realtek: add support for Ubiquiti UniFi USW Aggregation

realtek: add support for Ubiquiti UniFi USW Aggregation

Add support for the RTL9303-based Ubiquiti UniFi USW Aggregation, an
8-port 10G SFP+ aggregation switch.

Hardware
========

  - RTL9303 SoC
  - 256 MiB DDR
  - 16 MiB SPI-NOR flash
  - 8x 1G/10G SFP+ cages
  - Per-port LEDs: 1x white LED per SFP+ cage
  - Buttons: 1x Reset
  - Console: TTL 3.3V, 115200 8N1, internal unpopulated 4-hole THT
    footprint (the device must be opened to solder a header)
    - pinout (with the front panel facing you, left to right):
      VCC/unused, RX, TX, GND
  - Front touch display (see below)
  - Software chain:
    - U-Boot (Ubiquiti-flavoured)
    - UniFi OS (OpenWrt-based)

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

Single MAC address derived from the eeprom partition. Applied to all
switch ports.

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

The unit has a touch-capable front display, driven by a dedicated
STM32-based MCU connected to the host via UART. 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. The display is therefore
not supported beyond what the stock LCM firmware offers.

Disclaimer
==========

Stock uses a dual-bank layout (kernel0/kernel1, 7 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.

The stock firmware blob is RSA-signed and cannot be flashed via the
UniFi web UI. Installation has to be done from a root shell on the
running UniFi OS.

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

1. Enable SSH on the stock UniFi OS and log in as root.

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-aggregation-squashfs-sysupgrade.bin
   K0_BLOCKS=$((0x710000 / 0x10000))

   dd if="$IMG" of=/dev/mtdblock2 bs=64k count=$K0_BLOCKS conv=fsync
   if [ "$(wc -c < "$IMG")" -gt $((0x710000)) ]; then
       dd if="$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.

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, re-run the installation
   procedure above (the dd block, with $IMG pointing at the image on
   /tmp).

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=US.rtl930x_X.Y.Z.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 both kernel banks (since
the bootselect mechanism is not yet decoded, this guarantees U-Boot
picks the stock image regardless of bank):

   dd if=kernel0.uImage of=/dev/mtdblock2 bs=64k conv=fsync
   dd if=kernel0.uImage of=/dev/mtdblock3 bs=64k conv=fsync

Link: https://github.com/openwrt/openwrt/pull/23506
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>