Documentation/arm/Booting - linux/kernel/git/tj/cgroup - Git at Google

 			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/kernel-parameters.txt.


 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.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, whilst 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.
	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/kernel-parameters.txt.


	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.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, whilst 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.