mirror of
git://projects.qi-hardware.com/xburst-tools.git
synced 2024-12-23 17:59:32 +02:00
112 lines
4.2 KiB
Plaintext
112 lines
4.2 KiB
Plaintext
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-device>.bin for example
|
|
/boot/uImage-GTA02.bin.
|
|
|
|
You can disable a rootfs for consideration for boot if you add a file
|
|
/boot/noboot-<device>, eg, /boot/noboot-GTA02.
|
|
|
|
The kernel commandline used is associated with the storage device and partition,
|
|
this allows the correct root= line to be arrived at without any work.
|
|
|
|
If no kernel image can be found, Qi falls back to doing a memory test.
|
|
|
|
|
|
Appending to commandline
|
|
========================
|
|
|
|
You can append to the Qi commandline by creating a file /boot/append-<device>,
|
|
eg, /boot/append-GTA02 containing the additional kernel commandline you want.
|
|
|
|
This means you can affect the boot per-rootfs, but that if you reimage the
|
|
rootfs you at the same time define what is appeneded. Because these files are
|
|
looked for with the <device> name in them, options can be selected depending on
|
|
the device the rootfs is run on.
|
|
|
|
|
|
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 and USB is not enabled until Linux starts after a few seconds
|
|
|
|
- No startup splash screen
|
|
|
|
- by default there is no boot spew on the LCM
|
|
|
|
- On GTAxx boots from first uSD ext2 / 3 partition containing
|
|
/boot/uImage-<devicename>.bin present, eg, /boot/uImage-GTA02.bin, it checks
|
|
first three partitions in turn
|
|
|
|
- On GTA01 and 02 if nothing is workable on the SD Card, or it is not present,
|
|
Qi will try to boot from NAND
|
|
|
|
- You can disable a partition for boot by creating /boot/noboot-<devicename>,
|
|
eg, /boot/noboot-GTA02, it will skip it and check the next partition
|
|
|
|
- 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.
|
|
|