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"Das U-Boot" Source Tree
Documentation: tee uclass and op-tee driver
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
authored by
Jens Wiklander
and committed by
Tom Rini
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doc/README.tee
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doc/README.tee
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=============
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TEE uclass
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=============
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This document describes the TEE uclass in U-Boot
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A TEE (Trusted Execution Environment) is a trusted OS running in some
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secure environment, for example, TrustZone on ARM CPUs, or a separate
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secure co-processor etc. A TEE driver handles the details needed to
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communicate with the TEE.
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This uclass deals with:
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- Registration of TEE drivers
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- Managing shared memory between U-Boot and the TEE
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- Providing a generic API to the TEE
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The TEE interface
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=================
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include/tee.h defines the generic interface to a TEE.
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A client finds the TEE device via tee_find_device(). Other important functions
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when interfacing with a TEE are:
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- tee_shm_alloc(), tee_shm_register() and tee_shm_free() to manage shared
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memory objects often needed when communicating with the TEE.
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- tee_get_version() lets the client know which the capabilities of the TEE
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device.
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- tee_open_session() opens a session to a Trusted Application
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- tee_invoke_func() invokes a function in a Trusted Application
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- tee_close_session() closes a session to a Trusted Application
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Much of the communication between clients and the TEE is opaque to the
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driver. The main job for the driver is to receive requests from the
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clients, forward them to the TEE and send back the results.
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OP-TEE driver
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=============
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The OP-TEE driver handles OP-TEE [1] based TEEs. Currently it is only the ARM
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TrustZone based OP-TEE solution that is supported.
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Lowest level of communication with OP-TEE builds on ARM SMC Calling
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Convention (SMCCC) [2], which is the foundation for OP-TEE's SMC interface
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[3] used internally by the driver. Stacked on top of that is OP-TEE Message
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Protocol [4].
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OP-TEE SMC interface provides the basic functions required by SMCCC and some
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additional functions specific for OP-TEE. The most interesting functions are:
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- OPTEE_SMC_FUNCID_CALLS_UID (part of SMCCC) returns the version information
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which is then returned by TEE_IOC_VERSION
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- OPTEE_SMC_CALL_GET_OS_UUID returns the particular OP-TEE implementation, used
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to tell, for instance, a TrustZone OP-TEE apart from an OP-TEE running on a
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separate secure co-processor.
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- OPTEE_SMC_CALL_WITH_ARG drives the OP-TEE message protocol
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- OPTEE_SMC_GET_SHM_CONFIG lets the driver and OP-TEE agree on which memory
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range to used for shared memory between Linux and OP-TEE.
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The GlobalPlatform TEE Client API [5] is implemented on top of the generic
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TEE API.
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Picture of the relationship between the different components in the
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OP-TEE architecture:
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U-Boot Secure world
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~~~~~~ ~~~~~~~~~~~~
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+------------+ +-------------+
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| Client | | Trusted |
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| | | Application |
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+------------+ +-------------+
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/\ /\
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|| ||
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\/ \/
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+------------+ +-------------+
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| TEE | | TEE Internal|
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| uclass | | API |
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+------------+ +-------------+
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| OP-TEE | | OP-TEE |
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| driver | | Trusted OS |
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+------------+-----------+-------------+
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| OP-TEE MSG |
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| SMCCC (OPTEE_SMC_CALL_*) |
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+--------------------------------------+
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RPC (Remote Procedure Call) are requests from secure world to the driver.
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An RPC is identified by a special range of SMCCC return values from
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OPTEE_SMC_CALL_WITH_ARG.
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References
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==========
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[1] https://github.com/OP-TEE/optee_os
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[2] http://infocenter.arm.com/help/topic/com.arm.doc.den0028a/index.html
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[3] drivers/tee/optee/optee_smc.h
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[4] drivers/tee/optee/optee_msg.h
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[5] http://www.globalplatform.org/specificationsdevice.asp look for
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"TEE Client API Specification v1.0" and click download.