mirror of
https://github.com/torvalds/linux.git
synced 2024-12-05 10:32:35 +00:00
691462f209
- Add a SPDX header; - Adjust document title; - Adjust document and section titles; - Some whitespace fixes and new line breaks; - Mark literal blocks as such; - Add table markups; - Add it to devicetree/index.rst. Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org> Link: https://lore.kernel.org/r/8dce14af13f66cb8393c21b58f0693e667a7045f.1592918949.git.mchehab+huawei@kernel.org Signed-off-by: Jonathan Corbet <corbet@lwn.net>
238 lines
7.9 KiB
ReStructuredText
238 lines
7.9 KiB
ReStructuredText
=================
|
|
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. Load initramfs.
|
|
6. 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/admin-guide/kernel-parameters.rst.
|
|
|
|
|
|
3. Detect the machine type
|
|
--------------------------
|
|
|
|
Existing boot loaders:
|
|
OPTIONAL
|
|
New boot loaders:
|
|
MANDATORY except for DT-only platforms
|
|
|
|
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). This
|
|
should be passed to the kernel in register r1.
|
|
|
|
For DT-only platforms, the machine type will be determined by device
|
|
tree. set the machine type to all ones (~0). This is not strictly
|
|
necessary, but assures that it will not match any existing 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.rst.
|
|
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, while remaining within the region which will be covered
|
|
by the kernel's low-memory mapping.
|
|
|
|
A safe location is just above the 128MiB boundary from start of RAM.
|
|
|
|
5. Load initramfs.
|
|
------------------
|
|
|
|
Existing boot loaders:
|
|
OPTIONAL
|
|
New boot loaders:
|
|
OPTIONAL
|
|
|
|
If an initramfs is in use then, as with the dtb, it must be placed in
|
|
a region of memory where the kernel decompressor will not overwrite it
|
|
while also with the region which will be covered by the kernel's
|
|
low-memory mapping.
|
|
|
|
A safe location is just above the device tree blob which itself will
|
|
be loaded just above the 128MiB boundary from the start of RAM as
|
|
recommended above.
|
|
|
|
6. 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 and called there. The
|
|
kernel should be placed in the first 128MiB of RAM. It is recommended
|
|
that it is loaded above 32MiB in order to avoid the need to relocate
|
|
prior to decompression, which will make the boot process slightly
|
|
faster.
|
|
|
|
When booting a raw (non-zImage) kernel the constraints are tighter.
|
|
In this case the kernel must be loaded at an offset into system equal
|
|
to TEXT_OFFSET - PAGE_OFFSET.
|
|
|
|
In any 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)
|
|
|
|
For CPUs which do not include the ARM virtualization extensions, the
|
|
CPU must be in SVC mode. (A special exception exists for Angel)
|
|
|
|
CPUs which include support for the virtualization extensions can be
|
|
entered in HYP mode in order to enable the kernel to make full use of
|
|
these extensions. This is the recommended boot method for such CPUs,
|
|
unless the virtualisations are already in use by a pre-installed
|
|
hypervisor.
|
|
|
|
If the kernel is not entered in HYP mode for any reason, it must be
|
|
entered in SVC mode.
|
|
|
|
- Caches, MMUs
|
|
|
|
The MMU must be off.
|
|
|
|
Instruction cache may be on or off.
|
|
|
|
Data cache must be off.
|
|
|
|
If the kernel is entered in HYP mode, the above requirements apply to
|
|
the HYP mode configuration in addition to the ordinary PL1 (privileged
|
|
kernel modes) configuration. In addition, all traps into the
|
|
hypervisor must be disabled, and PL1 access must be granted for all
|
|
peripherals and CPU resources for which this is architecturally
|
|
possible. Except for entering in HYP mode, the system configuration
|
|
should be such that a kernel which does not include support for the
|
|
virtualization extensions can boot correctly without extra help.
|
|
|
|
- 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.
|