U-boot adaptation guide

UNDER CONSRUCTION!

PAGE STILL UNDER CONSTRUCTION...

Basic assumptions / scope

• This guide is meant for converting a working u-boot to boot to mer / nemo from SD card or internal eMMC.
• It is assumed that you have some way of accessing u-boot console (serial / adb / etc)
• Mer / Nemo / etc image with separate FAT or Ext2 boot partition and a root partition on your boot media (SD or eMMC)
  • in case of device internal eMMC you need HW specific flashing tools to transfer the image to it

• u-boot features needed:
  • working mmc/SD card driver for your board
  • "fat load" or ext2load support (CONFIG_CMD_FAT, CONFIG_CMD_EXT2)
  • hush shell support (CONFIG_SYS_HUSH_PARSER)

Minimum path to boot

What are the absolute minimum things to get u-boot boot a linux-kernel from a SD card or eMMC for mer/nemo?

Here is the list:

• load the kernel uImage from a disk to memory
  • fat load / ext2load / mmc load
• start the kernel
  • bootm ${loadaddr}
• pass right parameters to kernel in "bootargs" environment variable
  • parameters specific for the HW (typically just re-use what is there already in bootargs)
  • Mer/Nemo root FS location
  • systemd.unit=start.getty

Hacker way

Quick & generic SD card boot

1. Boot the device to u-boot console

Then we need to setup the above minimum things in u-boot environment in place. Start with

printenv

Now you see what is the current environment, it's good idea to copy-paste this somewhere safe just in case you make a mess of it ;)

Now then, first step was to load the kernel, so we set up a generic script to do that:

setenv nemo_bootcmd 'for j in 2 1 0; do \
	echo stepj $j; \
		mmc rescan $j;
		for i in 0 1 2 3 4 5 6 7 8 9 a b c d e f 10 11 12 13 14 15 16 17 18 19; do \
			mmc rescan $j; \
			if fat load mmc $j:$i ${loadaddr} /uImage; then \
				bootm ${loadaddr}; \
			fi; \
			if fat load mmc $j:$i ${loadaddr} /boot/uImage; then \
				bootm ${loadaddr}; \
			fi; \
			if ext2load mmc $j:$i ${loadaddr} /uImage; then \
				bootm ${loadaddr}; \
			fi; \
			if ext2load mmc $j:$i ${loadaddr} /boot/uImage; then \
				bootm ${loadaddr}; \
			fi; \
			echo no device found in mmc $j partition $i; \
		done; \
	done'

TO BE CONTINUED ;)