|ARM Linux Kernel Boot Requirements|
Booting ARM Linux
In order to boot ARM Linux, you require a boot loader, which is a small program that runs before the main kernel. The boot loader is expected to initialise various devices, and eventually call the Linux kernel, passing information to the kernel.
Essentially, the boot loader should provide (as a minimum) the following:
Existing boot loaders: MANDATORY
The boot loader is expected to find and initialise all RAM that the kernel will use for volatile data storage in the system. It performs this in a machine dependent manner. (It may use internal algorithms to automatically locate and size all RAM, or it may use knowledge of the RAM in the machine, or any other method the boot loader designer sees fit.)
Existing boot loaders: OPTIONAL, RECOMMENDED
The boot loader should initialise and enable one serial port on the target. This allows the kernel serial driver to automatically detect which serial port it should use for the kernel console (generally used for debugging purposes, or communication with the target.)
As an alternative, the boot loader can pass the relevant 'console=' option to the kernel via the tagged lists specifing the port, and serial format options as described in
Existing boot loaders: OPTIONAL
The boot loader should detect the machine type its running on by some method. Whether this is a hard coded value or some algorithm that looks at the connected hardware is beyond the scope of this document. The boot loader must ultimately be able to provide a MACH_TYPE_xxx value to the kernel. (see linux/arch/arm/tools/mach-types).
Existing boot loaders: OPTIONAL, HIGHLY RECOMMENDED
The boot loader must create and initialise the kernel tagged list. A valid tagged list starts with ATAG_CORE and ends with ATAG_NONE. The ATAG_CORE tag may or may not be empty. An empty ATAG_CORE tag has the size field set to '2' (0x00000002). The ATAG_NONE must set the size field to zero.
Any number of tags can be placed in the list. It is undefined whether a repeated tag appends to the information carried by the previous tag, or whether it replaces the information in its entirety; some tags behave as the former, others the latter.
The boot loader must pass at a minimum the size and location of the system memory, and root filesystem location. Therefore, the minimum tagged list should look:
+-----------+ base -> | ATAG_CORE | | +-----------+ | | ATAG_MEM | | increasing address +-----------+ | | ATAG_NONE | | +-----------+ vThe tagged list should be stored in system RAM.
The tagged list must be placed in a region of memory where neither the kernel decompressor nor initrd 'bootp' program will overwrite it. The recommended placement is in the first 16KiB of RAM.
Existing boot loaders: MANDATORY
There are two options for calling the kernel zImage. If the zImage is stored in flash, and is linked correctly to be run from flash, then it is legal for the boot loader to call the zImage in flash directly.
The zImage may also be placed in system RAM (at any location) and called there. Note that the kernel uses 16K of RAM below the image to store page tables. The recommended placement is 32KiB into RAM.
In either case, the following conditions must be met:
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