Brain (i.MX28) Linux boot flow notes

This is a consolidated boot-flow summary based on local sources:

buildbrain (boot image scripts, BrainLILO, u-boot-brain)
linux-brain kernel tree
brainux wiki pages (build + USB gadget)

1) Boot chain overview

i.MX28 ROM loads Windows CE from internal storage (vendor default).
Windows CE launches the BrainLILO app from the SD card.
BrainLILO selects a model-specific U-Boot binary from the SD card and jumps to it (soft reset / memory remap).
U-Boot loads the Linux kernel (zImage) + DTB from SD/eMMC and boots Linux with bootz.

Key locations:

BrainLILO loads from \\Storage Card\\loader\\<model>.bin (see sources/buildbrain/brainlilo/models.h).
Boot image creation logic is in sources/buildbrain/image/build_image.sh.

2) SD card layout (buildbrain convention)

From sources/buildbrain/image/build_image.sh:

Partition 1: FAT32 (~64 MiB)
- zImage and DTBs (imx28-pwsh*.dtb)
- nk/ and loader/ containing U-Boot binaries
- BrainLILO app directory (Windows CE): アプリ/Launch Linux
  - AppMain.exe (exeopener)
  - AppMain_.exe (BrainLILO.exe)
  - *.dll, index.din, AppMain.cfg
Partition 2: ext4 rootfs

3) U-Boot behavior (brain_mx28_common.h)

sources/buildbrain/u-boot-brain/include/configs/brain_mx28_common.h:

bootorder=emmc sd, sddev=1 (SD), emmcdev=0 (eMMC)
Loads zImage + fdt_file from FAT (fatload), then bootz
Optional uEnv.txt import on SD/eMMC via preboot
Default DTB is set by defconfig, e.g. CONFIG_DEFAULT_FDT_FILE=imx28-pwsh5.dtb

4) Kernel/DTB

linux-brain provides the kernel and DTBs:

Kernel: arch/arm/boot/zImage
DTB: arch/arm/boot/dts/imx28-pwsh{1..7}.dtb, imx28-pwa7200.dtb, etc.
Base DTS: arch/arm/boot/dts/imx28-brain.dtsi

Kernel version in this tree: 5.4.149 (see sources/linux-brain-brain/Makefile).

5) USB gadget (RNDIS)

Default imx28-brain.dtsi sets USB controller to host mode:

usb@80080000 { dr_mode = "host"; }

Brainux wiki describes switching to gadget mode by changing dr_mode = "peripheral" and then enabling RNDIS via configfs. A reference script/service exists in:

sources/buildbrain/os-brainux/override/usr/bin/enable_rndis_gadget
sources/buildbrain/os-brainux/override/lib/systemd/system/rndis_gadget.service

6) Recovery / USB boot

Buildbrain uses u-boot.sb for i.MX28 USB recovery mode (uuu). See sources/buildbrain/README.md and sources/buildbrain/Makefile.

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