On Wed, Feb 12, 2025 at 04:05:48PM -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..c70f8904a93d > --- /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) Does that also cover ARM CCA? Just a question (not a review question). > + > +- 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_ffa_crb 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_ffa_crb 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 ffa_crb_start() funnction > + in the tpm_ffa_crb 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
