Xiaomi Mi 8 (dipper) — Mainline Linux 7.1 postmarketOS Port

Mainline Linux 7.1.0-rc1 (sdm845) and postmarketOS edge port for the Xiaomi Mi 8 (xiaomi-dipper), running from the internal 128 GB UFS 2.1 storage (userdata) with full 8-core Kryo 385 CPU scaling, Samsung EA8074 AMOLED DRM/KMS display, Adreno 630 (freedreno) GPU acceleration, ST FTS5 capacitive touch, TI TAS2557 + WCD9340 audio (speaker, earpiece, and dual microphones), Bluetooth (WCN3990), and automated 60–80% USB charge-hysteresis supervision.

Companion Technical Deep-Dive: Read Bringing Up Mainline Linux 7.1 on the Xiaomi Mi 8 (dipper) Without UART for the full debugging story—including how an orphaned bootloader continuous-splash MDSS scan-out caused a 16-second interconnect sync_state() hang when entering cluster power collapse.

This physical unit has a board-level dead RF transceiver (rflm_qlnk.cpp:1082: RF stuck in QLINK start state even on stock Android). Because Snapdragon 845 runs its Wi-Fi firmware inside the modem Q6 DSP (mss), cellular and Wi-Fi are intentionally left disabled in the active Device Tree and the phone runs as a fast, low-power 6 GB RAM / 128 GB UFS Linux node over USB CDC NCM.

Hardware & Software Specifications

Subsystem Support Matrix

Three Non-Obvious Mainline Bring-Up Findings

All kernel debugging on this device was performed without UART serial, using early pstore/ramoops memory carves, an initramfs /dev/kmsg logger that survives switch_root by staging its own busybox onto the UFS cache partition (sda17), and binary-search device tree bisection.

1. TrustZone SPI GPIO Reservation (gpio-reserved-ranges)

&tlmm {
	gpio-reserved-ranges = <0 4>, <81 4>;
};

GPIOs 0..3 and 81..84 are routed to TrustZone-protected SPI controllers (fingerprint sensor and embedded Secure Element). Without gpio-reserved-ranges, pinctrl-msm probes every pin during GPIO chip registration, triggering an immediate TrustZone access violation that resets the SoC before USB or UFS ever initializes.

2. Why cluster-sleep-0 Hung the SoC 16 Seconds Into Boot (Orphaned Bootloader MDSS Scan-Out)

During initial bring-up with a minimal headless Device Tree (&mdss disabled), enabling the deep cluster idle state (cluster_sleep_0, PSCI parameter 0x4100c244) caused a deterministic hard hang ~16 seconds into every boot—even with clk_ignore_unused and pd_ignore_unused on the kernel command line.

Tracing every driver probe and sync_state callback revealed the exact trigger chain:

1. At ~16 seconds into boot, userland (mdev/udev) finishes loading all modular platform drivers.
2. Once the final consumer probes, qnoc-sdm845 (1500000.interconnect, 17900000.interconnect) invokes its sync_state() callback and releases the boot-time interconnect bandwidth votes.
3. However, the Xiaomi bootloader (XBL/ABL) leaves the MDSS display controller actively scanning out the Mi boot splash image from DDR memory (cont_splash at 0x9d400000).
4. When &mdss was disabled in Linux, no DRM driver ever took ownership of MDSS or shut down that DMA scan-out path. The moment qnoc-sdm845 dropped its boot bandwidth floors and the CPU cluster entered cluster-sleep-0 (activating the RPMh sleep set), MDSS attempted an unvoted DDR fetch and locked the SoC bus hard.

The Fix: Enabling &mdss, &mdss_mdp, &mdss_dsi0, and the generated Samsung EA8074 DRM panel driver (panel-samsung-ea8074.c) hands MDSS ownership to Linux (msm_dpu). msm_dpu properly votes for mdp0-mem interconnect bandwidth when the screen is active and cleanly shuts down the DSI/DPU pipes when the screen blanks—making cluster-sleep-0 (C1, C2, and s2idle) 100% stable with zero interconnect workarounds.

3. ST FTS5 Touch Regulator (vdd_l28) & WCD9340 Audio Capture MCLK

• Touchscreen (vdd_l28): Describing vdd_l28 (3.0 V touch avdd) as a fixed 3.3 V rail with regulator-boot-on and REGULATOR_MODE_HPM crashed RPMh before USB enumeration. Describing vdd_l28 with its true hardware range (2.856–3.312 V) and no boot-time override allowed the upstream stmfts driver (patched with .compatible = "st,stmfts5") to enable the rail cleanly at module probe time.
• Microphones (WCD9340 MCLK DAPM): Recording from the microphones initially returned all-zero PCM samples whenever speaker playback was not running simultaneously. Root cause: in sound/soc/qcom/sdm845.c, the WCD9340 MCLK supply widget is only wired to RX_BIAS (playback). Adding "ADC1", "MCLK", "ADC2", "MCLK", and "ADC3", "MCLK" to audio-routing in sdm845-xiaomi-dipper.dts—alongside AMIC1 <- MIC BIAS4 and AMIC3 <- MIC BIAS1—enabled standalone microphone capture.

Always-On USB Power & Thermal Profile

The phone is designed to stay plugged into a host over USB 24/7 without swelling the lithium-ion cell:

• charge-limit OpenRC service: Polls /sys/class/power_supply/qcom-battery/capacity every 60 seconds and toggles /sys/class/power_supply/qcom-smbchg-usb/input_suspend:
  • At >= 80% SOC: writes 1 (USBIN_SUSPEND), cutting charger current completely (status = Discharging).
  • At <= 60% SOC: writes 0, resuming charging up to 80%.
• Measured Power Draw:
  • Screen ON (idle): ~200 mA (~0.85 W)
  • Screen OFF (fb0/blank = 1): ~120 mA (~0.50 W)
  • System Suspend (s2idle): ~40 mA (~0.17 W)

Build & Flash Workflow

# 1. Compile the device tree (sdm845-xiaomi-dipper.dtb) and package boot.img
dtc -I dts -O dtb -o sdm845-xiaomi-dipper.dtb dts/sdm845-xiaomi-dipper.dts
cat vmlinuz-7.1.0-rc1-sdm845 sdm845-xiaomi-dipper.dtb > Image.gz-dtb

mkbootimg \
  --kernel Image.gz-dtb \
  --ramdisk initramfs \
  --base 0x0 --kernel_offset 0x8000 --ramdisk_offset 0x1000000 --tags_offset 0x100 --pagesize 4096 \
  --cmdline "console=tty0 root=/dev/sda21 rw rootwait clk_ignore_unused pd_ignore_unused" \
  -o boot-dipper.img

# 2. Flash to boot_a over fastboot
fastboot flash boot_a boot-dipper.img
fastboot reboot

Device Tags: