0001 =================
0002 Booting ARM Linux
0003 =================
0004
0005 Author: Russell King
0006
0007 Date : 18 May 2002
0008
0009 The following documentation is relevant to 2.4.18-rmk6 and beyond.
0010
0011 In order to boot ARM Linux, you require a boot loader, which is a small
0012 program that runs before the main kernel. The boot loader is expected
0013 to initialise various devices, and eventually call the Linux kernel,
0014 passing information to the kernel.
0015
0016 Essentially, the boot loader should provide (as a minimum) the
0017 following:
0018
0019 1. Setup and initialise the RAM.
0020 2. Initialise one serial port.
0021 3. Detect the machine type.
0022 4. Setup the kernel tagged list.
0023 5. Load initramfs.
0024 6. Call the kernel image.
0025
0026
0027 1. Setup and initialise RAM
0028 ---------------------------
0029
0030 Existing boot loaders:
0031 MANDATORY
0032 New boot loaders:
0033 MANDATORY
0034
0035 The boot loader is expected to find and initialise all RAM that the
0036 kernel will use for volatile data storage in the system. It performs
0037 this in a machine dependent manner. (It may use internal algorithms
0038 to automatically locate and size all RAM, or it may use knowledge of
0039 the RAM in the machine, or any other method the boot loader designer
0040 sees fit.)
0041
0042
0043 2. Initialise one serial port
0044 -----------------------------
0045
0046 Existing boot loaders:
0047 OPTIONAL, RECOMMENDED
0048 New boot loaders:
0049 OPTIONAL, RECOMMENDED
0050
0051 The boot loader should initialise and enable one serial port on the
0052 target. This allows the kernel serial driver to automatically detect
0053 which serial port it should use for the kernel console (generally
0054 used for debugging purposes, or communication with the target.)
0055
0056 As an alternative, the boot loader can pass the relevant 'console='
0057 option to the kernel via the tagged lists specifying the port, and
0058 serial format options as described in
0059
0060 Documentation/admin-guide/kernel-parameters.rst.
0061
0062
0063 3. Detect the machine type
0064 --------------------------
0065
0066 Existing boot loaders:
0067 OPTIONAL
0068 New boot loaders:
0069 MANDATORY except for DT-only platforms
0070
0071 The boot loader should detect the machine type its running on by some
0072 method. Whether this is a hard coded value or some algorithm that
0073 looks at the connected hardware is beyond the scope of this document.
0074 The boot loader must ultimately be able to provide a MACH_TYPE_xxx
0075 value to the kernel. (see linux/arch/arm/tools/mach-types). This
0076 should be passed to the kernel in register r1.
0077
0078 For DT-only platforms, the machine type will be determined by device
0079 tree. set the machine type to all ones (~0). This is not strictly
0080 necessary, but assures that it will not match any existing types.
0081
0082 4. Setup boot data
0083 ------------------
0084
0085 Existing boot loaders:
0086 OPTIONAL, HIGHLY RECOMMENDED
0087 New boot loaders:
0088 MANDATORY
0089
0090 The boot loader must provide either a tagged list or a dtb image for
0091 passing configuration data to the kernel. The physical address of the
0092 boot data is passed to the kernel in register r2.
0093
0094 4a. Setup the kernel tagged list
0095 --------------------------------
0096
0097 The boot loader must create and initialise the kernel tagged list.
0098 A valid tagged list starts with ATAG_CORE and ends with ATAG_NONE.
0099 The ATAG_CORE tag may or may not be empty. An empty ATAG_CORE tag
0100 has the size field set to '2' (0x00000002). The ATAG_NONE must set
0101 the size field to zero.
0102
0103 Any number of tags can be placed in the list. It is undefined
0104 whether a repeated tag appends to the information carried by the
0105 previous tag, or whether it replaces the information in its
0106 entirety; some tags behave as the former, others the latter.
0107
0108 The boot loader must pass at a minimum the size and location of
0109 the system memory, and root filesystem location. Therefore, the
0110 minimum tagged list should look::
0111
0112 +-----------+
0113 base -> | ATAG_CORE | |
0114 +-----------+ |
0115 | ATAG_MEM | | increasing address
0116 +-----------+ |
0117 | ATAG_NONE | |
0118 +-----------+ v
0119
0120 The tagged list should be stored in system RAM.
0121
0122 The tagged list must be placed in a region of memory where neither
0123 the kernel decompressor nor initrd 'bootp' program will overwrite
0124 it. The recommended placement is in the first 16KiB of RAM.
0125
0126 4b. Setup the device tree
0127 -------------------------
0128
0129 The boot loader must load a device tree image (dtb) into system ram
0130 at a 64bit aligned address and initialize it with the boot data. The
0131 dtb format is documented at https://www.devicetree.org/specifications/.
0132 The kernel will look for the dtb magic value of 0xd00dfeed at the dtb
0133 physical address to determine if a dtb has been passed instead of a
0134 tagged list.
0135
0136 The boot loader must pass at a minimum the size and location of the
0137 system memory, and the root filesystem location. The dtb must be
0138 placed in a region of memory where the kernel decompressor will not
0139 overwrite it, while remaining within the region which will be covered
0140 by the kernel's low-memory mapping.
0141
0142 A safe location is just above the 128MiB boundary from start of RAM.
0143
0144 5. Load initramfs.
0145 ------------------
0146
0147 Existing boot loaders:
0148 OPTIONAL
0149 New boot loaders:
0150 OPTIONAL
0151
0152 If an initramfs is in use then, as with the dtb, it must be placed in
0153 a region of memory where the kernel decompressor will not overwrite it
0154 while also with the region which will be covered by the kernel's
0155 low-memory mapping.
0156
0157 A safe location is just above the device tree blob which itself will
0158 be loaded just above the 128MiB boundary from the start of RAM as
0159 recommended above.
0160
0161 6. Calling the kernel image
0162 ---------------------------
0163
0164 Existing boot loaders:
0165 MANDATORY
0166 New boot loaders:
0167 MANDATORY
0168
0169 There are two options for calling the kernel zImage. If the zImage
0170 is stored in flash, and is linked correctly to be run from flash,
0171 then it is legal for the boot loader to call the zImage in flash
0172 directly.
0173
0174 The zImage may also be placed in system RAM and called there. The
0175 kernel should be placed in the first 128MiB of RAM. It is recommended
0176 that it is loaded above 32MiB in order to avoid the need to relocate
0177 prior to decompression, which will make the boot process slightly
0178 faster.
0179
0180 When booting a raw (non-zImage) kernel the constraints are tighter.
0181 In this case the kernel must be loaded at an offset into system equal
0182 to TEXT_OFFSET - PAGE_OFFSET.
0183
0184 In any case, the following conditions must be met:
0185
0186 - Quiesce all DMA capable devices so that memory does not get
0187 corrupted by bogus network packets or disk data. This will save
0188 you many hours of debug.
0189
0190 - CPU register settings
0191
0192 - r0 = 0,
0193 - r1 = machine type number discovered in (3) above.
0194 - r2 = physical address of tagged list in system RAM, or
0195 physical address of device tree block (dtb) in system RAM
0196
0197 - CPU mode
0198
0199 All forms of interrupts must be disabled (IRQs and FIQs)
0200
0201 For CPUs which do not include the ARM virtualization extensions, the
0202 CPU must be in SVC mode. (A special exception exists for Angel)
0203
0204 CPUs which include support for the virtualization extensions can be
0205 entered in HYP mode in order to enable the kernel to make full use of
0206 these extensions. This is the recommended boot method for such CPUs,
0207 unless the virtualisations are already in use by a pre-installed
0208 hypervisor.
0209
0210 If the kernel is not entered in HYP mode for any reason, it must be
0211 entered in SVC mode.
0212
0213 - Caches, MMUs
0214
0215 The MMU must be off.
0216
0217 Instruction cache may be on or off.
0218
0219 Data cache must be off.
0220
0221 If the kernel is entered in HYP mode, the above requirements apply to
0222 the HYP mode configuration in addition to the ordinary PL1 (privileged
0223 kernel modes) configuration. In addition, all traps into the
0224 hypervisor must be disabled, and PL1 access must be granted for all
0225 peripherals and CPU resources for which this is architecturally
0226 possible. Except for entering in HYP mode, the system configuration
0227 should be such that a kernel which does not include support for the
0228 virtualization extensions can boot correctly without extra help.
0229
0230 - The boot loader is expected to call the kernel image by jumping
0231 directly to the first instruction of the kernel image.
0232
0233 On CPUs supporting the ARM instruction set, the entry must be
0234 made in ARM state, even for a Thumb-2 kernel.
0235
0236 On CPUs supporting only the Thumb instruction set such as
0237 Cortex-M class CPUs, the entry must be made in Thumb state.
