1                        Booting ARM Linux
   2                        =================
   4Author: Russell King
   5Date  : 18 May 2002
   7The following documentation is relevant to 2.4.18-rmk6 and beyond.
   9In order to boot ARM Linux, you require a boot loader, which is a small
  10program that runs before the main kernel.  The boot loader is expected
  11to initialise various devices, and eventually call the Linux kernel,
  12passing information to the kernel.
  14Essentially, the boot loader should provide (as a minimum) the
  171. Setup and initialise the RAM.
  182. Initialise one serial port.
  193. Detect the machine type.
  204. Setup the kernel tagged list.
  215. Call the kernel image.
  241. Setup and initialise RAM
  27Existing boot loaders:          MANDATORY
  28New boot loaders:               MANDATORY
  30The boot loader is expected to find and initialise all RAM that the
  31kernel will use for volatile data storage in the system.  It performs
  32this in a machine dependent manner.  (It may use internal algorithms
  33to automatically locate and size all RAM, or it may use knowledge of
  34the RAM in the machine, or any other method the boot loader designer
  35sees fit.)
  382. Initialise one serial port
  41Existing boot loaders:          OPTIONAL, RECOMMENDED
  42New boot loaders:               OPTIONAL, RECOMMENDED
  44The boot loader should initialise and enable one serial port on the
  45target.  This allows the kernel serial driver to automatically detect
  46which serial port it should use for the kernel console (generally
  47used for debugging purposes, or communication with the target.)
  49As an alternative, the boot loader can pass the relevant 'console='
  50option to the kernel via the tagged lists specifying the port, and
  51serial format options as described in
  53       Documentation/kernel-parameters.txt.
  563. Detect the machine type
  59Existing boot loaders:          OPTIONAL
  60New boot loaders:               MANDATORY
  62The boot loader should detect the machine type its running on by some
  63method.  Whether this is a hard coded value or some algorithm that
  64looks at the connected hardware is beyond the scope of this document.
  65The boot loader must ultimately be able to provide a MACH_TYPE_xxx
  66value to the kernel. (see linux/arch/arm/tools/mach-types).
  684. Setup boot data
  71Existing boot loaders:          OPTIONAL, HIGHLY RECOMMENDED
  72New boot loaders:               MANDATORY
  74The boot loader must provide either a tagged list or a dtb image for
  75passing configuration data to the kernel.  The physical address of the
  76boot data is passed to the kernel in register r2.
  784a. Setup the kernel tagged list
  81The boot loader must create and initialise the kernel tagged list.
  82A valid tagged list starts with ATAG_CORE and ends with ATAG_NONE.
  83The ATAG_CORE tag may or may not be empty.  An empty ATAG_CORE tag
  84has the size field set to '2' (0x00000002).  The ATAG_NONE must set
  85the size field to zero.
  87Any number of tags can be placed in the list.  It is undefined
  88whether a repeated tag appends to the information carried by the
  89previous tag, or whether it replaces the information in its
  90entirety; some tags behave as the former, others the latter.
  92The boot loader must pass at a minimum the size and location of
  93the system memory, and root filesystem location.  Therefore, the
  94minimum tagged list should look:
  96        +-----------+
  97base -> | ATAG_CORE |  |
  98        +-----------+  |
  99        | ATAG_MEM  |  | increasing address
 100        +-----------+  |
 101        | ATAG_NONE |  |
 102        +-----------+  v
 104The tagged list should be stored in system RAM.
 106The tagged list must be placed in a region of memory where neither
 107the kernel decompressor nor initrd 'bootp' program will overwrite
 108it.  The recommended placement is in the first 16KiB of RAM.
 1104b. Setup the device tree
 113The boot loader must load a device tree image (dtb) into system ram
 114at a 64bit aligned address and initialize it with the boot data.  The
 115dtb format is documented in Documentation/devicetree/booting-without-of.txt.
 116The kernel will look for the dtb magic value of 0xd00dfeed at the dtb
 117physical address to determine if a dtb has been passed instead of a
 118tagged list.
 120The boot loader must pass at a minimum the size and location of the
 121system memory, and the root filesystem location.  The dtb must be
 122placed in a region of memory where the kernel decompressor will not
 123overwrite it.  The recommended placement is in the first 16KiB of RAM
 124with the caveat that it may not be located at physical address 0 since
 125the kernel interprets a value of 0 in r2 to mean neither a tagged list
 126nor a dtb were passed.
 1285. Calling the kernel image
 131Existing boot loaders:          MANDATORY
 132New boot loaders:               MANDATORY
 134There are two options for calling the kernel zImage.  If the zImage
 135is stored in flash, and is linked correctly to be run from flash,
 136then it is legal for the boot loader to call the zImage in flash
 139The zImage may also be placed in system RAM (at any location) and
 140called there.  Note that the kernel uses 16K of RAM below the image
 141to store page tables.  The recommended placement is 32KiB into RAM.
 143In either case, the following conditions must be met:
 145- Quiesce all DMA capable devices so that memory does not get
 146  corrupted by bogus network packets or disk data. This will save
 147  you many hours of debug.
 149- CPU register settings
 150  r0 = 0,
 151  r1 = machine type number discovered in (3) above.
 152  r2 = physical address of tagged list in system RAM, or
 153       physical address of device tree block (dtb) in system RAM
 155- CPU mode
 156  All forms of interrupts must be disabled (IRQs and FIQs)
 158  For CPUs which do not include the ARM virtualization extensions, the
 159  CPU must be in SVC mode.  (A special exception exists for Angel)
 161  CPUs which include support for the virtualization extensions can be
 162  entered in HYP mode in order to enable the kernel to make full use of
 163  these extensions.  This is the recommended boot method for such CPUs,
 164  unless the virtualisations are already in use by a pre-installed
 165  hypervisor.
 167  If the kernel is not entered in HYP mode for any reason, it must be
 168  entered in SVC mode.
 170- Caches, MMUs
 171  The MMU must be off.
 172  Instruction cache may be on or off.
 173  Data cache must be off.
 175  If the kernel is entered in HYP mode, the above requirements apply to
 176  the HYP mode configuration in addition to the ordinary PL1 (privileged
 177  kernel modes) configuration.  In addition, all traps into the
 178  hypervisor must be disabled, and PL1 access must be granted for all
 179  peripherals and CPU resources for which this is architecturally
 180  possible.  Except for entering in HYP mode, the system configuration
 181  should be such that a kernel which does not include support for the
 182  virtualization extensions can boot correctly without extra help.
 184- The boot loader is expected to call the kernel image by jumping
 185  directly to the first instruction of the kernel image.
 187  On CPUs supporting the ARM instruction set, the entry must be
 188  made in ARM state, even for a Thumb-2 kernel.
 190  On CPUs supporting only the Thumb instruction set such as
 191  Cortex-M class CPUs, the entry must be made in Thumb state.
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