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build_system.md

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Build system

The OP-TEE build system is based on GNU make. It 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/$(ARCH)/plat-$(PLATFORM)/platform_flags.mk Arch and platform-specific compiler flags
core/arch/$(ARCH)/plat-$(PLATFORM)/conf.mk Arch and platform-specific compiler flags and configuration variables for the TEE Core only
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 ARMv7.

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

PLATFORM / PLATFORM_FLAVOR (hardware platform)

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=orly2 will build for the ST Microelectronics Orly2 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-orly2 is a shortcut for make PLATFORM=stm PLATFORM_FLAVOR=orly2. 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 twice: once 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 build directories: user_ta-lib and core-lib.

The output directory also has an export-user_ta 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 libmpa.a (which implements multiple precision arithmetic and is required by libutee.a).
    • 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 and user_ta_elf_arm.lds: source file and linker script 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-user_ta/host_include.

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

CROSS_COMPILE (cross-compiler selection)

$(CROSS_COMPILE) is the prefix used to invoke the 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-"

The CROSS_COMPILE variable can be overridden with CROSS_COMPILE_core and CROSS_COMPILE_user_ta for TEE Core and Trusted Applications respectively. This is needed if TEE Core and Trusted Application libraries need to be compiled with different compilers due to a mix of 64bit and 32bit code.

Platform-specific configuration and flags

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

  • $(platform-aflags), $(platform-cflags) and $(platform-cppflags) are added to the assembler / C compiler / preprocessor flags for all source files
  • $(user_ta-platform-aflags), $(user_ta-platform-cflags) and $(user_ta-platform-cppflags) are added to the assembler / C compiler / preprocessor flags when building the user-mode libraries (libutee.a, libutils.a, libmpa.a) or Trusted Applications.

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

  • $(core-platform-aflags), $(core-platform-cflags) and $(core-platform-cppflags) are added to the assembler / C compiler / preprocessor flags when building files that will run in kernel mode in the TEE Core. This applies to core-only files, but also to the kernel versions of libmpa.a and libutils.a.
  • $(core-platform-subdirs) is the list of the subdirectories that are added to the TEE Core.

Platform-specific link recipes for the TEE Core

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 Configuration 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-user_ta/include and export-user_ta/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:

PLATFORM ?= stm
PLATFORM_FLAVOR ?= default

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)/core/include/generated/conf.h, after all the included makefiles have been processed. conf.h is automatically included by the preprocessor when a source file belonging to the TEE core 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, an empty value or an undefined variable means disabled. For instance, the following commands are equivalent and would disable feature CFG_CRYPTO_GCM:

$ make CFG_CRYPTO_GCM=n
$ make CFG_CRYPTO_GCM=
$ CFG_CRYPTO_GCM=n make
$ 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 */
};
#endif

Non-boolean configuration variables

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

$ make CFG_TEE_CORE_LOG_LEVEL=4
/* out/arm-plat-vexpress/core/include/generated/conf.h */

#define CFG_TEE_CORE_LOG_LEVEL 4 /* '4' */

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.