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. Load initramfs.
  22 6. Call the kernel image.
  23
  24
  25 1. Setup and initialise RAM
  26 ---------------------------
  27
  28 Existing boot loaders:          MANDATORY
  29 New boot loaders:               MANDATORY
  30
  31 The boot loader is expected to find and initialise all RAM that the
  32 kernel will use for volatile data storage in the system.  It performs
  33 this in a machine dependent manner.  (It may use internal algorithms
  34 to automatically locate and size all RAM, or it may use knowledge of
  35 the RAM in the machine, or any other method the boot loader designer
  36 sees fit.)
  37
  38
  39 2. Initialise one serial port
  40 -----------------------------
  41
  42 Existing boot loaders:          OPTIONAL, RECOMMENDED
  43 New boot loaders:               OPTIONAL, RECOMMENDED
  44
  45 The boot loader should initialise and enable one serial port on the
  46 target.  This allows the kernel serial driver to automatically detect
  47 which serial port it should use for the kernel console (generally
  48 used for debugging purposes, or communication with the target.)
  49
  50 As an alternative, the boot loader can pass the relevant 'console='
  51 option to the kernel via the tagged lists specifying the port, and
  52 serial format options as described in
  53
  54        Documentation/admin-guide/kernel-parameters.rst.
  55
  56
  57 3. Detect the machine type
  58 --------------------------
  59
  60 Existing boot loaders:          OPTIONAL
  61 New boot loaders:               MANDATORY except for DT-only platforms
  62
  63 The boot loader should detect the machine type its running on by some
  64 method.  Whether this is a hard coded value or some algorithm that
  65 looks at the connected hardware is beyond the scope of this document.
  66 The boot loader must ultimately be able to provide a MACH_TYPE_xxx
  67 value to the kernel. (see linux/arch/arm/tools/mach-types).  This
  68 should be passed to the kernel in register r1.
  69
  70 For DT-only platforms, the machine type will be determined by device
  71 tree.  set the machine type to all ones (~0).  This is not strictly
  72 necessary, but assures that it will not match any existing types.
  73
  74 4. Setup boot data
  75 ------------------
  76
  77 Existing boot loaders:          OPTIONAL, HIGHLY RECOMMENDED
  78 New boot loaders:               MANDATORY
  79
  80 The boot loader must provide either a tagged list or a dtb image for
  81 passing configuration data to the kernel.  The physical address of the
  82 boot data is passed to the kernel in register r2.
  83
  84 4a. Setup the kernel tagged list
  85 --------------------------------
  86
  87 The boot loader must create and initialise the kernel tagged list.
  88 A valid tagged list starts with ATAG_CORE and ends with ATAG_NONE.
  89 The ATAG_CORE tag may or may not be empty.  An empty ATAG_CORE tag
  90 has the size field set to '2' (0x00000002).  The ATAG_NONE must set
  91 the size field to zero.
  92
  93 Any number of tags can be placed in the list.  It is undefined
  94 whether a repeated tag appends to the information carried by the
  95 previous tag, or whether it replaces the information in its
  96 entirety; some tags behave as the former, others the latter.
  97
  98 The boot loader must pass at a minimum the size and location of
  99 the system memory, and root filesystem location.  Therefore, the
 100 minimum tagged list should look:
 101
 102         +-----------+
 103 base -> | ATAG_CORE |  |
 104         +-----------+  |
 105         | ATAG_MEM  |  | increasing address
 106         +-----------+  |
 107         | ATAG_NONE |  |
 108         +-----------+  v
 109
 110 The tagged list should be stored in system RAM.
 111
 112 The tagged list must be placed in a region of memory where neither
 113 the kernel decompressor nor initrd 'bootp' program will overwrite
 114 it.  The recommended placement is in the first 16KiB of RAM.
 115
 116 4b. Setup the device tree
 117 -------------------------
 118
 119 The boot loader must load a device tree image (dtb) into system ram
 120 at a 64bit aligned address and initialize it with the boot data.  The
 121 dtb format is documented in Documentation/devicetree/booting-without-of.txt.
 122 The kernel will look for the dtb magic value of 0xd00dfeed at the dtb
 123 physical address to determine if a dtb has been passed instead of a
 124 tagged list.
 125
 126 The boot loader must pass at a minimum the size and location of the
 127 system memory, and the root filesystem location.  The dtb must be
 128 placed in a region of memory where the kernel decompressor will not
 129 overwrite it, whilst remaining within the region which will be covered
 130 by the kernel's low-memory mapping.
 131
 132 A safe location is just above the 128MiB boundary from start of RAM.
 133
 134 5. Load initramfs.
 135 ------------------
 136
 137 Existing boot loaders:          OPTIONAL
 138 New boot loaders:               OPTIONAL
 139
 140 If an initramfs is in use then, as with the dtb, it must be placed in
 141 a region of memory where the kernel decompressor will not overwrite it
 142 while also with the region which will be covered by the kernel's
 143 low-memory mapping.
 144
 145 A safe location is just above the device tree blob which itself will
 146 be loaded just above the 128MiB boundary from the start of RAM as
 147 recommended above.
 148
 149 6. Calling the kernel image
 150 ---------------------------
 151
 152 Existing boot loaders:          MANDATORY
 153 New boot loaders:               MANDATORY
 154
 155 There are two options for calling the kernel zImage.  If the zImage
 156 is stored in flash, and is linked correctly to be run from flash,
 157 then it is legal for the boot loader to call the zImage in flash
 158 directly.
 159
 160 The zImage may also be placed in system RAM and called there.  The
 161 kernel should be placed in the first 128MiB of RAM.  It is recommended
 162 that it is loaded above 32MiB in order to avoid the need to relocate
 163 prior to decompression, which will make the boot process slightly
 164 faster.
 165
 166 When booting a raw (non-zImage) kernel the constraints are tighter.
 167 In this case the kernel must be loaded at an offset into system equal
 168 to TEXT_OFFSET - PAGE_OFFSET.
 169
 170 In any case, the following conditions must be met:
 171
 172 - Quiesce all DMA capable devices so that memory does not get
 173   corrupted by bogus network packets or disk data. This will save
 174   you many hours of debug.
 175
 176 - CPU register settings
 177   r0 = 0,
 178   r1 = machine type number discovered in (3) above.
 179   r2 = physical address of tagged list in system RAM, or
 180        physical address of device tree block (dtb) in system RAM
 181
 182 - CPU mode
 183   All forms of interrupts must be disabled (IRQs and FIQs)
 184
 185   For CPUs which do not include the ARM virtualization extensions, the
 186   CPU must be in SVC mode.  (A special exception exists for Angel)
 187
 188   CPUs which include support for the virtualization extensions can be
 189   entered in HYP mode in order to enable the kernel to make full use of
 190   these extensions.  This is the recommended boot method for such CPUs,
 191   unless the virtualisations are already in use by a pre-installed
 192   hypervisor.
 193
 194   If the kernel is not entered in HYP mode for any reason, it must be
 195   entered in SVC mode.
 196
 197 - Caches, MMUs
 198   The MMU must be off.
 199   Instruction cache may be on or off.
 200   Data cache must be off.
 201
 202   If the kernel is entered in HYP mode, the above requirements apply to
 203   the HYP mode configuration in addition to the ordinary PL1 (privileged
 204   kernel modes) configuration.  In addition, all traps into the
 205   hypervisor must be disabled, and PL1 access must be granted for all
 206   peripherals and CPU resources for which this is architecturally
 207   possible.  Except for entering in HYP mode, the system configuration
 208   should be such that a kernel which does not include support for the
 209   virtualization extensions can boot correctly without extra help.
 210
 211 - The boot loader is expected to call the kernel image by jumping
 212   directly to the first instruction of the kernel image.
 213
 214   On CPUs supporting the ARM instruction set, the entry must be
 215   made in ARM state, even for a Thumb-2 kernel.
 216
 217   On CPUs supporting only the Thumb instruction set such as
 218   Cortex-M class CPUs, the entry must be made in Thumb state.