_crb.rst
> new file mode 100644
> index 000000000000..0184193da3c7
> --- /dev/null
> +++ b/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/
> -- 
> 2.34.1
> 

Reviewed-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>

BR, Jarkko