Documentation/arm/Booting

   1                         Booting ARM Linux
   2                         =================
   3
   4 Author: Russell King
   5 Date  : 18 May 2002
   6
   7 The following documentation is relevant to 2.4.18-rmk6 and beyond.
   8
   9 In order to boot ARM Linux, you require a boot loader, which is a small
  10 program that runs before the main kernel.  The boot loader is expected
  11 to initialise various devices, and eventually call the Linux kernel,
  12 passing information to the kernel.
  13
  14 Essentially, the boot loader should provide (as a minimum) the
  15 following:
  16
  17 1. Setup and initialise the RAM.
  18 2. Initialise one serial port.
  19 3. Detect the machine type.
  20 4. Setup the kernel tagged list.
  21 5. Call the kernel image.
  22
  23
  24 1. Setup and initialise RAM
  25 ---------------------------
  26
  27 Existing boot loaders:          MANDATORY
  28 New boot loaders:               MANDATORY
  29
  30 The boot loader is expected to find and initialise all RAM that the
  31 kernel will use for volatile data storage in the system.  It performs
  32 this in a machine dependent manner.  (It may use internal algorithms
  33 to automatically locate and size all RAM, or it may use knowledge of
  34 the RAM in the machine, or any other method the boot loader designer
  35 sees fit.)
  36
  37
  38 2. Initialise one serial port
  39 -----------------------------
  40
  41 Existing boot loaders:          OPTIONAL, RECOMMENDED
  42 New boot loaders:               OPTIONAL, RECOMMENDED
  43
  44 The boot loader should initialise and enable one serial port on the
  45 target.  This allows the kernel serial driver to automatically detect
  46 which serial port it should use for the kernel console (generally
  47 used for debugging purposes, or communication with the target.)
  48
  49 As an alternative, the boot loader can pass the relevant 'console='
  50 option to the kernel via the tagged lists specifying the port, and
  51 serial format options as described in
  52
  53        Documentation/kernel-parameters.txt.
  54
  55
  56 3. Detect the machine type
  57 --------------------------
  58
  59 Existing boot loaders:          OPTIONAL
  60 New boot loaders:               MANDATORY
  61
  62 The boot loader should detect the machine type its running on by some
  63 method.  Whether this is a hard coded value or some algorithm that
  64 looks at the connected hardware is beyond the scope of this document.
  65 The boot loader must ultimately be able to provide a MACH_TYPE_xxx
  66 value to the kernel. (see linux/arch/arm/tools/mach-types).
  67
  68 4. Setup boot data
  69 ------------------
  70
  71 Existing boot loaders:          OPTIONAL, HIGHLY RECOMMENDED
  72 New boot loaders:               MANDATORY
  73
  74 The boot loader must provide either a tagged list or a dtb image for
  75 passing configuration data to the kernel.  The physical address of the
  76 boot data is passed to the kernel in register r2.
  77
  78 4a. Setup the kernel tagged list
  79 --------------------------------
  80
  81 The boot loader must create and initialise the kernel tagged list.
  82 A valid tagged list starts with ATAG_CORE and ends with ATAG_NONE.
  83 The ATAG_CORE tag may or may not be empty.  An empty ATAG_CORE tag
  84 has the size field set to '2' (0x00000002).  The ATAG_NONE must set
  85 the size field to zero.
  86
  87 Any number of tags can be placed in the list.  It is undefined
  88 whether a repeated tag appends to the information carried by the
  89 previous tag, or whether it replaces the information in its
  90 entirety; some tags behave as the former, others the latter.
  91
  92 The boot loader must pass at a minimum the size and location of
  93 the system memory, and root filesystem location.  Therefore, the
  94 minimum tagged list should look:
  95
  96         +-----------+
  97 base -> | ATAG_CORE |  |
  98         +-----------+  |
  99         | ATAG_MEM  |  | increasing address
 100         +-----------+  |
 101         | ATAG_NONE |  |
 102         +-----------+  v
 103
 104 The tagged list should be stored in system RAM.
 105
 106 The tagged list must be placed in a region of memory where neither
 107 the kernel decompressor nor initrd 'bootp' program will overwrite
 108 it.  The recommended placement is in the first 16KiB of RAM.
 109
 110 4b. Setup the device tree
 111 -------------------------
 112
 113 The boot loader must load a device tree image (dtb) into system ram
 114 at a 64bit aligned address and initialize it with the boot data.  The
 115 dtb format is documented in Documentation/devicetree/booting-without-of.txt.
 116 The kernel will look for the dtb magic value of 0xd00dfeed at the dtb
 117 physical address to determine if a dtb has been passed instead of a
 118 tagged list.
 119
 120 The boot loader must pass at a minimum the size and location of the
 121 system memory, and the root filesystem location.  The dtb must be
 122 placed in a region of memory where the kernel decompressor will not
 123 overwrite it.  The recommended placement is in the first 16KiB of RAM
 124 with the caveat that it may not be located at physical address 0 since
 125 the kernel interprets a value of 0 in r2 to mean neither a tagged list
 126 nor a dtb were passed.
 127
 128 5. Calling the kernel image
 129 ---------------------------
 130
 131 Existing boot loaders:          MANDATORY
 132 New boot loaders:               MANDATORY
 133
 134 There are two options for calling the kernel zImage.  If the zImage
 135 is stored in flash, and is linked correctly to be run from flash,
 136 then it is legal for the boot loader to call the zImage in flash
 137 directly.
 138
 139 The zImage may also be placed in system RAM (at any location) and
 140 called there.  Note that the kernel uses 16K of RAM below the image
 141 to store page tables.  The recommended placement is 32KiB into RAM.
 142
 143 In 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
 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)
 160
 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.
 166
 167   If the kernel is not entered in HYP mode for any reason, it must be
 168   entered in SVC mode.
 169
 170 - Caches, MMUs
 171   The MMU must be off.
 172   Instruction cache may be on or off.
 173   Data cache must be off.
 174
 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.
 183
 184 - The boot loader is expected to call the kernel image by jumping
 185   directly to the first instruction of the kernel image.
 186
 187   On CPUs supporting the ARM instruction set, the entry must be
 188   made in ARM state, even for a Thumb-2 kernel.
 189
 190   On CPUs supporting only the Thumb instruction set such as
 191   Cortex-M class CPUs, the entry must be made in Thumb state.