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115 lines
4.4 KiB
Plaintext
115 lines
4.4 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-device>.bin for example
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/boot/uImage-GTA02.bin.
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You can disable a rootfs for consideration for boot if you add a file
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/boot/noboot-<device>, eg, /boot/noboot-GTA02. This differs from renaming or
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deleting the kernel image because updating the kernel package would give you a
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working kernel again and allow boot, whereas the noboot indication will remain
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until you remove it.
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The kernel commandline used is associated with the storage device and partition,
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this allows the correct root= line to be arrived at without any work.
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If no kernel image can be found, Qi falls back to doing a memory test.
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Appending to commandline
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========================
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You can append to the Qi commandline by creating a file /boot/append-<device>,
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eg, /boot/append-GTA02 containing the additional kernel commandline you want.
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This means you can affect the boot per-rootfs, but that if you reimage the
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rootfs you at the same time define what is appeneded. Because these files are
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looked for with the <device> name in them, options can be selected depending on
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the device the rootfs is run on.
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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 and USB is not enabled until Linux starts after a few seconds
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- No startup splash screen
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- by default there is no boot spew on the LCM
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- On GTAxx boots from first uSD ext2 / 3 partition containing
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/boot/uImage-<devicename>.bin present, eg, /boot/uImage-GTA02.bin, it checks
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first three partitions in turn
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- On GTA01 and 02 if nothing is workable on the SD Card, or it is not present,
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Qi will try to boot from NAND
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- You can disable a partition for boot by creating /boot/noboot-<devicename>,
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eg, /boot/noboot-GTA02, it will skip it and check the next partition
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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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