CMake buildsystem design document

Change-Id: I9b69f2731b0d43ead4cacfa9844c6137c57f5aec
Signed-off-by: Balint Dobszay <balint.dobszay@arm.com>

docs/design_documents/cmake_framework.rst

+TF-A CMake buildsystem
+======================
+
+:Author: Balint Dobszay
+:Organization: Arm Limited
+:Contact: Balint Dobszay <balint.dobszay@arm.com>
+:Status: Accepted
+
+.. contents:: Table of Contents
+
+Abstract
+--------
+This document presents a proposal for a new buildsystem for TF-A using CMake,
+and as part of this a reusable CMake framework for embedded projects. For a
+summary about the proposal, please see the `Phabricator wiki page
+<https://developer.trustedfirmware.org/w/tf_a/cmake-buildsystem-proposal/>`_. As
+mentioned there, the proposal consists of two phases. The subject of this
+document is the first phase only.
+
+Introduction
+------------
+The current Makefile based buildsystem of TF-A has become complicated and hard
+to maintain, there is a need for a new, more flexible solution. The proposal is
+to use CMake language for the new buildsystem. The main reasons of this decision
+are the following:
+
+* It is a well-established, mature tool, widely accepted by open-source
+  projects.
+* TF-M is already using CMake, reducing fragmentation for tf.org projects can be
+  beneficial.
+* CMake has various advantages over Make, e.g.:
+
+  * Host and target system agnostic project.
+  * CMake project is scalable, supports project modularization.
+  * Supports software integration.
+  * Out-of-the-box support for integration with several tools (e.g. project
+    generation for various IDEs, integration with cppcheck, etc).
+
+Of course there are drawbacks too:
+
+* Language is problematic (e.g. variable scope).
+* Not embedded approach.
+
+To overcome these and other problems, we need to create workarounds for some
+tasks, wrap CMake functions, etc. Since this functionality can be useful in
+other embedded projects too, it is beneficial to collect the new code into a
+reusable framework and store this in a separate repository. The following
+diagram provides an overview of the framework structure:
+
+|Framework structure|
+
+Main features
+-------------
+
+Structured configuration description
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+In the current Makefile system the build configuration description, validation,
+processing, and the target creation, source file description are mixed and
+spread across several files. One of the goals of the framework is to organize
+this.
+
+The framework provides a solution to describe the input build parameters, flags,
+macros, etc. in a structured way. It contains two utilities for this purpose:
+
+* Map: simple key-value pair implementation.
+* Group: collection of related maps.
+
+The related parameters shall be packed into a group (or "setting group"). The
+setting groups shall be defined and filled with content in config files.
+Currently the config files are created and edited manually, but later a
+configuration management tool (e.g. Kconfig) shall be used to generate these
+files. Therefore, the framework does not contain parameter validation and
+conflict checking, these shall be handled by the configuration tool.
+
+Target description
+^^^^^^^^^^^^^^^^^^
+The framework provides an API called STGT ('simple target') to describe the
+targets, i.e. what is the build output, what source files are used, what
+libraries are linked, etc. The API wraps the CMake target functions, and also
+extends the built-in functionality, it can use the setting groups described in
+the previous section. A group can be applied onto a target, i.e. a collection of
+macros, flags, etc. can be applied onto the given output executable/library.
+This provides a more granular way than the current Makefile system where most of
+these are global and applied onto each target.
+
+Compiler abstraction
+^^^^^^^^^^^^^^^^^^^^
+Apart from the built-in CMake usage of the compiler, there are some common tasks
+that CMake does not solve (e.g. preprocessing a file). For these tasks the
+framework uses wrapper functions instead of direct calls to the compiler. This
+way it is not tied to one specific compiler.
+
+External tools
+^^^^^^^^^^^^^^
+In the TF-A buildsystem some external tools are used, e.g. fiptool for image
+generation or dtc for device tree compilation. These tools have to be found
+and/or built by the framework. For this, the CMake find_package functionality is
+used, any other necessary tools can be added later.
+
+Workflow
+--------
+The following diagram demonstrates the development workflow using the framework:
+
+|Framework workflow|
+
+The process can be split into two main phases:
+
+In the provisioning phase, first we have to obtain the necessary resources, i.e.
+clone the code repository and other dependencies. Next we have to do the
+configuration, preferably using a config tool like KConfig.
+
+In the development phase first we run CMake, which will generate the buildsystem
+using the selected generator backend (currently only the Makefile generator is
+supported). After this we run the selected build tool which in turn calls the
+compiler, linker, packaging tool, etc. Finally we can run and debug the output
+executables.
+
+Usually during development only the steps in this second phase have to be
+repeated, while the provisioning phase needs to be done only once (or rarely).
+
+Example
+-------
+This is a short example for the basic framework usage.
+
+First, we create a setting group called *mem_conf* and fill it with several
+parameters. It is worth noting the difference between *CONFIG* and *DEFINE*
+types: the former is only a CMake domain option, the latter is only a C language
+macro.
+
+Next, we create a target called *fw1* and add the *mem_conf* setting group to
+it. This means that all source and header files used by the target will have all
+the parameters declared in the setting group. Then we set the target type to
+executable, and add some source files. Since the target has the parameters from
+the settings group, we can use it for conditionally adding source files. E.g.
+*dram_controller.c* will only be added if MEM_TYPE equals dram.
+
+.. code-block:: cmake
+
+   group_new(NAME mem_conf)
+   group_add(NAME mem_conf TYPE DEFINE KEY MEM_SIZE VAL 1024)
+   group_add(NAME mem_conf TYPE CONFIG DEFINE KEY MEM_TYPE VAL dram)
+   group_add(NAME mem_conf TYPE CFLAG KEY -Os)
+
+   stgt_create(NAME fw1)
+   stgt_add_setting(NAME fw1 GROUPS mem_conf)
+   stgt_set_target(NAME fw1 TYPE exe)
+
+   stgt_add_src(NAME fw1 SRC
+       ${CMAKE_SOURCE_DIR}/main.c
+   )
+
+   stgt_add_src_cond(NAME fw1 KEY MEM_TYPE VAL dram SRC
+       ${CMAKE_SOURCE_DIR}/dram_controller.c
+   )
+
+.. |Framework structure| image::
+   ../resources/diagrams/cmake_framework_structure.png
+   :width: 75 %
+
+.. |Framework workflow| image::
+   ../resources/diagrams/cmake_framework_workflow.png
+
+--------------
+
+*Copyright (c) 2019-2020, Arm Limited and Contributors. All rights reserved.*

docs/design_documents/index.rst

+Design Documents
+================
+
+.. toctree::
+   :maxdepth: 1
+   :caption: Contents
+   :numbered:
+
+   cmake_framework
+
+--------------
+
+*Copyright (c) 2020, Arm Limited and Contributors. All rights reserved.*

docs/index.rst

@@ -14,6 +14,7 @@ Trusted Firmware-A Documentation
    plat/index
    perf/index
    security_advisories/index
+   design_documents/index
    change-log
    change-log-upcoming
    glossary
@@ -82,7 +83,7 @@ have previously been raised against the software.
 
 --------------
 
-*Copyright (c) 2013-2019, Arm Limited and Contributors. All rights reserved.*
+*Copyright (c) 2013-2020, Arm Limited and Contributors. All rights reserved.*
 
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