OP-TEE is a Trusted Execution Environment (TEE) designed as companion to a non-secure Linux kernel running on Arm; Cortex-A cores using the TrustZone technology. OP-TEE implements TEE Internal Core API v1.1.x which is the API exposed to Trusted Applications and the TEE Client API v1.0, which is the API describing how to communicate with a TEE. Those APIs are defined in the GlobalPlatform API specifications.
Getting the OP-TEE Source Code
$ git clone https://github.com/OP-TEE/optee_os.git
$ make CROSS_COMPILE64=aarch64-none-linux-gnu- PLATFORM=k3-j721e CFG_ARM64_core=y