linux/Documentation/arm/Booting
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   1                        Booting ARM Linux
   2                        =================
   3
   4Author: Russell King
   5Date  : 18 May 2002
   6
   7The following documentation is relevant to 2.4.18-rmk6 and beyond.
   8
   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.
  13
  14Essentially, the boot loader should provide (as a minimum) the
  15following:
  16
  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.
  22
  23
  241. Setup and initialise RAM
  25---------------------------
  26
  27Existing boot loaders:          MANDATORY
  28New boot loaders:               MANDATORY
  29
  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.)
  36
  37
  382. Initialise one serial port
  39-----------------------------
  40
  41Existing boot loaders:          OPTIONAL, RECOMMENDED
  42New boot loaders:               OPTIONAL, RECOMMENDED
  43
  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.)
  48
  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
  52
  53       Documentation/kernel-parameters.txt.
  54
  55
  563. Detect the machine type
  57--------------------------
  58
  59Existing boot loaders:          OPTIONAL
  60New boot loaders:               MANDATORY
  61
  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).
  67
  684. Setup boot data
  69------------------
  70
  71Existing boot loaders:          OPTIONAL, HIGHLY RECOMMENDED
  72New boot loaders:               MANDATORY
  73
  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.
  77
  784a. Setup the kernel tagged list
  79--------------------------------
  80
  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.
  86
  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.
  91
  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:
  95
  96        +-----------+
  97base -> | ATAG_CORE |  |
  98        +-----------+  |
  99        | ATAG_MEM  |  | increasing address
 100        +-----------+  |
 101        | ATAG_NONE |  |
 102        +-----------+  v
 103
 104The tagged list should be stored in system RAM.
 105
 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.
 109
 1104b. Setup the device tree
 111-------------------------
 112
 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.
 119
 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.
 127
 1285. Calling the kernel image
 129---------------------------
 130
 131Existing boot loaders:          MANDATORY
 132New boot loaders:               MANDATORY
 133
 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
 137directly.
 138
 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.
 142
 143In either case, the following conditions must be met:
 144
 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.
 148
 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
 154
 155- CPU mode
 156  All forms of interrupts must be disabled (IRQs and FIQs)
 157  The CPU must be in SVC mode.  (A special exception exists for Angel)
 158
 159- Caches, MMUs
 160  The MMU must be off.
 161  Instruction cache may be on or off.
 162  Data cache must be off.
 163
 164- The boot loader is expected to call the kernel image by jumping
 165  directly to the first instruction of the kernel image.
 166
 167  On CPUs supporting the ARM instruction set, the entry must be
 168  made in ARM state, even for a Thumb-2 kernel.
 169
 170  On CPUs supporting only the Thumb instruction set such as
 171  Cortex-M class CPUs, the entry must be made in Thumb state.
 172
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