sdlamp2/docs/rg35xx-plus.md

Anbernic RG35XX Plus

Device-specific reference for the target hardware. For build instructions see TOOLCHAIN.md.

Hardware

• Device: Anbernic RG35XX Plus
• SoC: Allwinner H700 — quad-core ARM Cortex-A53 @ 1.5 GHz (AArch64 / ARMv8-A)
• GPU: Mali G31 MP2
• RAM: 1 GB LPDDR4
• Display: 640×480 IPS

Software

• OS: Ubuntu 22.04 LTS (Jammy Jellyfish) — modified stock firmware
• Kernel: Linux 4.9.170 aarch64 (proprietary, no source released by Anbernic)
• Architecture: aarch64 / arm64 (confirmed via dpkg --print-architecture)
• glibc: 2.35
• Userland: Debian/Ubuntu-based (dpkg, apt-get, systemd)

Libraries on Device

All required shared libraries are pre-installed. Most are at /usr/lib/, some at /usr/lib/aarch64-linux-gnu/ (Debian multiarch path):

Library	Soname	Notes
SDL2	libSDL2-2.0.so.0.12.0	SDL 2.0.12
SDL2_image	libSDL2_image-2.0.so.0.900.0	SDL2_image 2.0.9, at multiarch path
libavcodec	libavcodec.so.58	FFmpeg ~4.x
libavformat	libavformat.so.58	FFmpeg ~4.x
libavutil	libavutil.so.56	FFmpeg ~4.x
libswresample	libswresample.so.3	FFmpeg ~4.x

Shared libraries are already present — no need to bundle or build them. A native aarch64 compile (e.g. inside the arm64 Docker container) produces working binaries. Must link against glibc ≤ 2.35.

Input Devices

Three input devices are registered via /proc/bus/input/devices:

Device	Handlers	Purpose
axp2202-pek	event0	Power button (KEY_POWER, code 116)
ANBERNIC-keys	event1, js0	Gamepad (d-pad, face buttons)
dierct-keys-polled	event2	Shoulder buttons, menu/function keys

• logind: HandlePowerKey=ignore in /etc/systemd/logind.conf — systemd does not act on the power button, leaving it free for userspace handling.
• evtest: Available at /usr/bin/evtest for debugging input events.

Partition Layout

Device	Mount point	Filesystem	Contents
/dev/mmcblk0p5	/	ext4	Root filesystem
/dev/mmcblk0p6	/mnt/vendor	ext4	Firmware, apps, scripts
/dev/mmcblk0p7	/mnt/data	ext4	User data
/dev/mmcblk0p8	/mnt/mmc	vfat	ROMs
SD card slot	/mnt/sdcard	(varies)	External storage

Boot Chain

The device uses systemd but boots its UI through a SysV init script that systemd auto-wraps via systemd-sysv-generator. The chain is intentionally layered: launcher.sh runs before partitions are mounted (hardware setup), loadapp.sh runs after mounts (filesystem/network setup + app restart loop), and dmenu_ln is the app dispatcher.

systemd (graphical.target)
  └─ launcher.service  (SysV → auto-generated by systemd-sysv-generator)
       └─ /etc/init.d/launcher.sh start
            ├── mounts /mnt/vendor
            ├── starts brightCtrl.bin (backlight daemon) &
            ├── starts cexpert &
            └── starts loadapp.sh &
                 ├── resizes root partition (first boot only)
                 ├── mounts /mnt/data, /mnt/mmc
                 ├── charging mode: if boot_mode==1 → charger UI → poweroff
                 ├── normal mode: starts NetworkManager, bluetooth, wifi
                 ├── syncs RTC, loads kernel modules, mounts SD card
                 ├── runs /mnt/mod/ctrl/autostart (if exists)
                 └── RESTART LOOP: runs /mnt/vendor/ctrl/dmenu_ln repeatedly
                      └─ /mnt/vendor/ctrl/dmenu_ln
                           ├── selects binary (default: dmenu.bin)
                           ├── config overrides: muos.ini → muos.bin, vpRun.ini → ...
                           ├── runs binary via app_scheduling()
                           └── on exit: executes /tmp/.next (app dispatch for menu)

Why the indirection?

• launcher.sh — Pre-mount hardware setup: mounts /mnt/vendor, starts backlight and system daemons. Runs as a SysV init script so it integrates with systemd's boot ordering.
• loadapp.sh — Post-mount filesystem/network setup: mounts data partitions, handles charging mode, starts networking, then enters the restart loop. Runs in the background (&) so launcher.sh can return.
• dmenu_ln — App dispatcher: selects which binary to run based on config files, wraps execution in app_scheduling() (which sleeps 30s on crash before retrying), and supports /tmp/.next for menu-driven app switching.

app_scheduling behavior

The app_scheduling function in dmenu_ln runs the selected binary. If it exits with a non-zero status, it sleeps 30 seconds before returning, which prevents crash loops from consuming all CPU. The outer while true loop in loadapp.sh then re-invokes dmenu_ln, restarting the application.

Framebuffer

Property	Value
Device	/dev/fb0
Visible size	640x480
Virtual size	640x960 (double-buf)
Bits per pixel	32 (BGRA)
Stride	2560 bytes (640 * 4)

No standard framebuffer image tools (fbv, fbi, psplash) are installed. To display a PNG on the framebuffer, decode with Python3+PIL and write raw BGRA pixels to /dev/fb0.

Backlight Control

There is no sysfs backlight interface (/sys/class/backlight/ is empty) and the stock brightCtrl.bin daemon does not expose a usable control mechanism. The backlight is controlled via the Allwinner /dev/disp ioctl interface:

Ioctl	Number	Description
DISP_SET_BRIGHTNESS	0x102	Set backlight brightness (0–255)
DISP_GET_BRIGHTNESS	0x103	Get current backlight brightness

Both ioctls take an argument buffer of 4 unsigned longs (struct { unsigned long args[4]; }):

• args[0] = screen index (always 0)
• args[1] = brightness value (for SET; ignored for GET)
• Return value (for GET) is in args[0] after the ioctl call

Setting brightness to 0 turns the screen off completely. The original value (typically ~50) can be restored to turn it back on. This is used by rg35xx-screen-monitor.py for idle timeout and power button toggle.

Stock Firmware Assets

Shutdown-related assets in /mnt/vendor/res1/shutdown/:

File	Description	Size
goodbye.png	Shutdown screen (RGB, 640x480)	Matches display
lowpower.png	Low battery warning	—

The boot logo is at /mnt/vendor/res1/boot/logo.png.

Deploying sdlamp2

Overview

The dmenu_ln script already supports switching the startup binary via config files (e.g. muos.ini → muos.bin). We follow the same pattern to launch sdlamp2 instead of the default menu.

Setup

1. Copy the binary, wrapper, and screen monitor to the device:

   scp build/sdlamp2 root@rg35xx:/mnt/vendor/bin/sdlamp2
   scp device/rg35xx/rg35xx-wrapper.sh root@rg35xx:/mnt/vendor/bin/rg35xx-wrapper.sh
   scp device/rg35xx/rg35xx-screen-monitor.py root@rg35xx:/mnt/vendor/bin/rg35xx-screen-monitor.py
   

2. Add the config check to /mnt/vendor/ctrl/dmenu_ln. In the section where CMD overrides are checked (after the existing muos.ini / vpRun.ini checks, before the app_scheduling call), add:

   if [ -f "/mnt/vendor/sdlamp2.ini" ];then
       CMD="/mnt/vendor/bin/rg35xx-wrapper.sh"
   fi
   

   The wrapper script handles device-specific concerns (WiFi hotspot, power button monitoring) and launches sdlamp2 as its main foreground process. See device/rg35xx/rg35xx-wrapper.sh for details.

3. Enable sdlamp2 on boot:

   touch /mnt/vendor/sdlamp2.ini
   

4. Disable (revert to normal menu):

   rm /mnt/vendor/sdlamp2.ini
   

What's preserved

Everything else in the boot chain continues to work:

• Charging mode — handled in loadapp.sh before the restart loop
• LED/backlight control — brightCtrl.bin started by launcher.sh
• Clean shutdown — sdlamp2 handles SIGTERM/SIGINT, saving position and volume before exit. The wrapper displays the stock goodbye.png on the framebuffer and calls poweroff
• Restart on exit — if sdlamp2 exits cleanly (status 0), the restart loop in loadapp.sh re-launches it immediately
• Crash recovery — if sdlamp2 crashes (non-zero exit), app_scheduling sleeps 30s then the loop retries
• Easy revert — removing sdlamp2.ini restores the stock menu on next boot