[RG35XX Plus revised board] Built-in WiFi (RTL8821CS) never enumerates — 8821cs.ko loads, power applied, but SDIO times out (cmd 52/5 RTO)

[Sylveex]

Knulli build version

scarab 2026/05/11 (20260511) — knulli-h700-rg35xx-plus-scarab-20260511

Your architecture

Anbernic RG35XX Plus (revised H700 hardware revision)

Issue description

Built-in WiFi is completely non-functional on the revised RG35XX Plus. No wlan0 interface; Network Settings show no WiFi technology; the Wireless_Hybrid_Fix toggle has no effect.

Unlike the older report (archived knulli-cfw/GPG[4cd7b1715e27], which assumed an RTL8723DS chip + a missing r8723bs module), the actual chip here is RTL8821CS, and its driver is already present and loaded. The failure is at SDIO enumeration: the chip is powered but never answers SDIO commands, so it never appears on the bus.

Detailed reproduction steps

1. Flash knulli-h700-rg35xx-plus-scarab-20260511.img.gz to a fresh SD card and boot.
2. Network Settings → Enable WiFi → scan → "No Wi-Fi Networks found".
3. Via ADB: no wlan0; /sys/bus/sdio/devices/ is empty; rfkill shows only sunxi-bt.

Details of any attempts to fix this yourself

Diagnostics via ADB root shell:

• Chip: RTL8821CS on SDIO bus sdc1 (mmc host mmc2).
• Driver 8821cs.ko IS present and loaded (/lib/modules/4.9.170/kernel/drivers/net/wireless/rtl8821cs/8821cs.ko) — identical to the official Anbernic stock firmware.
• Power rail axp2202-cldo4 is enabled; platform device soc@03000000:wlan is bound to sunxi-wlan (DT node: compatible="allwinner,sunxi-wlan", wlan_busnum=<1>, wlan_power="axp2202-cldo4", wlan_regon=<0x4b 06 12 …>, status="okay").
• dmesg shows power is applied but the chip never answers SDIO:

sunxi-wlan soc@03000000:wlan: bus_index: 1
sunxi-wlan soc@03000000:wlan: check wlan wlan_power voltage: 3300000
sunxi-mmc sdc1: sdc set ios:clk 400000Hz ...
sunxi-mmc sdc1: smc 2 p1 err, cmd 52, RTO !!
sunxi-mmc sdc1: smc 2 p1 err, cmd 5, RTO !!    (repeated)
sunxi-mmc sdc1: ... vdd 0 ... OFF

• A runtime driver rebind (unbind/bind on sunxi-wlan) + rmmod/insmod 8821cs reproduces the exact same CMD52/CMD5 RTO — so it is not a one-shot boot-timing fluke.

Comparison with working stock firmware: The official Anbernic stock firmware V1.2.3 (RG35XX+-P-V1.2.3-EN16GB-251225) uses the same kernel (4.9.170) and the same 8821cs.ko, and has working WiFi. I extracted its device tree; the wlan node is:

wlan {
    compatible = "allwinner,sunxi-wlan";
    wlan_busnum = <0x01>;
    wlan_power = "axp2202-cldo4";
    wlan_regon = <0x4b 0x06 0x12 0x01 ...>;
    wlan_hostwake = <0x4b 0x06 0x0f 0x06 ...>;
    status = "okay";
};

Stock also loads the module late via /etc/rc.local (insmod .../8821cs.ko).

Since the driver and kernel are identical to stock and stock works, this looks like a wlan_regon/reset-timing or sunxi-mmc (sdc1) config difference in Knulli's device tree for the revised board — the chip is powered but effectively held in reset / not clocked correctly, so SDIO init times out. Request: align the revised RG35XX Plus DT / mmc (sdc1) config with stock V1.2.3 so the RTL8821CS enumerates.

Details of any modifications you have made to Knulli.

None — fresh install of Scarab. All diagnostics were gathered via ADB (USB Mode → ADB), no modifications.

Logs and data

Key dmesg lines are quoted above. Related (archived, less-accurate) report with multiple affected RG35XX Plus users: knulli-cfw/distribution#463 — note that one assumed the wrong chip (RTL8723DS / missing r8723bs); on the revised board the chip is actually RTL8821CS and its driver is already present, so the fix is a DT/mmc timing issue rather than a missing module. I have the extracted stock V1.2.3 .dtb/.dts and can provide any additional ADB diagnostics on request.

[Sylveex]

Update — ruled out the device tree.

I extracted Knulli's device tree from the SCARAB image (knulli-h700-rg35xx-plus-scarab-20260511) and diffed it against the official Anbernic stock V1.2.3 device tree. The WiFi-relevant nodes are byte-identical:

• wlan node (allwinner,sunxi-wlan): identical — same wlan_power = "axp2202-cldo4", wlan_busnum = <0x01>, wlan_regon = <0x4b 0x06 0x12 …>, wlan_hostwake = <0x4b 0x06 0x0f …>, status = "okay".
• sdc1 / sdmmc@04021000 (the WiFi SDIO controller): identical — same compatible = "allwinner,sunxi-mmc-v4p1x", clocks, max-frequency, sunxi-dly-* params, and SDIO caps (sdio-used-1v8, cap-sdio-irq, sd-uhs-*, etc.).
• No difference in any axp2202 regulator or WiFi pinctrl node.

The only DT differences between Knulli and stock are unrelated to WiFi (USB wakeup flags, SD-card slot card-detect, LCD timings, controller button ADC).

So this is not a device-tree problem. Same kernel version (4.9.170) and an identical WiFi/SDIO device tree, yet stock associates fine while Knulli times out at SDIO init (cmd 52/5 RTO) with power already applied (3.3 V). The remaining difference is the compiled driver/kernel build: Knulli's 8821cs.ko (3,889,232 bytes, built Mar 2026) differs from stock's (3,889,256 bytes, Dec 2025) — same driver, different build.

Suggested direction: compare Knulli's 8821cs driver source/version and kernel .config against Anbernic stock V1.2.3. A CMD52/CMD5 timeout with power applied and a byte-identical DT points at the driver's SDIO init / mmc-host handling (or a kernel .config/patch difference), not the device tree. I have the extracted .dtb/.dts from both builds and can share them if useful.