1 ================= 2 Booting ARM Linux 3 ================= 4 5 Author: Russell King 6 7 Date : 18 May 2002 8 9 The following documentation is relevant to 2.4.18-rmk6 and beyond. 10 11 In order to boot ARM Linux, you require a boot loader, which is a small 12 program that runs before the main kernel. The boot loader is expected 13 to initialise various devices, and eventually call the Linux kernel, 14 passing information to the kernel. 15 16 Essentially, the boot loader should provide (as a minimum) the 17 following: 18 19 1. Setup and initialise the RAM. 20 2. Initialise one serial port. 21 3. Detect the machine type. 22 4. Setup the kernel tagged list. 23 5. Load initramfs. 24 6. Call the kernel image. 25 26 27 1. Setup and initialise RAM 28 --------------------------- 29 30 Existing boot loaders: 31 MANDATORY 32 New boot loaders: 33 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. Initialise one serial port 44 ----------------------------- 45 46 Existing boot loaders: 47 OPTIONAL, RECOMMENDED 48 New boot loaders: 49 OPTIONAL, RECOMMENDED 50 51 The boot loader should initialise and enable one serial port on the 52 target. This allows the kernel serial driver to automatically detect 53 which serial port it should use for the kernel console (generally 54 used for debugging purposes, or communication with the target.) 55 56 As an alternative, the boot loader can pass the relevant 'console=' 57 option to the kernel via the tagged lists specifying the port, and 58 serial format options as described in 59 60 Documentation/admin-guide/kernel-parameters.rst. 61 62 63 3. Detect the machine type 64 -------------------------- 65 66 Existing boot loaders: 67 OPTIONAL 68 New boot loaders: 69 MANDATORY except for DT-only platforms 70 71 The boot loader should detect the machine type its running on by some 72 method. Whether this is a hard coded value or some algorithm that 73 looks at the connected hardware is beyond the scope of this document. 74 The boot loader must ultimately be able to provide a MACH_TYPE_xxx 75 value to the kernel. (see linux/arch/arm/tools/mach-types). This 76 should be passed to the kernel in register r1. 77 78 For DT-only platforms, the machine type will be determined by device 79 tree. set the machine type to all ones (~0). This is not strictly 80 necessary, but assures that it will not match any existing types. 81 82 4. Setup boot data 83 ------------------ 84 85 Existing boot loaders: 86 OPTIONAL, HIGHLY RECOMMENDED 87 New boot loaders: 88 MANDATORY 89 90 The boot loader must provide either a tagged list or a dtb image for 91 passing configuration data to the kernel. The physical address of the 92 boot data is passed to the kernel in register r2. 93 94 4a. Setup the kernel tagged list 95 -------------------------------- 96 97 The boot loader must create and initialise the kernel tagged list. 98 A valid tagged list starts with ATAG_CORE and ends with ATAG_NONE. 99 The ATAG_CORE tag may or may not be empty. An empty ATAG_CORE tag 100 has the size field set to '2' (0x00000002). The ATAG_NONE must set 101 the size field to zero. 102 103 Any number of tags can be placed in the list. It is undefined 104 whether a repeated tag appends to the information carried by the 105 previous tag, or whether it replaces the information in its 106 entirety; some tags behave as the former, others the latter. 107 108 The boot loader must pass at a minimum the size and location of 109 the system memory, and root filesystem location. Therefore, the 110 minimum tagged list should look:: 111 112 +-----------+ 113 base -> | ATAG_CORE | | 114 +-----------+ | 115 | ATAG_MEM | | increasing address 116 +-----------+ | 117 | ATAG_NONE | | 118 +-----------+ v 119 120 The tagged list should be stored in system RAM. 121 122 The tagged list must be placed in a region of memory where neither 123 the kernel decompressor nor initrd 'bootp' program will overwrite 124 it. The recommended placement is in the first 16KiB of RAM. 125 126 4b. Setup the device tree 127 ------------------------- 128 129 The boot loader must load a device tree image (dtb) into system ram 130 at a 64bit aligned address and initialize it with the boot data. The 131 dtb format is documented at https://www.devicetree.org/specifications/. 132 The kernel will look for the dtb magic value of 0xd00dfeed at the dtb 133 physical address to determine if a dtb has been passed instead of a 134 tagged list. 135 136 The boot loader must pass at a minimum the size and location of the 137 system memory, and the root filesystem location. The dtb must be 138 placed in a region of memory where the kernel decompressor will not 139 overwrite it, while remaining within the region which will be covered 140 by the kernel's low-memory mapping. 141 142 A safe location is just above the 128MiB boundary from start of RAM. 143 144 5. Load initramfs. 145 ------------------ 146 147 Existing boot loaders: 148 OPTIONAL 149 New boot loaders: 150 OPTIONAL 151 152 If an initramfs is in use then, as with the dtb, it must be placed in 153 a region of memory where the kernel decompressor will not overwrite it 154 while also with the region which will be covered by the kernel's 155 low-memory mapping. 156 157 A safe location is just above the device tree blob which itself will 158 be loaded just above the 128MiB boundary from the start of RAM as 159 recommended above. 160 161 6. Calling the kernel image 162 --------------------------- 163 164 Existing boot loaders: 165 MANDATORY 166 New boot loaders: 167 MANDATORY 168 169 There are two options for calling the kernel zImage. If the zImage 170 is stored in flash, and is linked correctly to be run from flash, 171 then it is legal for the boot loader to call the zImage in flash 172 directly. 173 174 The zImage may also be placed in system RAM and called there. The 175 kernel should be placed in the first 128MiB of RAM. It is recommended 176 that it is loaded above 32MiB in order to avoid the need to relocate 177 prior to decompression, which will make the boot process slightly 178 faster. 179 180 When booting a raw (non-zImage) kernel the constraints are tighter. 181 In this case the kernel must be loaded at an offset into system equal 182 to TEXT_OFFSET - PAGE_OFFSET. 183 184 In any case, the following conditions must be met: 185 186 - Quiesce all DMA capable devices so that memory does not get 187 corrupted by bogus network packets or disk data. This will save 188 you many hours of debug. 189 190 - CPU register settings 191 192 - r0 = 0, 193 - r1 = machine type number discovered in (3) above. 194 - r2 = physical address of tagged list in system RAM, or 195 physical address of device tree block (dtb) in system RAM 196 197 - CPU mode 198 199 All forms of interrupts must be disabled (IRQs and FIQs) 200 201 For CPUs which do not include the ARM virtualization extensions, the 202 CPU must be in SVC mode. (A special exception exists for Angel) 203 204 CPUs which include support for the virtualization extensions can be 205 entered in HYP mode in order to enable the kernel to make full use of 206 these extensions. This is the recommended boot method for such CPUs, 207 unless the virtualisations are already in use by a pre-installed 208 hypervisor. 209 210 If the kernel is not entered in HYP mode for any reason, it must be 211 entered in SVC mode. 212 213 - Caches, MMUs 214 215 The MMU must be off. 216 217 Instruction cache may be on or off. 218 219 Data cache must be off. 220 221 If the kernel is entered in HYP mode, the above requirements apply to 222 the HYP mode configuration in addition to the ordinary PL1 (privileged 223 kernel modes) configuration. In addition, all traps into the 224 hypervisor must be disabled, and PL1 access must be granted for all 225 peripherals and CPU resources for which this is architecturally 226 possible. Except for entering in HYP mode, the system configuration 227 should be such that a kernel which does not include support for the 228 virtualization extensions can boot correctly without extra help. 229 230 - The boot loader is expected to call the kernel image by jumping 231 directly to the first instruction of the kernel image. 232 233 On CPUs supporting the ARM instruction set, the entry must be 234 made in ARM state, even for a Thumb-2 kernel. 235 236 On CPUs supporting only the Thumb instruction set such as 237 Cortex-M class CPUs, the entry must be made in Thumb state.
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