RK3326 Device Tree Panel Debug: LCD Backlight On but Black Screen After Kernel Upgrade

Date: 2026-08-07

1. Trigger: Black Screen After Kernel Upgrade

An embedded Linux device based on RK3326 (Cortex-A35 / Mali-G31 / 640x480 MIPI-DSI) booted its stock kernel 5.10 with a working display. To upgrade the system, a new image with kernel 6.12 was written to the storage card. After power-on: backlight lit, but the screen stayed black and the system never came up.

Timeline (key: symptoms are staged)

1. After writing the image: black screen, no backlight, power LED off — thought the image was written wrong
2. After copying the bootloader region (16MB after MBR): backlight came on, but screen black, status LED off — progress, U-Boot running
3. Tried swapping device trees / configs repeatedly: backlight flickered, screen always black
4. After ruling out power issues: still black
5. The stock system (kernel 5.10) on the same card/device always lit the screen

Readout tips (the most valuable part)

• Backlight on = U-Boot running (boot chain OK)
• Status LED on = system booted deeper
• No backlight + charge LED on = power/battery issue (not the image)
• System still running after card removal (backlight controllable) = kernel/rootfs in RAM

2. Misdiagnoses: Three Wrong Assumptions

Trap 1: “Bootloader region not fully copied”

The RK3326 boot chain (idbloader/U-Boot/trust) lives in the 16MB region after MBR, before partition 1. Writing only partitions (loop0p1/p2) does cause black screen — after copying dd bs=512 skip=1 seek=1 count=32767, backlight came on, seemingly solved. But screen still black: bootloader was not the only problem.

Trap 2: “The right device tree will light it”

The new image ships 60+ DTBs (panel1-4 / type2-4 variants). Swapping all of them, even reusing the stock generic DTB, all failed. Two reasons:

• DTB is tightly bound to kernel version: stock DTB pairs with 5.10 kernel, loading it on 6.12 fails
• Panel driver ignores the DTB init sequence: new image DTB uses a custom driver (gameconsole,r36s-panel), stock DTB uses the standard driver (simple-panel-dsi)

Trap 3: “Injecting the stock init sequence into the new DTB will save it”

Injected the stock 1134-byte st7703 panel-init-sequence into the new r36s.dtb and changed compatible to simple-panel-dsi — compiled, file in place, still black. The new image’s display driver stack is completely different; changing DTB properties cannot change driver behavior.

3. Source Verification: Why Incompatible (Measured Evidence)

3.1 Boot chain location

dd if=card bs=1M skip=8 count=1 | head -c8   # should show "LOADER"
dd if=card bs=1M skip=12 count=1 | head -c8  # should show "BL3X"
dd if=image of=card bs=512 skip=1 seek=1 count=32767  # copy bootloader

3.2 DTB panel init sequence comparison (decisive)

dtc -I dtb -O dts stock.dtb > /tmp/m.dts
dtc -I dtb -O dts new-r36s.dtb > /tmp/r.dts
grep -c "panel-init-sequence" /tmp/m.dts   # stock: 1 (1134 bytes)
for f in /mnt/*.dtb; do dtc -I dtb -O dts "$f" | grep -c "panel-init-sequence"; done  # new: all 0

Stock panel node: compatible = "sitronix,st7703", "simple-panel-dsi" + panel-init-sequence + display-timings. New image panel node: compatible = "gameconsole,r36s-panel", "sitronix,st7703", no init sequence, no timings — relies on driver-built-in sequence, mismatched with this panel.

3.3 Kernel/DTB binding

strings KERNEL | grep "Linux version"   # stock 5.10.160 vs new 6.12.79

3.4 Fix attempts (all failed)

3.5 Counter-example: stock system works

Stock EmuELEC (5.10 + generic rk3326-evb-lp3-v12-linux.dtb) on the same card/device always boots with display — card, machine, power are all fine; the problem is the new image’s incompatibility with the hardware.

4. Symptom Grading (locate the fault layer fast)

5. Conclusions: Reusable Approach

Pre-flight compatibility check (in order)

1. Bootloader present: dd if=card bs=1M skip=8 | head -c8 shows LOADER, skip=12 shows BL3X
2. Panel init sequence: dtc -I dtb -O dts image.dtb | grep -c "panel-init-sequence" should be >0
3. DTB/kernel match: strings KERNEL | grep "Linux version" vs DTB source
4. Power: charge LED on with no backlight = battery, charge until LED off

Troubleshooting order

Bootloader → UUID/partitions → DTB panel sequence → finally suspect hardware. Measured here: after fixing the bootloader, U-Boot ran (backlight), but missing panel sequence + driver stack incompatibility left the screen permanently black — not fixable at config level. Abandon the image, return to stock.

Scope

• RK3326 embedded Linux devices upgrading kernels / replacing system images
• “Backlight on but screen black / status LED off” troubleshooting
• Deciding whether an image is worth further effort

Key lessons

• Stock systems (generic DTB) usually have the best compatibility
• “Backlight on” is the watershed: U-Boot is through; everything after is kernel/driver layer
• Panel driver match (compatible) and init sequence are the two requirements for LCD

6. Quick Reference

Human developers

dd if=card bs=1M skip=8 count=1 | head -c8      # check LOADER
dtc -I dtb -O dts image.dtb | grep -c "panel-init-sequence"
strings KERNEL | grep "Linux version"

Conversational AI

Question: “RK3326 device black screen after kernel upgrade, backlight on, how to debug?” Answer: backlight on = U-Boot running; decompile DTB and check panel-init-sequence (absent means driver-built-in sequence, possibly mismatched); verify DTB/kernel version pairing.

Code agents

• Readout: backlight on = U-Boot OK; status LED off + black = kernel/panel not up
• Check: dtc -I dtb -O dts X.dtb | grep -c "panel-init-sequence" should be >0; strings KERNEL | grep "Linux version"
• Stop-loss: missing panel sequence + driver incompatibility = not fixable at config level; revert to stock image

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