Exper Easypad 8 (Model T8E) — Mainline Linux 7.2 & Alpine Bring-Up

Standalone bring-up project and reproducible build tree for running modern mainline Linux 7.2 and Alpine Linux 3.20 on the Exper Easypad 8 (Model T8E) quad-core tablet.

Internal NAND/eMMC flash remains completely untouched; the entire system boots cleanly from an external MicroSD card. Removing the SD card restores the factory stock Android system.

Hardware Specifications

Hardware Support Status

PowerVR SGX544 MP2 GPU Bring-Up on Linux 7.2

The Allwinner A31s integrates a PowerVR SGX544 MP2 rev 1.1.5 (CORE_ID 0x01191201, CORE_REVISION 0x00010105 at 0x01c40000, IRQ SPI 97, powered by AXP221 dcdc2 at 1.2 V). While mainline Linux includes the Device Tree binding (allwinner,sun6i-a31-gpu), there is no mainline kernel driver.

Key Hardware & Driver Findings

1. Bank Layout & Early Bus Hang Root Cause:
   • The SGX544 MP2 register space is divided into 0x0000–0x3fff (broadcast bank — writes broadcast to all cores, reads hang the AXI/AHB bus), 0x4000–0x7fff (MP master bank), and 0x8000 + n*0x4000 (per-core banks).
   • Early bring-up probes hung on the first register read because they read offset 0x0020 in the broadcast bank before the master bank (0x4000) and per-core clock gating controls were sequenced.
2. Clock & Power Sequencing (sunxi Platform Layer):
   • Sequencing dcdc2 (1.2 V), PRCM power-off gating release, PLL8/PLL9, and the GPU core/hyd/mem/AHB clocks brings the MP master bank (0x4010 / 0x4014) and both SGX cores online cleanly.
3. Kernel Module (pvrsrvkm) & Mesa Integration:
   • Ported the open-source eurasia_km kernel module to Mainline Linux 7.2, paired with a patched Mesa GBM/EGL bridge and an OpenRC pvr init service (modprobe pvrsrvkm + pvrsrvctl --start).
   • Cold-boot verification: kernel module loads at ~4.96 s, SGX microkernel initializes at ~7.69 s.
4. Measured GPU Benchmarks on the 1024×768 Panel:
   • kms-gles (GBM + EGL + GLES2 on sun4i-drm): 55.0 fps (vsync-locked to the 54.87 Hz LVDS panel) for single-layer rendering; 27.4 fps constant across 20 blended near-full-screen layers (~430 Mpixel/s) over a 180-second soak test (4,940 frames, 32k SGX interrupts, zero hardware recoveries/lockups).
   • glmark2-es2-drm: Completed all default benchmark scenes at 1024×768 with a score of 211 (after tuning /etc/powervr.ini ParamBufferSize=33554432 and MaxParamBufferSize=67108864 to avoid TA parameter buffer exhaustion on complex scenes).
   • Wayland (weston 10.0.1): Runs on the DRM backend using the SGX544 MP2 GL renderer with a custom gles-unpack-shim preload library providing GL_EXT_unpack_subimage; weston-simple-egl runs at ~55 fps vsync-locked with Goodix multi-touch input.

Built on This Port: Wireless Android Auto Head Unit

Repository: Mechres/exper-t8e-android-auto

Using the mainline Linux 7.2 DRM, ALSA, Wi-Fi, and Goodix touch stack from this port, the tablet also serves as a standalone wireless Android Auto head unit:

 phone (hotspot, AA head unit server :5277)
        ▲ Wi-Fi (tablet = client, phone = default gateway)
        │ TLS, Android Auto protocol
 ┌──────┴────────────────────────────────────────────────────────────┐
 │ exper-auto ── H.264 ──► ffmpeg (decode, yuv420p) ──► drm-show      │
 │     │                                    NEON bilinear scale+crop   │
 │     │ touch (Goodix GT911, evdev)         ──► DRM overlay plane ──► LVDS panel
 │     └ audio PCM ──► aplay ──► ALSA ──► 3.5 mm jack                  │
 │ exper-powerd: power key / external power (AXP221)                   │
 └─────────────────────────────────────────────────────────────────────┘

• Custom ARMv7 NEON Scaler (drm-show): Because the Allwinner A31s DE1 hardware frontend scaler (sun4i-frontend) does not produce output on this SoC, drm-show implements a hand-tuned ARMv7 NEON bilinear scaler and YUV420p-to-XRGB converter that renders 800×480 H.264 streams onto a sun4i-drm plane at ~60 fps (compared to 25–40 fps with FFmpeg swscale).
• Native 4:3 Projection: Requests a 640×480 active UI region (margin_width = 160) and scales it 1.6× to fill the 1024×768 panel without black bars or distortion.
• Automotive Power Daemon (exper-powerd): Monitors AXP221 VBUS/AC presence and the physical power key, blanking the backlight when ignition power cuts and shutting down cleanly after a configurable delay.

Boot Architecture

[BROM] → [boot0 / SPL] (MicroSD offset 8 KB)
           ↓
[U-Boot 2024.x] (MicroSD offset 40 KB, PSCI / Non-Secure HYP)
  ├── LCD init & splash screen (SimpleFB @ 1024x768x32)
  └── Loads boot.scr → sun6i-a31s-exper-t8e-drm.dtb, zImage (cma=64M)
        ↓
[Linux Kernel 7.2 Mainline] (init at ~1.8 s)
  ├── 4× Cortex-A7 (SMP), 8250_dw serial console (PH20/PH21)
  ├── AXP221s PMIC on P2WI @ 0x68 (regulators, battery, power key)
  ├── sunxi-mmc root mount (root=/dev/mmcblk0p2), mmc1 SDIO Wi-Fi (wlan0)
  ├── goodix touchscreen (I2C1 0x5d), musb-sunxi USB gadget (usb0 CDC Ethernet)
  └── sun4i-drm (/dev/dri/card0, LVDS-1 + HDMI-A-1) & pvrsrvkm (SGX544 MP2)
        ↓
[Alpine Linux 3.20 (ARMv7 OpenRC)]
  ├── /sbin/getty 115200 ttyS0
  ├── sshd listening on 172.16.42.1:22
  └── wlan0 & DRM/Wayland/Android-Auto services ready

Reproducible Build Quick Start

# 1. Fetch Bootlin ARMv7 hard-float toolchain, mainline U-Boot, and Linux 7.2
bash scripts/fetch-sources.sh

# 2. Build U-Boot SPL + u-boot-sunxi-with-spl.bin
bash scripts/25-uboot-build.sh

# 3. Build mainline Linux 7.2 kernel, sun6i-a31s-exper-t8e-drm.dtb, and modules
bash scripts/30-kernel-build.sh

# 4. Assemble the Alpine Linux 3.20 ARMv7 rootfs and write bootable MicroSD image
bash scripts/45-alpine-stage1.sh
sudo bash scripts/45-alpine-stage2.sh

Device Tags: