The Fairphone 3 as a Nerves device

The phone

The Fairphone 3 and 3+ share one mainboard and run the same firmware.

At boot the kernel detects which camera and loudspeaker modules are fitted, so one image works on both models:

{out, _} = System.cmd("dmesg", [])
out |> String.split("\n") |> Enum.filter(&(&1 =~ "fp3-module-slots"))

What you can use from Nerves

Not available yet: classic Bluetooth. The radio supports it, but this firmware drives Bluetooth with BlueHeron, which only speaks Bluetooth LE, so Bluetooth headphones, call headsets and serial adapters won't work. Wi-Fi is set up from the Wi-Fi notebook.

How it boots

The stock Fairphone bootloader only boots Android images, so the boot chain has a few extra steps:

1. Stock bootloader loads lk2nd from the Android boot partition. It first applies the device-tree overlays found in the dtbo partition; Android's overlays don't match lk2nd, so dtbo must hold a dummy overlay.
2. lk2nd is a second-stage bootloader. It reads extlinux.conf from the Nerves boot partition and starts the Linux kernel.
3. An initramfs maps the Nerves partitions (they live inside Android's userdata, see below), mounts the read-only root filesystem and hands over to it.
4. erlinit starts the Erlang VM, and your application.

Partitions

The Fairphone bootloader doesn't allow adding partitions, so the whole Nerves layout is nested inside Android's userdata partition:

userdata (/dev/mmcblk0p62)
├── firmware metadata (U-Boot environment format)
├── Boot A / Boot B     ext2, 100 MiB each: kernel, device tree, initramfs
├── Rootfs A / Rootfs B squashfs, 250 MiB each: the firmware, read-only
└── Application data    f2fs, rest of the disk, mounted at /root (= /data)

A/B slots let mix upload write the inactive slot and switch to it. Your data (notebooks, models) lives on the application partition and survives firmware updates.

Three Android partitions are mounted read-only because they hold the firmware the phone's DSPs and modem need: vendor, persist and dsp.

File.read!("/proc/mounts") |> String.split("\n") |> Enum.filter(&(&1 =~ ~r"/(root|boot|mnt)"))

First-time setup (from a stock Fairphone 3)

This wipes the phone. You need fastboot on your computer and these three files:

• lk2nd-msm8953.img from the lk2nd releases
• the dummy dtbo.img from z3ntu/dtbo-fp3
• the firmware image, built with mix firmware.image (nerves_livebook_fp3.img)

Steps:

1. In Android, enable Developer options → OEM unlocking.
2. Power the phone off, connect USB, then hold Volume Down while powering on to enter fastboot mode.
3. Run:

fastboot flashing unlock        # confirm on the phone; erases it
fastboot set_active a
fastboot flash dtbo dtbo.img
fastboot flash boot lk2nd-msm8953.img
fastboot flash userdata nerves_livebook_fp3.img
fastboot reboot

The first boot grows the data partition to fill the phone's storage. The AI notebooks download their models when you first run them, so connect the phone to Wi-Fi (Wi-Fi notebook) before trying them.

After that

• Update the firmware over the network with mix upload.
• Reach the phone at http://nerves.local:4000 (Livebook) or ssh nerves.local.
• To reflash from scratch, hold Volume Down while powering on: once lk2nd is installed, that opens lk2nd's fastboot mode. Wait until the screen turns on before pressing the button. Then repeat the fastboot flash userdata step.