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+=============
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+TEE uclass
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+=============
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+
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+This document describes the TEE uclass in U-Boot
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+
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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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+
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+This uclass deals with:
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+
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+- Registration of TEE drivers
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+
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+- Managing shared memory between U-Boot and the TEE
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+
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+- Providing a generic API to the TEE
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+
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+The TEE interface
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+=================
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+
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+include/tee.h defines the generic interface to a TEE.
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+
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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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+
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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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+
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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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+
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+- tee_open_session() opens a session to a Trusted Application
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+
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+- tee_invoke_func() invokes a function in a Trusted Application
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+
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+- tee_close_session() closes a session to a Trusted Application
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+
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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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+
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+OP-TEE driver
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+=============
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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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+
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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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+
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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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+
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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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+
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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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+
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+- OPTEE_SMC_CALL_WITH_ARG drives the OP-TEE message protocol
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+
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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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+
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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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+
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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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+
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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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+
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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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+
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+References
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+==========
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+
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+[1] https://github.com/OP-TEE/optee_os
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+
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+[2] http://infocenter.arm.com/help/topic/com.arm.doc.den0028a/index.html
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+
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+[3] drivers/tee/optee/optee_smc.h
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+
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+[4] drivers/tee/optee/optee_msg.h
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+
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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.
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