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98 lines
3.7 KiB
Plaintext
98 lines
3.7 KiB
Plaintext
Qi
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==
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Qi (named by Alan Cox on Openmoko kernel list) is a minimal bootloader that
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"breathes life" into Linux. Its goal is to stay close to the minimum needed
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to "load" and then "boot" Linux -- no boot menus, additional peripheral init
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or private states.
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What's wrong with U-Boot, they keep telling people to not reinvent the wheel?
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=============================================================================
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U-Boot is gradually becoming a simplified knockoff of Linux. After using it a
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while, it became clear we were cutting and pasting drivers into U-Boot from
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Linux, cutting them down and not having a plan to maintain the U-Boot versions
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as the Linux ones were changed.
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We decided that we would use full Linux for things that Linux is good at and
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only have the bootloader do the device init that is absolutely required before
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Linux can be pulled into memory and started. In practice since we had a working
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U-Boot implementation it meant cutting that right down to the bone (start.S
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mainly for s3c2442) and then building it up from scratch optimized to just do
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load and boot.
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Samsung - specific boot sequence
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================================
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Right now Qi supports Samsung "steppingstone" scheme devices, but in fact it's
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the same in processors like iMX31 that there is a small area of SRAM that is
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prepped with NAND content via ROM on the device. There's nothing that stops Qi
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use on processors without steppingstone, although the ATAG stuff assumes we deal
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with ARM based processor.
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Per-CPU Qi
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==========
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Qi has a concept of a single bootloader binary created per CPU type. The
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different devices that use that CPU are all supported in the same binary. At
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runtime after the common init is done, Qi asks each supported device code in
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turn if it recognizes the device as being handled by it, the first one to reply
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that it knows the device gets to control the rest of the process.
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Consequently, there is NO build-time configuration file as found on U-Boot
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except a make env var that sets the CPU type being built, eg:
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make CPU=s3c6410
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Booting Heuristics
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==================
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Qi has one or more ways to fetch a kernel depending on the device it finds it is
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running on, for example on GTA02 it can use NAND and SD card devices. It goes
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through these device-specific storage devices in order and tries to boot the
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first viable kernel it finds, usually /boot/uImage.bin.
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The kernel commandline used is associated with the storage device, this allows
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the correct root= line to be arrived at without any work. The inability to set
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the Qi kernel commandline externally is deliberate, two otherwise identical
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devices differing by the kernel commandline or other "environment" is not good.
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A whole class of bugs and support issues around private bootloader state are
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therefore avoided.
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If no kernel image can be found, Qi falls back to doing a memory test.
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Initrd support
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==============
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Initrd or initramfs in separate file is supported to be loaded at given
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memory address in addition to kernel image. The ATAGs are issued accordingly.
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Functional Differences from U-Boot on GTA02
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===========================================
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- Backlight is not enabled until Linux starts after a few seconds
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- kernel loglevel is set to NOT output gobs of text to the screen
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- On GTA02 will ALWAYS boot from uSD if first partition is ext2 and contains
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/boot/uImage.bin, otherwise boots from NAND
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- On GTA03 will ALWAYS boot from uSD second partition if /boot/uImage.bin is
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present otherwise try the third / backup partition
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- No startup splash screen
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- Way faster
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- There is no concept of "staying in the bootloader". The bootloader exits to
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Linux as fast as possible, that's all it does.
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- USB is not started until Linux starts around 5 seconds after boot, there is
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no DFU.
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