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