The TEE core of optee_os is provided under the BSD 2-Clause license. But there are also other software such as libraries included in optee_os. This “other” software will have different licenses that are compatible with BSD 2-Clause (i.e., non-contaminating licenses unlike GPL-v2 for example).

Build instructions

You can build the code in this git only or build it as part of the entire system, i.e. as a part of a full OP-TEE developer setup. For the latter, please refer to instructions at the build page. For standalone builds optee_os uses only regular GNU Makefiles (i.e. no CMake support here unlike the other OP-TEE gits).

Configure the toolchain

First step is to download and configure a toolchain, see the Toolchains page for instructions.

Clone optee_os

$ git clone https://github.com/OP-TEE/optee_os
$ cd optee_os

Build using GNU Make

Since optee_os supports many devices and configurations it’s impossible to give a examples to all variants. But below is how you for example would build for QEMU running Armv7-A (AArch32), with debugging enabled and the benchmark framework disabled and will put all built files in a folder name out/arm in the root of the git.

$ make \
    CROSS_COMPILE=arm-linux-gnueabihf- \
    CROSS_COMPILE_core=arm-linux-gnueabihf- \
    CROSS_COMPILE_ta_arm32=arm-linux-gnueabihf- \
    CROSS_COMPILE_ta_arm64=aarch64-linux-gnu- \
    DEBUG=1 \
    O=out/arm \

The same for an QEMU Armv8-A (AArch64) would look like this:

$ make \
    CFG_ARM64_core=y \
    CROSS_COMPILE=aarch64-linux-gnu- \
    CROSS_COMPILE_core=aarch64-linux-gnu- \
    CROSS_COMPILE_ta_arm32=arm-linux-gnueabihf- \
    CROSS_COMPILE_ta_arm64=aarch64-linux-gnu- \
    DEBUG=1 \
    O=out/arm \


To be able to see all commands when building you could build with:

$ make V=1

Build using LLVM/clang

optee_os can be compiled using llvm/clang. Start by downloading the toolchain (see LLVM / Clang). After that you can compile by running.


On line one you need to adjust the path so it matches the version of clang you are using.

$ export PATH=<optee-project>/toolchains/clang-v9.0.1/bin:$PATH
$ make COMPILER=clang

Coding standards

See Coding standards.

Build system

The build system in optee_os consists of a main Makefile in the root of the project together with sub.mk files in all source directories. In addition, some supporting files are used to recursively process all sub.mk files and generate the build rules.

Name Description
core/core.mk Included from Makefile to build the TEE Core
ta/ta.mk Included from Makefile to create the TA devkit
mk/compile.mk Create rules to make objects from source files
mk/lib.mk Create rules to make a libraries (.a)
mk/subdir.mk Process sub.mk files recursively
mk/config.mk Global configuration variable
core/arch/$(ARCH)/$(ARCH).mk Arch-specific compiler flags
core/arch/$(ARCH)/plat-$(PLATFORM)/conf.mk Platform-specific compiler flags and configuration variables
core/arch/$(ARCH)/plat-$(PLATFORM)/link.mk Make recipes to link the TEE Core
ta/arch/arm/link.mk Make recipes to link Trusted Applications
ta/mk/ta_dev_kit.mk Main Makefile to be included when building Trusted Applications
mk/checkconf.mk Utility functions to manipulate configuration variables and generate a C header file
sub.mk List source files and define compiler flags

make is always invoked from the top-level directory; there is no recursive invocation of make itself.

Choosing the build target

The target architecture, platform and build directory may be selected by setting environment or make variables (VAR=value make or make VAR=value).

ARCH - CPU architecture

$(ARCH) is the CPU architecture to be built. Currently, the only supported value is arm for 32-bit or 64-bit Armv7-A or Armv8-A. Please note that contrary to the Linux kernel, $(ARCH) should not be set to arm64 for 64-bit builds. The ARCH variable does not need to be set explicitly before building either, because the proper instruction set is selected from the $(PLATFORM) value. For platforms that support both 32-bit and 64-bit builds, CFG_ARM64_core=y should be set to select 64-bit and not set (or set to n) to select 32-bit.

Architecture-specific source code belongs to sub-directories that follow the arch/$(ARCH) pattern, such as: core/arch/arm, lib/libutee/arch/arm and so on.


$(CROSS_COMPILE) is the prefix used to invoke the (32-bit) cross-compiler toolchain. The default value is arm-linux-gnueabihf-. This is the variable you want to change in case you want to use ccache to speed you recompilations:

$ make CROSS_COMPILE="ccache arm-linux-gnueabihf-"

If the build includes a mix of 32-bit and 64-bit code, for instance if you set CFG_ARM64_core=y to build a 64-bit secure kernel, then two different toolchains are used, that are controlled by $(CROSS_COMPILE32) and $(CROSS_COMPILE64). The default value of $(CROSS_COMPILE32) is the value of CROSS_COMPILE, which defaults to arm-linux-gnueabihf- as mentioned above. The default value of $(CROSS_COMPILE64) is aarch64-linux-gnu-. Examples:

# For this example, select HiKey which supports both 32- and 64-bit builds
$ export PLATFORM=hikey

# 1. Build everything 32-bit
$ make

# 2. Same as (1.) but override the toolchain
$ make CROSS_COMPILE="ccache arm-linux-gnueabihf-"

# 3. Same as (2.)
$ make CROSS_COMPILE32="ccache arm-linux-gnueabihf-"

# 4. Select 64-bit secure 'core' (and therefore both 32- and 64-bit
# Trusted Application libraries)
$ make CFG_ARM64_core=y

# 5. Same as (4.) but override the toolchains
$ make CFG_ARM64_core=y \
       CROSS_COMPILE32="ccache arm-linux-gnueabihf-" \
       CROSS_COMPILE64="ccache aarch64-linux-gnu-"


A platform is a family of closely related hardware configurations. A platform flavor is a variant of such configurations. When used together they define the target hardware on which OP-TEE will be run.

For instance PLATFORM=stm PLATFORM_FLAVOR=b2260 will build for the ST Microelectronics 96boards/cannes2 board, while PLATFORM=vexpress PLATFORM_FLAVOR=qemu_virt will generate code for a para-virtualized Arm Versatile Express board running on QEMU.

For convenience, the flavor may be appended to the platform name with a dash, so make PLATFORM=stm-b2260 is a shortcut for make PLATFORM=stm PLATFORM_FLAVOR=b2260. Note that in both cases the value of $(PLATFORM) is stm in the makefiles.

Platform-specific source code belongs to core/arch/$(ARCH)/plat-$(PLATFORM), for instance: core/arch/arm/plat-vexpress or core/arch/arm/plat-stm.

O - output directory

All output files go into a platform-specific build directory, which is by default out/$(ARCH)-plat-$(PLATFORM).

The output directory has basically the same structure as the source tree. For instance, assuming ARCH=arm PLATFORM=stm, core/kernel/panic.c will compile into out/arm-plat-stm/core/kernel/panic.o.

However, some libraries are compiled several times: once or twice for user mode, and once for kernel mode. This is because they may be used by the TEE Core as well as by the Trusted Applications. As a result, the lib source directory gives two or three build directories: ta_arm{32,64}-lib and core-lib.

The output directory also has an export-ta_arm{32,64} directory, which contains:

  • All the files needed to build Trusted Applications.

    • In lib/: libutee.a (the GlobalPlatform Internal API), libutils.a (which implements a part of the standard C library), and other libraries which provide additional APIs.
    • In include/: header files for the above libraries
    • In mk/: ta_dev_kit.mk, which is a Make include file with suitable rules to build a TA, and its dependencies
    • scripts/sign.py: a Python script used by ta_dev_kit.mk to sign TAs.
    • In src: user_ta_header.c: source file to add a suitable header to the Trusted Application (as expected by the loader code in the TEE Core).
  • Some files needed to build host applications (using the Client API), under export-ta_arm{32,64}/host_include.

Finally, the build directory contains the auto-generated configuration file for the TEE Core: $(O)/include/generated/conf.h (see below).

Configuration and flags

The following variables are defined in core/arch/$(ARCH)/$(ARCH).mk:

  • $(core-platform-aflags), $(core-platform-cflags) and $(core-platform-cppflags) are added to the assembler / C compiler / preprocessor flags for all source files compiled for TEE Core including the kernel versions of libraries such as libutils.a.

  • $(ta_arm{32,64}-platform-aflags), $(ta_arm{32,64}-platform-cflags) and $(ta_arm{32,64}-platform-cppflags) are added to the assembler / C compiler / preprocessor flags when building the user-mode libraries (libutee.a, libutils.a) or Trusted Applications.

    The following variables are defined in core/arch/$(ARCH)/plat-$(PLATFORM)/conf.mk:

  • If $(arm{32,64}-platform-cflags), $(arm{32,64}-platform-aflags) and $(arm{32,64}-platform-cppflags) are defined their content will be added to $(\*-platform-\*flags) when they are are initialized in core/arch/$(ARCH)/$(ARCH).mk as described above.

  • $(core-platform-subdirs) is the list of the subdirectories that are added to the TEE Core.

Linker scripts

The file core/arch/$(ARCH)/plat-$(PLATFORM)/link.mk contains the rules to link the TEE Core and perform any related tasks, such as running objdump to produce a dump file. link.mk adds files to the all: target.

Source files

Each directory that contains source files has a file called sub.mk. This makefile defines the source files that should be included in the build, as well as any subdirectories that should be processed, too. For example:

# core/arch/arm/sm/sub.mk
srcs-y += sm_asm.S
srcs-y += sm.c
# core/sub.mk
subdirs-y += kernel
subdirs-y += mm
subdirs-y += tee
subdirs-y += drivers

The -y suffix is meant to facilitate conditional compilation. See section Configuration variables below.

srcs-y and subdirs-y are often not used together in the same sub.mk, because source files are usually alone in leaf directories. But this is not a hard rule.

In addition to source files, sub.mk may define compiler flags, include directories and/or configuration variables as explained below.

Compiler flags

Default compiler flags are defined in mk/compile.mk. Note that platform-specific flags must not appear in this file which is common to all platforms.

To add flags for a given source file, you may use the following variables in sub.mk:

  • cflags-<filename>-y for C files (*.c)
  • aflags-<filename>-y for assembler files (*.S)
  • cppflags-<filename>-y for both C and assembler

For instance:

# core/lib/libtomcrypt/src/pk/dh/sub.mk
srcs-y += dh.c
cflags-dh.c-y := -Wno-unused-variable

Compiler flags may also be removed, as follows:

# lib/libutils/isoc/newlib/sub.mk
srcs-y += memmove.c
cflags-remove-memmove.c-y += -Wcast-align

Some variables apply to libraries only (that is, when using mk/lib.mk) and affect all the source files that belong to the library: cppflags-lib-y and cflags-lib-y.

Include directories

Include directories may be added to global-incdirs-y, in which case they will be accessible from all the source files and will be copied to export-ta_arm{32,64}/include and export-ta_arm{32,64}/host_include.

When sub.mk is used to build a library, incdirs-lib-y may receive additional directories that will be used for that library only.

Configuration variables

Some features may be enabled, disabled or otherwise controlled at compile time through makefile variables. Default values are normally provided in makefiles with the ?= operator so that their value may be easily overridden by environment variables. For instance:


Some global configuration variables are defined in mk/config.mk, but others may be defined in sub.mk when then pertain to a specific library for instance.

Variables with the CFG_ prefix are treated in a special way: their value is automatically reflected in the generated header file $(out-dir)/include/generated/conf.h, after all the included makefiles have been processed. conf.h is automatically included by the preprocessor when a source file is built.

Depending on their value, variables may be considered either boolean or non-boolean, which affects how they are translated into conf.h.

Boolean configuration variables

When a configuration variable controls the presence or absence of a feature, y means enabled, while n, empty value or an undefined variable means disabled. For instance, the following commands are equivalent and would disable feature CFG_CRYPTO_GCM:

$ export CFG_CRYPTO_GCM=n
$ make

Configuration variables may then be used directly in sub.mk to trigger conditional compilation:

# core/lib/libtomcrypt/src/encauth/sub.mk
subdirs-$(CFG_CRYPTO_CCM) += ccm
subdirs-$(CFG_CRYPTO_GCM) += gcm

When a configuration variable is enabled (y), <generated/conf.h> contains a macro with the same name as the variable and the value 1. If it is disabled, however, no macro definition is output. This allows the C code to use constructs like:

/* core/lib/libtomcrypt/src/tee_ltc_provider.c */

/* ... */

#if defined(CFG_CRYPTO_GCM)
struct tee_gcm_state {
        gcm_state ctx;      /* the gcm state as defined by LTC */
        size_t tag_len;     /* tag length */

Non-boolean configuration variables

Configuration variables that are not recognized as booleans are simply output unchanged into <generated/conf.h>. For instance:

/* out/arm-plat-vexpress/include/generated/conf.h */

#define CFG_TEE_CORE_LOG_LEVEL 4 /* '4' */

The ‘force’ macro

Some platforms may require specific values for some of their configuration variables. For instance, the number of CPU cores in a system is fixed so the value of CFG_TEE_CORE_NB_CORE should generally not be changed. Or some feature may not be supported by the hardware, in which case the corresponding configuration variable should always be disabled.

In such cases, the force macro should be used. It sets the variable to the specified value while reporting about any conflicting value that may have been set previously, either via a previous assignment in the makefiles or via the command line or environment variables. For example:

$(call force,CFG_TEE_CORE_NB_CORE,8)
$(call force,CFG_ARM64_core,n)
$(call force,CFG_ARM_GICV3,y)
$ make -j10 PLATFORM=hikey CFG_TEE_CORE_NB_CORE=4
core/arch/arm/plat-hikey/conf.mk:5: *** CFG_TEE_CORE_NB_CORE is set to '4' (from command line) but its value must be '8'.  Stop.

Configuration dependencies

Some combinations of configuration variables may not be valid. This should be dealt with by custom checks in makefiles. mk/checkconf.h provides functions to help detect and deal with such situations.

Import branches

This section is more specifically intended for maintainers.

The optee_os repository contains branches with the import/ prefix, which we call import branches below. This section describes their purpose and how they are used.

Import branches are meant to help import external libraries into the optee_os repository and maintain them:

  • Import means copy source files from a given upstream version of the library and commit them locally (typically under optee_os/lib or optee_os/core/lib), along with OP-TEE specific changes (build and configuration files for instance)
  • Maintain means carry local bug fixes or improvements that did not make their way upstream, and periodically upgrade the library by importing changes from upstream.

Import branches have the version of the imported library in their names. For example: import/mbedtls-2.6.1. They are forked from master. They record the history of all changes made for OP-TEE to a library for a given library version. For example, the import branch for Mbed TLS 2.6.1 illustrates how the Mbed TLS library was initially imported and later modified:

$ BRANCH=github/import/mbedtls-2.6.1
$ BASE=`git merge-base master $BRANCH`
$ git log --oneline --no-merges $BASE..$BRANCH
8ff963a6 (github/import/mbedtls-2.6.1) mbedtls: fix memory leak in mpi_miller_rabin()
213cce52 libmedtls: mpi_miller_rabin: increase count limit
f934e291 mbedtls: add mbedtls_mpi_init_static()
782fddd1 libmbedtls: add mbedtls_mpi_init_mempool()
33873834 libmbedtls: make mbedtls_mpi_mont*() available
e0186224 libmbedtls: refine mbedtls license header
215609ae mbedtls: configure mbedtls to reach for config
6916dcd9 mbedtls: remove default include/mbedtls/config.h
b60fc42a Import mbedtls-2.6.1

Commit b60fc42a imports the library under lib/libmbedtls/mbedtls with no modification to the code (not the whole library is imported however, since some files are not needed they are deleted as mentioned in the commit description). Then a couple of adjustments are made in commits 6916dcd9 and 215609ae in order to be able to build with optee_os. At this point the initial import is done and subsequent commits are local improvements or bug fixes made later on.

The initial import (commits b60fc42a, 6916dcd9 and 215609ae) is merged into master as a “squashed” commit to preserve bisectability – in other words, so that no commit in master breaks the build:

$ git show --quiet 817466cb
commit 817466cb476de705a8e3dabe1ef165fe27a18c2f
Author: Jens Wiklander <jens.wiklander@linaro.org>
Date:   Tue May 22 13:49:31 2018 +0200

    Squashed commit importing mbedtls-2.6.1 source

    Squash merging branch import/mbedtls-2.6.1

    215609ae4d8c ("mbedtls: configure mbedtls to reach for config")
    6916dcd9b9cd ("mbedtls: remove default include/mbedtls/config.h")
    b60fc42a5cd5 ("Import mbedtls-2.6.1")

    Acked-by: Joakim Bech <joakim.bech@linaro.org>
    Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>

Later changes are reviewed and merged into master normally, and are also recorded on top of the import branch via pull requests against the import branch. Consider for example the two following commits, one is in the master branch and the other is applied on top of the import branch.

18c5148d357e ("mbedtls: add mbedtls_mpi_init_static()")  # In master
f934e2913b7b ("mbedtls: add mbedtls_mpi_init_static()")  # In import/mbedtls-2.6.1

The master branch is occasionally merged into the import branches. Otherwise, some patches cherry-picked from master would not apply. Note that it is a “normal” merge (not a rebase), so the commits that are on master can easily be filtered out (git log --oneline --first-parent import/...).

When it is time to upgrade a library, a new import branch is created from master, for example: import/mbedtls-2.16.0. A pull request is created against this branch with the following commits:

  • The first commit deletes the “old” version of the library and imports the new upstream version.
  • The subsequent commits are cherry-picked from the previous import branch and adjusted as needed. These commits are effectively “rebased” onto the new library.
  • Build files are updated if needed.

Here is the history of the import/mbedtls-2.16.0 branch, for comparison with the initial import:

$ BRANCH=github/import/mbedtls-2.16.0
$ BASE=`git merge-base master $BRANCH`
$ git log --oneline --no-merges $BASE..$BRANCH
68df6eb0 libmbedtls: mbedtls_mpi_exp_mod(): reduce stack usage
f58facc6 libutee: increase MPI mempool size
be040a3e libmbedtls: preserve mempool usage on reinit
ae499f6a libmbedtls: mbedtls_mpi_exp_mod() initialize W
b95a6c5d libmbedtls: fix no CRT issue
ac34734a libmbedtls: add interfaces in mbedtls for context memory operation
9ee2a92d libmbedtls: compile new files added with 2.16.0
9b0818d4 mbedtls: fix memory leak in mpi_miller_rabin()
2d6644ee libmedtls: mpi_miller_rabin: increase count limit
d831db4c libmbedtls: add mbedtls_mpi_init_mempool()
df0f4886 libmbedtls: make mbedtls_mpi_mont*() available
7b079206 libmbedtls: refine mbedtls license header
2616e2d9 mbedtls: configure mbedtls to reach for config
d686ab1c mbedtls: remove default include/mbedtls/config.h
50a57cfa Import mbedtls-2.16.0
8bfc3de4 libutee: lessen dependency on mbedtls internals

Note that the first commit 8bfc3de4 (“libutee: lessen dependency on mbedtls internals”) can be ignored, it was applied to master in anticipation of the 2.16.0 upgrade but the import/mbedtls-2.16.0 was forked before.

The upgrade from 2.6.1 to 2.16.0 is made of all the commits up to and including commit 9ee2a92d (“libmbedtls: compile new files added with 2.16.0”):

  • Commit 50a57cfa (“Import mbedtls-2.16.0”) deletes the “old” files and library imports the new ones.
  • Commits d686ab1c..9b0818d4 are cherry-picked from the previous import branch.
  • Commit 9ee2a92d (“libmbedtls: compile new files added with 2.16.0”) adapts the build files to the new version.

The master branch contains a squashed equivalent of the above:

$ git show --quiet 3d3b0591
commit 3d3b05918ec9052ba13de82fbcaba204766eb636
Author: Jens Wiklander <jens.wiklander@linaro.org>
Date:   Wed Mar 20 15:30:29 2019 +0100

    Squashed commit upgrading to mbedtls-2.16.0

    Squash merging branch import/mbedtls-2.16.0

    9ee2a92de51f ("libmbedtls: compile new files added with 2.16.0")
    9b0818d48d29 ("mbedtls: fix memory leak in mpi_miller_rabin()")
    2d6644ee0bbe ("libmedtls: mpi_miller_rabin: increase count limit")
    d831db4c238a ("libmbedtls: add mbedtls_mpi_init_mempool()")
    df0f4886b663 ("libmbedtls: make mbedtls_mpi_mont*() available")
    7b0792062b65 ("libmbedtls: refine mbedtls license header")
    2616e2d9709f ("mbedtls: configure mbedtls to reach for config")
    d686ab1c51b7 ("mbedtls: remove default include/mbedtls/config.h")
    50a57cfac892 ("Import mbedtls-2.16.0")

    Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
    Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>

Subsequent commits in the import/mbedtls-2.16.0 branch are modifications that happened later in master as a result of OP-TEE development.