Documentation/arm64/booting.txt

   1                         Booting AArch64 Linux
   2                         =====================
   3
   4 Author: Will Deacon <will.deacon@arm.com>
   5 Date  : 07 September 2012
   6
   7 This document is based on the ARM booting document by Russell King and
   8 is relevant to all public releases of the AArch64 Linux kernel.
   9
  10 The AArch64 exception model is made up of a number of exception levels
  11 (EL0 - EL3), with EL0 and EL1 having a secure and a non-secure
  12 counterpart.  EL2 is the hypervisor level and exists only in non-secure
  13 mode. EL3 is the highest priority level and exists only in secure mode.
  14
  15 For the purposes of this document, we will use the term `boot loader'
  16 simply to define all software that executes on the CPU(s) before control
  17 is passed to the Linux kernel.  This may include secure monitor and
  18 hypervisor code, or it may just be a handful of instructions for
  19 preparing a minimal boot environment.
  20
  21 Essentially, the boot loader should provide (as a minimum) the
  22 following:
  23
  24 1. Setup and initialise the RAM
  25 2. Setup the device tree
  26 3. Decompress the kernel image
  27 4. Call the kernel image
  28
  29
  30 1. Setup and initialise RAM
  31 ---------------------------
  32
  33 Requirement: MANDATORY
  34
  35 The boot loader is expected to find and initialise all RAM that the
  36 kernel will use for volatile data storage in the system.  It performs
  37 this in a machine dependent manner.  (It may use internal algorithms
  38 to automatically locate and size all RAM, or it may use knowledge of
  39 the RAM in the machine, or any other method the boot loader designer
  40 sees fit.)
  41
  42
  43 2. Setup the device tree
  44 -------------------------
  45
  46 Requirement: MANDATORY
  47
  48 The device tree blob (dtb) must be placed on an 8-byte boundary and must
  49 not exceed 2 megabytes in size. Since the dtb will be mapped cacheable
  50 using blocks of up to 2 megabytes in size, it must not be placed within
  51 any 2M region which must be mapped with any specific attributes.
  52
  53 NOTE: versions prior to v4.2 also require that the DTB be placed within
  54 the 512 MB region starting at text_offset bytes below the kernel Image.
  55
  56 3. Decompress the kernel image
  57 ------------------------------
  58
  59 Requirement: OPTIONAL
  60
  61 The AArch64 kernel does not currently provide a decompressor and
  62 therefore requires decompression (gzip etc.) to be performed by the boot
  63 loader if a compressed Image target (e.g. Image.gz) is used.  For
  64 bootloaders that do not implement this requirement, the uncompressed
  65 Image target is available instead.
  66
  67
  68 4. Call the kernel image
  69 ------------------------
  70
  71 Requirement: MANDATORY
  72
  73 The decompressed kernel image contains a 64-byte header as follows:
  74
  75   u32 code0;                    /* Executable code */
  76   u32 code1;                    /* Executable code */
  77   u64 text_offset;              /* Image load offset, little endian */
  78   u64 image_size;               /* Effective Image size, little endian */
  79   u64 flags;                    /* kernel flags, little endian */
  80   u64 res2      = 0;            /* reserved */
  81   u64 res3      = 0;            /* reserved */
  82   u64 res4      = 0;            /* reserved */
  83   u32 magic     = 0x644d5241;   /* Magic number, little endian, "ARM\x64" */
  84   u32 res5;                     /* reserved (used for PE COFF offset) */
  85
  86
  87 Header notes:
  88
  89 - As of v3.17, all fields are little endian unless stated otherwise.
  90
  91 - code0/code1 are responsible for branching to stext.
  92
  93 - when booting through EFI, code0/code1 are initially skipped.
  94   res5 is an offset to the PE header and the PE header has the EFI
  95   entry point (efi_stub_entry).  When the stub has done its work, it
  96   jumps to code0 to resume the normal boot process.
  97
  98 - Prior to v3.17, the endianness of text_offset was not specified.  In
  99   these cases image_size is zero and text_offset is 0x80000 in the
 100   endianness of the kernel.  Where image_size is non-zero image_size is
 101   little-endian and must be respected.  Where image_size is zero,
 102   text_offset can be assumed to be 0x80000.
 103
 104 - The flags field (introduced in v3.17) is a little-endian 64-bit field
 105   composed as follows:
 106   Bit 0:        Kernel endianness.  1 if BE, 0 if LE.
 107   Bits 1-63:    Reserved.
 108
 109 - When image_size is zero, a bootloader should attempt to keep as much
 110   memory as possible free for use by the kernel immediately after the
 111   end of the kernel image. The amount of space required will vary
 112   depending on selected features, and is effectively unbound.
 113
 114 The Image must be placed text_offset bytes from a 2MB aligned base
 115 address near the start of usable system RAM and called there. Memory
 116 below that base address is currently unusable by Linux, and therefore it
 117 is strongly recommended that this location is the start of system RAM.
 118 The region between the 2 MB aligned base address and the start of the
 119 image has no special significance to the kernel, and may be used for
 120 other purposes.
 121 At least image_size bytes from the start of the image must be free for
 122 use by the kernel.
 123
 124 Any memory described to the kernel (even that below the start of the
 125 image) which is not marked as reserved from the kernel (e.g., with a
 126 memreserve region in the device tree) will be considered as available to
 127 the kernel.
 128
 129 Before jumping into the kernel, the following conditions must be met:
 130
 131 - Quiesce all DMA capable devices so that memory does not get
 132   corrupted by bogus network packets or disk data.  This will save
 133   you many hours of debug.
 134
 135 - Primary CPU general-purpose register settings
 136   x0 = physical address of device tree blob (dtb) in system RAM.
 137   x1 = 0 (reserved for future use)
 138   x2 = 0 (reserved for future use)
 139   x3 = 0 (reserved for future use)
 140
 141 - CPU mode
 142   All forms of interrupts must be masked in PSTATE.DAIF (Debug, SError,
 143   IRQ and FIQ).
 144   The CPU must be in either EL2 (RECOMMENDED in order to have access to
 145   the virtualisation extensions) or non-secure EL1.
 146
 147 - Caches, MMUs
 148   The MMU must be off.
 149   Instruction cache may be on or off.
 150   The address range corresponding to the loaded kernel image must be
 151   cleaned to the PoC. In the presence of a system cache or other
 152   coherent masters with caches enabled, this will typically require
 153   cache maintenance by VA rather than set/way operations.
 154   System caches which respect the architected cache maintenance by VA
 155   operations must be configured and may be enabled.
 156   System caches which do not respect architected cache maintenance by VA
 157   operations (not recommended) must be configured and disabled.
 158
 159 - Architected timers
 160   CNTFRQ must be programmed with the timer frequency and CNTVOFF must
 161   be programmed with a consistent value on all CPUs.  If entering the
 162   kernel at EL1, CNTHCTL_EL2 must have EL1PCTEN (bit 0) set where
 163   available.
 164
 165 - Coherency
 166   All CPUs to be booted by the kernel must be part of the same coherency
 167   domain on entry to the kernel.  This may require IMPLEMENTATION DEFINED
 168   initialisation to enable the receiving of maintenance operations on
 169   each CPU.
 170
 171 - System registers
 172   All writable architected system registers at the exception level where
 173   the kernel image will be entered must be initialised by software at a
 174   higher exception level to prevent execution in an UNKNOWN state.
 175
 176   For systems with a GICv3 interrupt controller:
 177   - If EL3 is present:
 178     ICC_SRE_EL3.Enable (bit 3) must be initialiased to 0b1.
 179     ICC_SRE_EL3.SRE (bit 0) must be initialised to 0b1.
 180   - If the kernel is entered at EL1:
 181     ICC.SRE_EL2.Enable (bit 3) must be initialised to 0b1
 182     ICC_SRE_EL2.SRE (bit 0) must be initialised to 0b1.
 183
 184 The requirements described above for CPU mode, caches, MMUs, architected
 185 timers, coherency and system registers apply to all CPUs.  All CPUs must
 186 enter the kernel in the same exception level.
 187
 188 The boot loader is expected to enter the kernel on each CPU in the
 189 following manner:
 190
 191 - The primary CPU must jump directly to the first instruction of the
 192   kernel image.  The device tree blob passed by this CPU must contain
 193   an 'enable-method' property for each cpu node.  The supported
 194   enable-methods are described below.
 195
 196   It is expected that the bootloader will generate these device tree
 197   properties and insert them into the blob prior to kernel entry.
 198
 199 - CPUs with a "spin-table" enable-method must have a 'cpu-release-addr'
 200   property in their cpu node.  This property identifies a
 201   naturally-aligned 64-bit zero-initalised memory location.
 202
 203   These CPUs should spin outside of the kernel in a reserved area of
 204   memory (communicated to the kernel by a /memreserve/ region in the
 205   device tree) polling their cpu-release-addr location, which must be
 206   contained in the reserved region.  A wfe instruction may be inserted
 207   to reduce the overhead of the busy-loop and a sev will be issued by
 208   the primary CPU.  When a read of the location pointed to by the
 209   cpu-release-addr returns a non-zero value, the CPU must jump to this
 210   value.  The value will be written as a single 64-bit little-endian
 211   value, so CPUs must convert the read value to their native endianness
 212   before jumping to it.
 213
 214 - CPUs with a "psci" enable method should remain outside of
 215   the kernel (i.e. outside of the regions of memory described to the
 216   kernel in the memory node, or in a reserved area of memory described
 217   to the kernel by a /memreserve/ region in the device tree).  The
 218   kernel will issue CPU_ON calls as described in ARM document number ARM
 219   DEN 0022A ("Power State Coordination Interface System Software on ARM
 220   processors") to bring CPUs into the kernel.
 221
 222   The device tree should contain a 'psci' node, as described in
 223   Documentation/devicetree/bindings/arm/psci.txt.
 224
 225 - Secondary CPU general-purpose register settings
 226   x0 = 0 (reserved for future use)
 227   x1 = 0 (reserved for future use)
 228   x2 = 0 (reserved for future use)
 229   x3 = 0 (reserved for future use)