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
following:
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
New 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
Documentation/kernel-parameters.txt.
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 boot data
------------------
Existing boot loaders: OPTIONAL, HIGHLY RECOMMENDED
New boot loaders: MANDATORY
The boot loader must provide either a tagged list or a dtb image for
passing configuration data to the kernel. The physical address of the
boot data is passed to the kernel in register r2.
4a. Setup the kernel tagged list
--------------------------------
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 | |
+-----------+ 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.
4b. Setup the device tree
-------------------------
The boot loader must load a device tree image (dtb) into system ram
at a 64bit aligned address and initialize it with the boot data. The
dtb format is documented in Documentation/devicetree/booting-without-of.txt.
The kernel will look for the dtb magic value of 0xd00dfeed at the dtb
physical address to determine if a dtb has been passed instead of a
tagged list.
The boot loader must pass at a minimum the size and location of the
system memory, and the root filesystem location. The dtb must be
placed in a region of memory where the kernel decompressor will not
overwrite it. The recommended placement is in the first 16KiB of RAM
with the caveat that it may not be located at physical address 0 since
the kernel interprets a value of 0 in r2 to mean neither a tagged list
nor a dtb were passed.
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
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:
- 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, or
physical address of device tree block (dtb) 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.
On CPUs supporting the ARM instruction set, the entry must be
made in ARM state, even for a Thumb-2 kernel.
On CPUs supporting only the Thumb instruction set such as
Cortex-M class CPUs, the entry must be made in Thumb state.