MediaTek Genio boot flow: from power-on to Linux

MediaTek Genio follows the standard ARM Trusted Firmware boot architecture, but the exact layout — which partitions hold what, how device tree overlays are loaded, and how boot mode is selected — is specific to the Genio platform. Understanding it fully matters when you’re bringing up a custom carrier board or debugging a failed flash.

Key Insights

• The boot chain is four stages: BL2 (TF-A, DRAM init) → BL31 (EL3 secure monitor, stays resident) → BL32 (OP-TEE) → BL33 (U-Boot, loads Linux).
• BL31+BL32+BL33 are packaged as fip.bin in the bootloaders partition. BL2 lives in mmc0boot0, loaded by the bootrom before the partition table is accessible.
• Set boot_conf to load device tree overlays — it accepts hash-delimited DTB/DTBO paths and can be set at Yocto build time, from the U-Boot shell, or from Linux via fw_setenv.
• U-Boot reads the fused chip ID at runtime and selects the correct DTB automatically via UBOOT_CHIP_DTB_MAP — one fip.bin can cover Genio 720 and 520 on a shared carrier board.
• External 12 kΩ strapping resistors are required on MT8391. Floating strapping pins cause random boot mode selection on every power cycle.

The four-stage Genio boot chain

Genio follows the standard ARM Trusted Firmware boot architecture. Each stage runs at a specific exception level:

Power-on
    ↓
BL2 (TF-A) — mmc0boot0
    DRAM initialization, OP-TEE setup
    ↓
BL31 (Trusted Firmware) — EL3, stays resident
    Secure monitor for system lifetime
    Handles all PSCI calls from Linux
    ↓
BL32 (OP-TEE) — TrustZone EL1S
    Secure OS, Trusted Applications
    ↓
BL33 (U-Boot) — EL1
    Loads fitImage, selects DTB via chip ID
    ↓
Linux kernel — EL1

BL31 is important to understand: it stays resident at EL3 and handles every PSCI call from Linux — CPU hotplug, suspend/resume, power domain control. If your Linux system has power management issues on Genio, BL31 is worth checking.

The entire BL31+BL32+BL33 chain is packaged as fip.bin (4MB) in the bootloaders partition. BL2 lives in mmc0boot0, a separate boot area the bootrom loads before the main partition table is accessible.

Partition layout

Partition	Size	Purpose
mmc0boot0	—	BL2 (TF-A)
mmc0boot1	—	U-Boot environment + backup BL2
bootloaders	4MB	fip.bin (BL31 + BL32 + BL33)
bootloaders_b	4MB	A/B backup of bootloaders
kernel	32MB	fitImage (kernel + DTB + DTBOs)
rootfs	varies	Linux rootfs
bootassets	32MB	DTBs, overlays, board assets
EFI	100MB	EFI system partition (optional)
misc	1MB	A/B update state tracking
dramk	512KB	DRAM calibration data
firmware	32MB	SCP, SOF, PMIC firmware
firmware_b	32MB	A/B backup of firmware

A/B update covers the bootloaders and firmware partitions. The misc partition tracks which slot is active. A failed bootloader update can be recovered from the _b slot — a real advantage over single-copy bootloader designs.

How U-Boot selects the right DTB

U-Boot does runtime chip ID detection and selects the correct DTB from the fitImage automatically via UBOOT_CHIP_DTB_MAP:

UBOOT_CHIP_DTB_MAP = "mt8366:mediatek/mt8366-genio-360-evk.dtb \
                      mt8367:mediatek/mt8367-genio-360p-evk.dtb"

At boot, U-Boot reads the fused chip ID from hardware and picks the matching DTB. A single fip.bin and fitImage can cover multiple SoC variants on the same carrier board — useful for products supporting both MT8391 and MT8371 (Genio 720 and 520) on a common PCB.

Loading device tree overlays — boot_conf

DTBOs are loaded via the boot_conf U-Boot environment variable:

boot_conf=#conf-mediatek_mt8391-genio-720-evk.dtb#conf-display-dsi.dtbo#conf-camera-imx214-csi0.dtbo

Three ways to set this:

1. At Yocto build time:

KERNEL_DEVICETREE_OVERLAYS_AUTOLOAD += "display-dsi.dtbo"

2. From the U-Boot shell (persistent):

setenv boot_conf "#conf-mediatek_mt8391-genio-720-evk.dtb#conf-display-dsi.dtbo"
saveenv

3. From Linux (persistent, no U-Boot shell needed):

fw_setenv boot_conf "#conf-mediatek_mt8391-genio-720-evk.dtb#conf-display-dsi.dtbo"

fw_setenv modifies the U-Boot environment stored in mmc0boot1 — changes take effect on the next boot.

Available overlays on the Genio 720 EVK include:

• display-dsi.dtbo — MIPI DSI display
• display-hdmi.dtbo — HDMI output
• display-dpoc.dtbo — DisplayPort over Type-C (required on Genio 520/720 — no HDMI on these SoCs)
• audio-sof.dtbo — SOF audio DSP
• camera-imx214-csi0.dtbo — IMX214 on CSI0
• camera-lt6911uxe-csi0-std.dtbo — LT6911 HDMI-to-CSI serializer

Boot mode strapping on custom carrier boards

This is where carrier board bring-up often stalls. Genio latches the boot mode on the RESETB rising edge — whatever the strapping pins read at that instant is the boot mode for this power cycle.

External 12 kΩ pull-up or pull-down resistors are required. Floating strapping pins cause the SoC to read random logic levels and boot into unpredictable modes. The failure pattern: USB download mode on some power cycles, eMMC boot on others, with no obvious cause.

MT8391 (Genio 720) supports: eMMC, UFS, SD, SPI NOR, and USB download mode. The strapping combination determines which. Get the resistors in place before first power-on — debugging a floating strapping pin after assembly is a significant setback.

We cover additional carrier board hardware design gotchas in the Genio hardware design post. For any Genio BSP work from a custom carrier board, we handle this from schematic review through first boot at ProventusNova.

Frequently Asked Questions

What is the boot sequence on MediaTek Genio?

BL2 (TF-A) from mmc0boot0 initializes DRAM, BL31 sets up the EL3 secure monitor, BL32 (OP-TEE) runs the TrustZone secure OS, and BL33 (U-Boot) loads the fitImage and starts Linux.

How do I load a device tree overlay on MediaTek Genio?

Set the boot_conf U-Boot environment variable with hash-delimited DTB and DTBO paths. Set it at Yocto build time, from the U-Boot shell with setenv/saveenv, or from Linux with fw_setenv.

What does the Genio fitImage contain?

The fitImage (32MB kernel partition) contains the kernel binary, main board DTB, and any DTBOs. U-Boot selects the correct DTB at runtime from the chip ID via UBOOT_CHIP_DTB_MAP.

What is the boot mode strapping on MediaTek Genio?

Boot mode is latched from strapping pins on the RESETB rising edge. External 12 kΩ pull-up or pull-down resistors are required. Floating strapping pins produce random boot mode selection.

How does A/B update work on MediaTek Genio?

A/B update covers the bootloaders and firmware partitions. The 1MB misc partition tracks the active slot. Kernel and rootfs A/B operate independently.