From 857f3cb52712538a9341796f8b9cb1f76225ab64 Mon Sep 17 00:00:00 2001
From: David Beck <david.beck@arm.com>
Date: Fri, 28 Sep 2018 16:19:47 +0100
Subject: IVGCVSW-1935 : inline code documentation for pluggable backends

Change-Id: I2a46e4dfffcc103bf228aa161bb2fba8f259076a
---
 README.md              |  2 ++
 src/backends/README.md | 81 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 83 insertions(+)
 create mode 100644 src/backends/README.md

diff --git a/README.md b/README.md
index bb016d39a9..3aa3a8c17f 100644
--- a/README.md
+++ b/README.md
@@ -10,6 +10,8 @@ There is a getting started guide here using Caffe: <https://developer.arm.com/te
 
 There is a getting started guide here using ONNX: [ONNX Support](src/armnnOnnxParser/README.md)
 
+There is a guide for backend development: [Backend development guide](src/backends/README.md)
+
 ### Build Instructions
 
 Arm tests the build system of Arm NN with the following build environments:
diff --git a/src/backends/README.md b/src/backends/README.md
new file mode 100644
index 0000000000..09b9e81539
--- /dev/null
+++ b/src/backends/README.md
@@ -0,0 +1,81 @@
+# Backend developer guide
+
+ArmNN allows adding new backends through the 'Pluggable Backend' mechanism.
+
+## How to add a new backend
+
+Backends reside under src/backends in separate subfolders. They must have a ```backend.cmake``` file
+which is read automatically by [src/backends/backends.cmake](backends.cmake). The ```backend.cmake``` file
+under the backend specific folder is then included by the main CMakeLists.txt file at the root of the
+ArmNN source tree.
+
+### The backend.cmake file
+
+The ```backend.cmake``` has two main purposes:
+
+1. It makes sure the artifact (typically a static library) is linked into the ArmNN shared library.
+2. It makes sure that the subdirectory where backend sources reside gets included in the build.
+
+To achieve this there are two requirements for the ```backend.cmake``` file
+(example taken from [reference/backend.cmake](reference/backend.cmake))
+
+```cmake
+#
+# Make sure the reference backend is included in the build.
+# By adding the subdirectory, cmake requires the presence of CMakeLists.txt
+# in the reference (backend) folder.
+#
+add_subdirectory(${PROJECT_SOURCE_DIR}/src/backends/reference)
+
+#
+# Add the static libraries built by the reference backend to the
+# list of libraries linked against the ArmNN shared library.
+#
+list(APPEND armnnLibraries armnnRefBackend armnnRefBackendWorkloads)
+```
+
+### The CMakeLists.txt file
+
+As described in the previous section, adding a new backend will require creating a CMakeLists.txt in
+the backend folder. This follows the standard cmake conventions, and is required to build an artifact
+to be linked into the ArmNN shared library. As with any cmake build, the code can be structured into
+subfolders and modules as the developer sees fit.
+
+Example can be found under [reference/CMakeLists.txt](reference/CMakeLists.txt).
+
+### The backend.mk file
+
+ArmNN on Android uses the native Android build system. New backends are integrated by creating a
+```backend.mk``` file which has a single variable called ```BACKEND_SOURCES``` listing all cpp
+files to be built by the Android build system.
+
+Example taken from [reference/backend.mk](reference/backend.mk):
+
+```make
+BACKEND_SOURCES := \
+        RefLayerSupport.cpp \
+        RefWorkloadFactory.cpp \
+        workloads/Activation.cpp \
+        workloads/ArithmeticFunction.cpp \
+        workloads/Broadcast.cpp \
+        ...
+```
+
+## How to add common code across backends
+
+For multiple backends that need common code, there is support for including them in the build
+similarly to the backend code. This requires adding three files under a subfolder at the same level
+as the backends folders. These are:
+
+1. common.cmake
+2. common.mk
+3. CMakeLists.txt
+
+They work the same way as the backend files. The only difference between them is that
+common code is built first, so the backend code can depend on them.
+
+[aclCommon](aclCommon) is an example for this concept and you can find the corresponding files:
+
+1. [aclCommon/common.cmake](aclCommon/common.cmake)
+2. [aclCommon/common.mk](aclCommon/common.mk)
+3. [aclCommon/CMakeLists.txt](aclCommon/CMakeLists.txt)
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