[PATCH rtems-docs] raspberrypi4.rst: Added Documentation for the new AArch64 Raspberry pi 4B BSP

[Mohd Noor Aman]

This patch adds the relevant documentations required for booting the new BSP. JTAG support is added for debugging. I have built the HTML docs and verified them.
---
 user/bsps/aarch64/raspberrypi4.rst | 92 ++++++++++++++++++++++++++++++
 user/bsps/bsps-aarch64.rst         |  1 +
 2 files changed, 93 insertions(+)
 create mode 100644 user/bsps/aarch64/raspberrypi4.rst

diff --git a/user/bsps/aarch64/raspberrypi4.rst b/user/bsps/aarch64/raspberrypi4.rst
new file mode 100644
index 0000000..94daacf
--- /dev/null
+++ b/user/bsps/aarch64/raspberrypi4.rst
@@ -0,0 +1,92 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+.. Copyright (C) 2022 Mohd Noor Aman
+
+.. _BSP_aarch64_Raspberrypi_4:
+
+Raspberry Pi 4B
+===============
+
+The 'raspberrypi4b' BSP currently supports only the LP64 ABI. Raspberry pi 4B
+and Raspberry Pi 400  are supported. The default bootloader which is used by the
+Raspbian OS or other OS can be used to boot the RTEMS. Currently, QEMU is not
+supported. This BSP supports the GICv2 interrupt controller.
+
+Clock Driver
+------------
+
+The clock driver uses the `ARM Generic Timer`.
+
+Console Driver
+--------------
+
+The console driver supports the default Qemu emulated ARM PL011 PrimeCell UART
+as well as the physical ARM PL011 PrimeCell UART in the raspberrypi hardware.
+Only UART0 is supported as of now.
+
+Preparing to boot
+------------------
+
+Raspberry Pi uses a different mechanism to boot. First the GPU initializes,
+loads the bootloader and then looks for the kernel img. By default the arm64
+mode looks for the ``kernel8.img``. Any other kernel can be loaded by adding
+`kernel=<img_name>` to the ``config.txt``.
+
+The Firmware files are required in order to boot RTEMS. The latest firmware can
+be downloaded from the `Raspberry Pi Firmware Repository
+<https://github.com/raspberrypi/firmware/>`_. USB boot is supported. All the
+files (Firmwares and kernel) must be place in the FAT32 partition only. Add
+``arm_64bit=1`` in the config.txt file in order to boot the BSP in 64bit kernel
+mode. 
+
+
+UART Setup
+^^^^^^^^^^
+
+Connect your serial device to the GPIO15 and GPIO14. Add the following to the
+config.txt file in order to use the PL011 UART0.
+
+.. code-block:: none
+
+  dtoverlay = disable-bt
+  enable_uart=1
+
+Generating kernel image 
+^^^^^^^^^^^^^^^^^^^^^^^
+
+The following steps show how to run ``hello.exe`` on the BSP. Other executables
+can be processed in a similar way.
+
+To create the kernel image:
+
+.. code-block:: shell
+
+  $ aarch64-rtems at rtems-ver-major@-objcopy -Obinary hello.exe kernel8.img
+
+Copy the kernel image to the SD card.
+
+JTAG Setup
+----------
+
+The Raspberry Pi 4 doesn't have dedicated JTAG pins. Instead, you must configure
+the GPIO pins (GPIO22-GPIO27) to activate the JTAG functionality. The RPi 4
+documentation refers to this as Alt4 functions of those pins. Alt5 does exist
+too, which goes from GPIO4, 5, 6, 12 and 13. you can check this out from
+`pinout.xyz <https://pinout.xyz/pinout/jtag#>`_ or `eLinux
+<https://elinux.org/RPi_BCM2835_GPIOs>`_
+
+One more thing to note on JTAG with Raspberry pi 4B is that, by default, All the
+GPIO pins are pulled down, according to the `BCM2711 documentation
+<https://datasheets.raspberrypi.com/bcm2711/bcm2711-peripherals.pdf>`_. This
+wasn't the case in the earlier models. So in order to let the data flow freely,
+we will have to disable them.
+
+.. code-block:: none
+
+  # Disable pull downs
+  gpio=22-27=np
+
+  # Enable jtag pins (i.e. GPIO22-GPIO27)
+  enable_jtag_gpio=1
+
+
diff --git a/user/bsps/bsps-aarch64.rst b/user/bsps/bsps-aarch64.rst
index 933370f..f3aa15c 100644
--- a/user/bsps/bsps-aarch64.rst
+++ b/user/bsps/bsps-aarch64.rst
@@ -9,3 +9,4 @@ aarch64 (AArch64)
 .. include:: aarch64/a72.rst
 .. include:: aarch64/xilinx-versal.rst
 .. include:: aarch64/xilinx-zynqmp.rst
+.. include:: aarch64/raspberrypi4.rst
\ No newline at end of file
-- 
2.34.1