Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.
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.