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Booting ARM Linux By Russell King

Booting ARM Linux

Author: Russell King
Date : 18 May 2002

The following documentation is relevant to 2.4.18-rmk6 and beyond.

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

1. Setup and initialise the RAM.
2. Initialise one serial port.
3. Detect the machine type.
4. Setup the kernel tagged list.
5. Call the kernel image.

1. Setup and initialise RAM

Existing boot loaders: MANDATORY
New 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.)

2. Initialise one serial port

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 specifying the port, and
serial format options as described in


3. Detect the machine type

Existing boot loaders: OPTIONAL
New boot loaders: MANDATORY

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).

4. Setup the kernel tagged list

New boot loaders: MANDATORY

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
+-----------+ |
+-----------+ v

The 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.

5. Calling the kernel image

Existing boot loaders: MANDATORY
New 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

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:

- Quiesce all DMA capable devices so that memory does not get
corrupted by bogus network packets or disk data. This will save
you many hours of debug.

- CPU register settings
r0 = 0,
r1 = machine type number discovered in (3) above.
r2 = physical address of tagged list in system RAM.

- CPU mode
All forms of interrupts must be disabled (IRQs and FIQs)
The CPU must be in SVC mode. (A special exception exists for Angel)

- Caches, MMUs
The MMU must be off.
Instruction cache may be on or off.
Data cache must be off.

- The boot loader is expected to call the kernel image by jumping
directly to the first instruction of the kernel image.


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