xburst-tools/qiboot

Qi
==

Qi (named by Alan Cox on Openmoko kernel list) is a minimal bootloader that
"breathes life" into Linux.  Its goal is to stay close to the minimum needed
to "load" and then "boot" Linux -- no boot menus, additional peripheral init
or private states.


What's wrong with U-Boot, they keep telling people to not reinvent the wheel?
=============================================================================

U-Boot is gradually becoming a simplified knockoff of Linux.  After using it a
while, it became clear we were cutting and pasting drivers into U-Boot from
Linux, cutting them down and not having a plan to maintain the U-Boot versions
as the Linux ones were changed.

We decided that we would use full Linux for things that Linux is good at and
only have the bootloader do the device init that is absolutely required before
Linux can be pulled into memory and started.  In practice since we had a working
U-Boot implementation it meant cutting that right down to the bone (start.S
mainly for s3c2442) and then building it up from scratch optimized to just do
load and boot.


Samsung - specific boot sequence
================================

Right now Qi supports Samsung "steppingstone" scheme devices, but in fact it's
the same in processors like iMX31 that there is a small area of SRAM that is
prepped with NAND content via ROM on the device.  There's nothing that stops Qi
use on processors without steppingstone, although the ATAG stuff assumes we deal
with ARM based processor.


Per-CPU Qi
==========

Qi has a concept of a single bootloader binary created per CPU type.  The
different devices that use that CPU are all supported in the same binary.  At
runtime after the common init is done, Qi asks each supported device code in
turn if it recognizes the device as being handled by it, the first one to reply
that it knows the device gets to control the rest of the process.

Consequently, there is NO build-time configuration file as found on U-Boot
except a make env var that sets the CPU type being built, eg:

  make CPU=s3c6410


Booting Heuristics
==================

Qi has one or more ways to fetch a kernel depending on the device it finds it is
running on, for example on GTA02 it can use NAND and SD card devices.  It goes
through these device-specific storage devices in order and tries to boot the
first viable kernel it finds, usually /boot/uImage.bin.

The kernel commandline used is associated with the storage device, this allows
the correct root= line to be arrived at without any work.  The inability to set
the Qi kernel commandline externally is deliberate, two otherwise identical
devices differing by the kernel commandline or other "environment" is not good.
A whole class of bugs and support issues around private bootloader state are
therefore avoided.

If no kernel image can be found, Qi falls back to doing a memory test.


Initrd support
==============

Initrd or initramfs in separate file is supported to be loaded at given
memory address in addition to kernel image.  The ATAGs are issued accordingly.


Functional Differences from U-Boot on GTA02
===========================================

 - Backlight is not enabled until Linux starts after a few seconds

 - kernel loglevel is set to NOT output gobs of text to the screen

 - On GTA02 will ALWAYS boot from uSD if first partition is ext2 and contains
    /boot/uImage.bin, otherwise boots from NAND

 - On GTA03 will ALWAYS boot from uSD second partition if /boot/uImage.bin is
    present otherwise try the third / backup partition

 - No startup splash screen

 - Way faster

 - There is no concept of "staying in the bootloader".  The bootloader exits to
    Linux as fast as possible, that's all it does.

 - USB is not started until Linux starts around 5 seconds after boot, there is
    no DFU.