How to access the boot menu, determine the boot order, relfash U-Boot, ...
The GTA04's new default "U-Boot" boot system (as of Oct. 2012) provides a graphical multi-boot selection and a very flexible way to multi-boot up to 5 systems installed on the device (p1, p2, p3, p4 on the uSD card and NAND flash): http://lists.goldelico.com/pipermail/gta04-owner/2012-October/003235.html
Therefore U-Boot is looking in different places to find the systems installed.
It is looking for a kernel (uImage) and/or a bootargs.scr script (which can be used to further tweak the booting process, e.g. to use custom boot parameters) on the microSD card (fat32 or ext[4,3,2] filesystems) in the following default boot order. This boot order can be modified by using the graphical boot menu (see below).
- Partition 1 (p1)
- Partition 2 (p2)
- Partition 3 (p3)
- Partition 4 (p4)
- NAND flash
If a kernel is found in the root of a partition (e.g. /uImage), it is assumed that the rootfs will be on partition 2 (e.g. root=/dev/mmcblk0p2).
This is used for legacy mode booting (e.g. uImage on fat32 partitioned p1 and rootfs on ext3 partitioned p2).
If a kernel is found in the /boot directory of a partition (e.g. /boot/uImage), it is assumed that the rootfs will be on this very same partition.
If no kernel is found at all, it will fall back to the kernel (pre-)installed in the NAND flash.
If a bootargs.scr script was found in one of the places, it is beeing executed. Otherwise the (pre-)installed bootargs from NAND flash will be used.
It is possible to have a bootargs.scr script but no kernel installed. In this case bootargs.scr will be executed, before booting the fallback kernel from the NAND flash.
If neither a kernel nor bootargs were found, the U-Boot will fall back to boot the system (pre-)installed in the NAND flash.
The bootargs.scr script can be created by running mkimage on a (modified) version of this bootargs.txt template, e.g.:
mkimage -A arm -O linux -T script -C none -a 0 -e 0 -n 'Boot Args' -d "bootargs.txt" bootargs.scr
Accessing & using the boot menu
You can easily access the graphical boot menu, by booting the device while keeping the AUX button pressed.
If you select one boot option ("SD:1", "SD:2", "SD:3", "SD:4" or "NAND"), the boot menu will remember this selection and use the selected boot order on all consecutive boots, until you select another option.
E.g. if you select "SD:3" U-Boot will first search for a system on partition 3, then p4, then p1, then p2 and finally NAND (-> SD:3 412N).
The first system found during this search will be booted.
If you select the "NAND" option, U-Boot will keep booting into the system installed in the NAND flash, until you select another option in the boot menu.
Reflashing U-Boot (update/recovery)
Flashing U-Boot can be done by booting from a hw-validation microSD card, which was prepared with the makesd script.
If the U-Boot from the hw-validation microSD card is newer than the one installed in the NAND flash, the system is automatically beeing updated (the screen will turn red while flasing).