NixOS on SHARP Brain (i.MX28) - build guide

This directory contains a Nix-based build setup for Brain (armv5). It assumes the following repositories exist in sources/:

sources/linux-brain-brain (kernel)
sources/buildbrain (boot tooling + u-boot-brain submodule)

See docs/brain-bootflow.md for the boot chain details.

0) Model and DTB name

Pick your Brain model and DTB name from:

sources/linux-brain-brain/arch/arm/boot/dts/

Examples:

PW-SH5 -> imx28-pwsh5.dtb
PW-SH6 -> imx28-pwsh6.dtb

DTB selection is done by U-Boot via the fdtfile environment variable, which is set from CONFIG_DEFAULT_FDT_FILE in each defconfig. Build the matching U-Boot defconfig for your model (pwshX / pwa7X00).

1) U-Boot with extlinux support (fork + patch)

Patch file:

patches/u-boot-brain-extlinux.patch

Apply it to your fork of brain-hackers/u-boot-brain and build for your model defconfig (pwshX / pwa7X00).

Nix build helper (cross-compiles to armv5):

nix-build nix/uboot.nix -A pwsh6

Output:

result/u-boot.bin

Rename it for BrainLILO:

gen3_6.bin (PW-SH6)

The mapping is defined in:

sources/buildbrain/brainlilo/models.h

2) Prepare bootfiles/ (FAT partition contents)

The latest BrainLILO release has already been downloaded and extracted to:

bootfiles/アプリ/BrainLILO/

Create the following layout in bootfiles/ (PW-SH6 example):

bootfiles/
  loader/
    gen3_6.bin
  nk/                     # optional
  アプリ/BrainLILO/
    AppMain.exe
    BrainLILO.dll
    BrainLILODrv.dll
    index.din

Notes:

BrainLILO release binaries are from: https://github.com/brain-hackers/brainlilo/releases
If you prefer the buildbrain layout (アプリ/Launch Linux), use sources/buildbrain/image/build_image.sh as the reference.

3) Build the NixOS SD image

Build from this repo root:

nix-build nix/brain-system.nix -A config.system.build.sdImage

The image will be in:

result/sd-image/brain-nixos-*.img.bz2

Flash it to SD:

bzcat result/sd-image/brain-nixos-*.img.bz2 > /dev/sdX

4) Boot

Insert the SD card into the Brain.
Run BrainLILO from Windows CE (SD card app list).
U-Boot loads extlinux from the ext4 root partition.

5) USB gadget (optional)

To use USB Ethernet gadget:

Change DTB dr_mode from host to peripheral in imx28-brain.dtsi.
Enable a configfs RNDIS gadget (see docs/brain-bootflow.md).

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