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The Build System
The LK build system thinks of the world in four layers.
Projects are what you ask make to build. They are a set of modules
(typically apps and libraries) built for a Target. They are defined
in project/<name>.mk. The Target is selected via TARGET := <name>
in this makefile or an included makefile (in the case where multiple
projects share a common configuration). You choose your project by
either passing it to make on the commandline (make <name>) or
putting a PROJECT := <name> directive in local.mk.
Targets are typically a specific PCB (or family of extremely closely
related PCBs, maybe differing just a little, detectable by some
revision resistors or the like). They belong to a Platform and are
defined in target/<name>/rules.mk. The Platform is selected via
PLATFORM := <name> in this makefile.
Platforms typically are Systems-on-Chip (either individual or related
families). They belong to an Architecture and are defined in
platform/<name>/rules.mk. The Architecture is selected via
ARCH := <name> in this makefile.
Architectures are a specific CPU or CPU family (ARM Cortex M3, Intel IA32, etc).
An example project is stm32f4-discovery-test. The Target there
is stm32f4-discovery (the ST development board of the same name).
Its Platform is stm32f4xx, a family of ST microcontrollers.
STM32_CHIP is set to stm32f407 for things that may be specific
to that chip. Some GLOBAL_DEFINES are configured here as well,
like HSE_VALUE which is the frequency of the crystal — a
platform-specific definition. The Platform stm32f4xx sets ARCH
as arm and ARM_CPU (specific to arm architecture) to cortex-m4,
as well as configuring other platform-specific values.
The above configuration information lives in the following files in
the lk source tree:
project/stm32f4-discovery-test.mk
target/stm32f4-discovery/rules.mk
platform/stm32f4xx/rules.mk
arch/arm/rules.mk