add text for downgrading firmware

arednHow-toGuides/firmware_tips.rst

@@ -25,15 +25,18 @@ Web browser cache and sessions
 PXE Server
   If you are using a `PXE <https://en.wikipedia.org/wiki/Preboot_Execution_Environment>`_ server to provide your device with an IP address and a new firmware image, be sure to allow the PXE server through your computer's firewall. If the PXE server does not display any activity when you begin your firmware install, check your firewall settings.  On the Windows control panel, for example, click *Advanced Settings* and look through the "Inbound Rules" to see if a rule exists for the PXE server. If a rule exists, make sure to "allow connection" for both private and public networks. If no rule exists, create a new rule allowing connection for both public and private networks.
 
-Tips for Upgrading
-------------------
+Tips for Upgrading Firmware
+---------------------------
 
 Upgrading an AREDN |trade| node is accomplished using the *Setup > Administration > Firmware Update* feature on the node's web interface. Follow the procedures documented in the **Downloading AREDN Firmware** section to ensure you have the correct firmware version from the AREDN |trade| website to install on your node.
 
-In rare cases the upgrade process can fail due to lack of memory, but such a failure will leave the node running its previous firmware version. The following tips help ensure that memory utilization is at a minimum on the node.
+In rare cases the upgrade process can fail due to lack of node resources, but such a failure will leave the node running its previous firmware version. The following tips help ensure that memory utilization is at a minimum on the node.
+
+Try to Load Local Firmware
+  The **Load Local Firmware** option is described in the *Configuration Deep Dive > Administration* documentation. This involves using a file copy utility on your computer to copy the firmware file to a specific directory and filename on your node. Once the new firmware file is available on the node, you can click the *Load Local Firmware* button to start the install process.
 
 Release node resources
-  Before starting the firmware upgrade on low memory devices, it may be necessary to stop, disable, or uninstall extra packages such as Meshchat, snmp, and tunneling. The goal of this step is to keep those processes from using RAM memory and to free as much RAM as possible before the upgrade. Rebooting the node before beginning the upgrade will ensure that RAM utilization is at a minimum.
+  Before starting the firmware upgrade on low memory devices, it may be necessary to stop, disable, or uninstall extra packages such as MeshChat, SNMP, and IperfSpeed. The goal of this step is to keep those processes from using RAM memory and to free as much RAM as possible before the upgrade. Rebooting the node before beginning the upgrade will ensure that RAM utilization is at a minimum.
 
   You may also want to stop node programs or services that are not needed during the upgrade. For example, you can telnet or ssh to the node and type the command ``wifi down`` to free the memory used by this driver.
 
@@ -58,3 +61,24 @@ Tips for legacy nodes with low memory (32mb)
     To transfer the image from a Windows computer you can use a *Secure Copy* program such as `WinSCP <https://winscp.net>`_. Then use a terminal program such as `PuTTY <https://www.chiark.greenend.org.uk/~sgtatham/putty/>`_ to connect to the node via ssh or telnet in order to run the sysupgrade command shown as the last line above.
 
   - As a last resort, use the TFTP procedure to load the *factory.bin* firmware image to the node. This procedure is described in the *First Install* sections of **Installing AREDN Firmware**.
+
+Tips for Downgrading Firmware
+-----------------------------
+
+Downgrading AREDN |trade| firmware is typically accomplished using the same procedure as for uploading firmware to your node. You are simply uploading a previous version of the firmware rather than the latest version.
+
+However, there is a difference if you are downgrading the firmware on a node which previously used a different target architecture. As explained in the **Downloading AREDN Firmware** section, the legacy ``ar71xx`` target has been retired and replaced by the ``ath79`` target. For example, you may have a node that was previously running an ``ar71xx`` firmware version but you installed the latest Nightly Build which upgraded it to an ``ath79`` firmware target. In this case you will need to do a fresh "first install" using the legacy architecture's firmware.
+
+1. Use the `AREDN Download website <http://downloads.arednmesh.org/firmware/html/stable.html>`_ to download the previous release's install files. For example, if your Ubiquiti Rocket M5 XW is currently running the Nightly Build which uses ``ath79``, then download the *factory* file for the a previous Stable Release (which uses ``ar71xx``) as shown below.
+
+  .. image:: _images/downgrade.png
+     :alt: Downgrading across target architectures
+     :align: center
+
+2. Review the **Installing AREDN Firmware** documentation and follow the steps for the *First Install* procedure that is appropriate for your node model.
+
+  - For Ubiquiti and TP-LINK models you will be uploading the *factory* firmware.
+  - For Mikrotik models you will boot using the *vmlinux-initramfs.elf* file (which you rename to *rb.elf*) and then immediately apply the *sysupgrade.bin* firmware image.
+  - For GL.iNet models you will use the `recovery procedure <https://docs.gl-inet.com/en/3/tutorials/debrick/>`_ to upload the *sysupgrade.bin* firmware image.
+
+After downgrading your node's firmware you will then continue the process for entering your callsign and configuring the node's settings, as explained in the **Basic Setup** section.