[PATCH v3] user/exe: Add Device Tree section

[Vijay Kumar Banerjee]

---
 user/bsps/arm/beagle.rst | 21 +--------
 user/exe/device-tree.rst | 92 ++++++++++++++++++++++++++++++++++++++++
 user/exe/index.rst       |  1 +
 3 files changed, 95 insertions(+), 19 deletions(-)
 create mode 100644 user/exe/device-tree.rst

diff --git a/user/bsps/arm/beagle.rst b/user/bsps/arm/beagle.rst
index eb4ecfb..efc1267 100644
--- a/user/bsps/arm/beagle.rst
+++ b/user/bsps/arm/beagle.rst
@@ -40,25 +40,8 @@ from the libbsd HEAD of freebsd-org. For example if the HEAD is at
 Then the right Device Tree Source (DTS) file is:
 https://github.com/freebsd/freebsd/blob/19a6ceb89dbacf74697d493e48c388767126d418/sys/gnu/dts/arm/am335x-boneblack.dts
 
-.. code-block:: shell
-   :linenos:
-
-     #building the dtb
-     #We will use the script from https://github.com/freebsd/freebsd/blob/19a6ceb89dbacf74697d493e48c388767126d418/sys/tools/fdt/make_dtb.sh
-
-     #The make_dtb.sh script uses environment variable MACHINE
-     export MACHINE='arm'
-
-     SCRIPT_DIR=$HOME/freebsd/sys/tools/fdt
-
-     #The arguments to the script are
-     # $1 -> Build Tree
-     # $2 -> DTS source file
-     # $3 -> output path of the DTB file
-
-     ${SCRIPT_DIR}/make_dtb.sh ${SCRIPT_DIR}/../../ \
-     ${SCRIPT_DIR}/../../gnu/dts/arm/am335x-boneblack.dts \
-     $(pwd)
+Please refer to the :ref:`device-tree` to know more about building and applying
+the device Trees.
 
 Writing the uEnv.txt file
 -------------------------
diff --git a/user/exe/device-tree.rst b/user/exe/device-tree.rst
new file mode 100644
index 0000000..167ebdc
--- /dev/null
+++ b/user/exe/device-tree.rst
@@ -0,0 +1,92 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+.. Copyright (C) 2019 Vijay Kumar Banerjee <vijaykumar9597 at gmail.com>
+
+Device Tree
+===========
+.. index:: Device Tree
+
+A Device Tree is a data structure that is used to describe properties of
+non-discoverable hardware instead of hardcoding them on the Kernel. The device
+tree data is generally stored in a `.dts` or a Device Tree Source (DTS) file.
+This file is then compiled into a binary format called Device Tree Blob (DTB)
+with `.dtb` extension. RTEMS preferably uses DTB built from the FreeBSD source
+tree matching the freebsd-org HEAD commit hash in libBSD.
+
+Building the DTB
+----------------
+
+A single device tree source (DTS) file can be built using the `dtc` tool in
+libfdt using the following command :
+
+.. code-block:: none
+
+    dtc -@ -I dts -O dtb -o my-devicetree.dtb my-devicetree.dts
+
+For building the DTS from the FreeBSD source, the `make_dtb.sh` script
+from `freebsd/sys/tools/fdt` must be used as most of the DTS files in FreeBSD
+have included `.dtsi` files from their source tree. An example is given below as
+a reference of how to build the device tree from the FreeBSD source.
+
+`NOTE: The following example uses FreeBSD master branch from github mirror as
+an example. It is advised to always use the source from the commit matching the
+freebsd-org HEAD in libBSD.`
+
+.. code-block:: shell
+   :linenos:
+
+     #We're using the script from freebsd/sys/tools/make_dtb.sh
+     #Target device: Beaglebone Black.
+     #Architecture: Arm.
+     #DTS source name: am335x-boneblack.dts
+
+     #The make_dtb.sh script uses environment variable MACHINE
+     export MACHINE='arm'
+
+     SCRIPT_DIR=$HOME/freebsd/sys/tools/fdt
+
+     #The arguments to the script are
+     # $1 -> Build Tree (This is the path to freebsd/sys/ directory)
+     # $2 -> DTS source file
+     # $3 -> output path of the DTB file
+
+     ${SCRIPT_DIR}/make_dtb.sh ${SCRIPT_DIR}/../../ \
+     ${SCRIPT_DIR}/../../gnu/dts/arm/am335x-boneblack.dts \
+     $(pwd)
+
+Using Device Tree Overlay
+-------------------------
+
+Device tree overlay is used either to add properties or devices to the existing
+device tree. Adding any property to DTS using an overlay will override the
+current values in the DTB. The Overlays enable us to modify the device tree
+using a small maintainable plugin without having to edit the whole Base Tree.
+
+There are two ways of applying an overlay on top of the built dtb.
+
+#. Use fdtoverlay from libfdt
+
+#. Add the overlay in the root partition of the SD card and apply it using U-Boot
+
+The fdtoverlay command can be used as follows:
+
+.. code-block:: none
+
+    fdtoverlay -i my-base-tree.dtb -o output-tree.dtb my-overlay.dtbo
+
+To apply it from U-Boot during system initialization we have to add the device
+tree overlay file in the root directory of the SD card and use U-Boot commands
+to apply the overlay.
+
+Below is given the series of U-Boot commands that can be used to aplly the
+overlay, given that the overlay blob (.dtbo) file is already in the card.
+
+.. code-block:: shell
+
+    fatload mmc 0:1 0x80800000 rtems-app.img
+    fatload mmc 0:1 0x88000000 my-base-tree.dtb
+    fdt addr 0x88000000
+    fatload mmc 0:1 0x88100000 my-overlay.dtbo
+    fdt resize 0x1000
+    fdt apply 0x88100000
+    bootm 0x80800000-0x88000000
diff --git a/user/exe/index.rst b/user/exe/index.rst
index 3e9b571..e22420a 100644
--- a/user/exe/index.rst
+++ b/user/exe/index.rst
@@ -21,3 +21,4 @@ execiutable, and creating and dynamically loading code.
    initialization
    debugging
    loader
+   device-tree
-- 
2.20.1