[PATCH rtems-docs v3] raspberrypi4.rst: Documentation for the new AArch64 Raspberry pi 4B BSP

[Kinsey Moore]

It seems like the typical contents for config.txt are scattered 
throughout several sections. It's good to call out the importance of 
those config options in those sections, but this isn't very useful for 
users trying to configure a system. It might be good to provide a direct 
description of the contents of the SD card and config.txt in particular 
since the values described for config.txt aren't planned to change based 
on user preferences.

Additional comments inline below.

On 10/6/2022 16:47, Mohd Noor Aman wrote:
> 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 | 104 +++++++++++++++++++++++++++++
>   user/bsps/bsps-aarch64.rst         |   1 +
>   2 files changed, 105 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..274ade9
> --- /dev/null
> +++ b/user/bsps/aarch64/raspberrypi4.rst
> @@ -0,0 +1,104 @@
> +.. 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. ILP32 is not
> +supported. Raspberry pi 4B all variants and Raspberry Pi 400  are supported. The
> +default bootloader which is used by the Raspbian OS or other OS can be used to
> +boot RTEMS. SMP is currently not supported.
> +
> +Raspberry Pi 4B has 2 types of interrupt controller, GIC-400 (GICv2) and ARM
> +legacy generic controller. Both are supported. By default, raspberrypi 4B uses
> +ARM legacy generic controller. Set ``enable_gic=1`` in the ``config.txt`` file
> +to enable GIC.
> +
> +Clock Driver
> +------------
> +
> +The clock driver uses the `ARM Generic Timer`.
> +
> +Console Driver
> +--------------
> +
> +Raspberry pi 4B has 2 types of UARTs, ARM PL011 and Mini-uart. The PL011 is a
> +capable, broadly 16550-compatible UART, while the mini UART has a reduced
> +feature set. 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. Mini-uart is not supported.
> +
> +Preparing to boot
> +------------------
> +
> +Raspberry Pi uses a different mechanism to boot. First the GPU initializes,

A different mechanism as compared to what? Is this sentence important to 
users?

Should GPU be CPU, instead?

> +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`` file.
> +
> +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 and thus disabling the
> +default Mini-uart.
> +
> +.. code-block:: none
> +
> +  dtoverlay = disable-bt
> +  enable_uart=1
> +
> +.. note::
> +  The Raspberry Pi 4B and 400 have an additional four PL011 UARTs. They are not
> +  supported.
> +
> +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