realtek: add support for Ubiquiti UniFi USW Pro Max 24 PoE

realtek: add support for Ubiquiti UniFi USW Pro Max 24 PoE

Add support for RTL9302B-based Ubiquiti UniFi USW Pro Max 24 PoE switch
with 16x GbE and 8x 2.5G RJ45 ports, 2x SFP+, and a front display.

Hardware
========

  - RTL9302B switch SoC
  - 512 MiB RAM
  - 32 MiB SPI-NOR flash
  - 16x 100M/1G RJ45 ports via 2x RTL8218E
  - 8x 100M/1G/2.5G RJ45 ports via 2x RTL8224
  - PoE:
      - 400W total budget
      - 8x 802.3at, 32W per port (ports 1-8)
      - 16x 802.3bt, 60W per port (ports 9-24)
  - 2x SFP+ ports
  - Buttons: 1x Reset
  - LEDs: RGBW LED per port (Etherlighting)
  - Front touch display via USB ACM (see below)
  - Console: TTL 3.3V, 115200 8N1 (internal pin header close to SoC;
    layout front to back: VCC RX TX GND)
  - Etherlighting feature (lighting patterns and color control)
  - Vendor firmware: U-Boot + LEDE-based Ubiquiti OS

MAC address
===========

Single MAC address in EEPROM partition, applied to all ports.

Front touch display
===================

The unit has a touch-capable front display, driven by a dedicated
STM32-based MCU. Unlike other Ubiquiti switches where the MCU is
connected to the SoC via UART directly, here it is exposed as a USB
CDC-ACM serial device through an on-board Genesys Logic GL850G USB hub.
The MCU runs Ubiquiti's LCM firmware and exposes a high-level JSON
protocol (page selection, button-press events, etc.); arbitrary
pixel-level control is not possible without replacing the MCU firmware.
Display support therefore depends on both USB host support and a driver
for the LCM protocol, neither of which is currently available.

Known issues
============

  - PoE not available, depends on WIP Realtek PSE MCU driver
  - Etherlighting not controllable, driver WIP. Port LEDs for link work
    though. By default, the controller keeps the LEDs in a breathing
    state, gated by the link state delivered by the Realtek SoC.

Disclaimer
==========

Stock firmware uses a dual-bank layout (kernel0/kernel1, ~15 MiB each).
OpenWrt replaces both banks with a single contiguous firmware partition.
Flashing OpenWrt overwrites both stock kernel slots; U-Boot remains
intact and can be used for recovery.

Installation
============

1. Enable SSH on the stock UniFi OS and log in with user account.

2. Copy the OpenWrt sysupgrade image to /tmp on the switch (e.g. via
   scp).

3. Adjust IMG below to point at the copied file, then run the block as a
   whole. It writes kernel0, splits into kernel1 if the image is larger
   than that slot (otherwise invalidates kernel1 so U-Boot cannot pick
   a stale bank), and reboots:

   IMG=/tmp/openwrt-realtek-rtl930x-ubnt_usw-pro-max-24-poe-squashfs-sysupgrade.bin
   K0_BLOCKS=$((0xec0000 / 0x10000))

   dd if="$IMG" of=/dev/mtdblock2 bs=64k count=$K0_BLOCKS conv=fsync
   if [ "$(wc -c < "$IMG")" -gt $((0xec0000)) ]; then
       dd if="$IMG" of=/dev/mtdblock3 bs=64k skip=$K0_BLOCKS conv=fsync
   else
       dd if=/dev/zero of=/dev/mtdblock3 bs=64k count=1 conv=fsync
   fi
   sync
   reboot

   The switch comes up in OpenWrt after reboot.

4. It is recommended to modify the bootcmd to speed up the boot and
   prevent any issues due to the dual-boot selection. Since U-Boot by
   default uses bootubnt which does a lot of (unneeded) RTK
   initialization, quite some time passes until Linux is started.
   Additionally, the U-Boot logic fiddles with some bits on flash which
   causes JFFS2 errors in OpenWrt. While this doesn't seem to cause
   issues yet, be defensive and set the bootcmd to:

     bootm 0xb4150000

   This directly boots the uImage from flash, without doing all the
   initialization. OpenWrt is able to bootstrap the networking
   completely on its own.

It does not matter which bank stock booted from when the dd block
runs: both banks are touched in the same pass (kernel0 written, kernel1
either written or invalidated). With kernel1 invalidated, U-Boot's
internal fallback kicks in and permanently switches to kernel0 on the
next boot, so the device stays on OpenWrt as long as kernel0 is
bootable.

Recovery
========

Since the installation procedure invalidates or partially overwrites
the second bank, recovery requires serial console access (see Hardware
above for pinout).

1. Interrupt U-Boot autoboot by spamming a key during early boot to
   drop into the U-Boot prompt.

2. Bring up networking:

   rtk network on

3. Transfer an OpenWrt initramfs image via TFTP and boot it:

   tftpboot 0x82000000 <server>:<initramfs.bin>
   bootm 0x82000000

4. From the running initramfs OpenWrt, do a sysupgrade to reflash
   OpenWrt or whatever you want to recover. There is no need for the
   complicated procedure from installation since OpenWrt sees the
   firmware partition already as a whole.

Return to stock firmware
========================

There is no fully-supported revert path. The stock firmware blob is a
Ubiquiti UBNT archive (header + parts, see firmware-utils' fw.h) that
embeds a u-boot and a kernel0 uImage payload; only the latter is
relevant when writing back to the kernel partitions.

The snippet below extracts the kernel0 uImage from such a blob by
locating the uImage magic and using the size carried in the uImage
header itself, without parsing any UBNT framing. It is provided as a
best-effort starting point; verify the result before flashing,
otherwise you're on your own:

   BLOB=<firmware.bin>
   OFF=$(grep -aboF $'\x27\x05\x19\x56' "$BLOB" | head -1 | cut -d: -f1)
   SIZE=$(( $(dd if="$BLOB" bs=1 skip=$((OFF + 12)) count=4 2>/dev/null \
               | hexdump -e '1/4 "%u"') + 64 ))
   dd if="$BLOB" of=kernel0.uImage bs=1 skip="$OFF" count="$SIZE"

Once you have a clean uImage, write it to the kernel partition from
within OpenWrt. If you adjusted the bootcmd during installation, make
sure to restore it to the default "bootcmd=bootubnt". After a reboot,
Ubiquiti's firmware should boot.

Or, if you made backups of the flash before installation, just write
the backup back to flash.

Link: https://github.com/openwrt/openwrt/pull/24110
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>