Based on kernel version 2.6.26. Page generated on 2008-07-16 21:12 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. Call the kernel image. 22 23 24 1. Setup and initialise RAM 25 --------------------------- 26 27 Existing boot loaders: MANDATORY 28 New boot loaders: MANDATORY 29 30 The boot loader is expected to find and initialise all RAM that the 31 kernel will use for volatile data storage in the system. It performs 32 this in a machine dependent manner. (It may use internal algorithms 33 to automatically locate and size all RAM, or it may use knowledge of 34 the RAM in the machine, or any other method the boot loader designer 35 sees fit.) 36 37 38 2. Initialise one serial port 39 ----------------------------- 40 41 Existing boot loaders: OPTIONAL, RECOMMENDED 42 New boot loaders: OPTIONAL, RECOMMENDED 43 44 The boot loader should initialise and enable one serial port on the 45 target. This allows the kernel serial driver to automatically detect 46 which serial port it should use for the kernel console (generally 47 used for debugging purposes, or communication with the target.) 48 49 As an alternative, the boot loader can pass the relevant 'console=' 50 option to the kernel via the tagged lists specifying the port, and 51 serial format options as described in 52 53 Documentation/kernel-parameters.txt. 54 55 56 3. Detect the machine type 57 -------------------------- 58 59 Existing boot loaders: OPTIONAL 60 New boot loaders: MANDATORY 61 62 The boot loader should detect the machine type its running on by some 63 method. Whether this is a hard coded value or some algorithm that 64 looks at the connected hardware is beyond the scope of this document. 65 The boot loader must ultimately be able to provide a MACH_TYPE_xxx 66 value to the kernel. (see linux/arch/arm/tools/mach-types). 67 68 69 4. Setup the kernel tagged list 70 ------------------------------- 71 72 Existing boot loaders: OPTIONAL, HIGHLY RECOMMENDED 73 New boot loaders: MANDATORY 74 75 The boot loader must create and initialise the kernel tagged list. 76 A valid tagged list starts with ATAG_CORE and ends with ATAG_NONE. 77 The ATAG_CORE tag may or may not be empty. An empty ATAG_CORE tag 78 has the size field set to '2' (0x00000002). The ATAG_NONE must set 79 the size field to zero. 80 81 Any number of tags can be placed in the list. It is undefined 82 whether a repeated tag appends to the information carried by the 83 previous tag, or whether it replaces the information in its 84 entirety; some tags behave as the former, others the latter. 85 86 The boot loader must pass at a minimum the size and location of 87 the system memory, and root filesystem location. Therefore, the 88 minimum tagged list should look: 89 90 +-----------+ 91 base -> | ATAG_CORE | | 92 +-----------+ | 93 | ATAG_MEM | | increasing address 94 +-----------+ | 95 | ATAG_NONE | | 96 +-----------+ v 97 98 The tagged list should be stored in system RAM. 99 100 The tagged list must be placed in a region of memory where neither 101 the kernel decompressor nor initrd 'bootp' program will overwrite 102 it. The recommended placement is in the first 16KiB of RAM. 103 104 5. Calling the kernel image 105 --------------------------- 106 107 Existing boot loaders: MANDATORY 108 New boot loaders: MANDATORY 109 110 There are two options for calling the kernel zImage. If the zImage 111 is stored in flash, and is linked correctly to be run from flash, 112 then it is legal for the boot loader to call the zImage in flash 113 directly. 114 115 The zImage may also be placed in system RAM (at any location) and 116 called there. Note that the kernel uses 16K of RAM below the image 117 to store page tables. The recommended placement is 32KiB into RAM. 118 119 In either case, the following conditions must be met: 120 121 - Quiesce all DMA capable devices so that memory does not get 122 corrupted by bogus network packets or disk data. This will save 123 you many hours of debug. 124 125 - CPU register settings 126 r0 = 0, 127 r1 = machine type number discovered in (3) above. 128 r2 = physical address of tagged list in system RAM. 129 130 - CPU mode 131 All forms of interrupts must be disabled (IRQs and FIQs) 132 The CPU must be in SVC mode. (A special exception exists for Angel) 133 134 - Caches, MMUs 135 The MMU must be off. 136 Instruction cache may be on or off. 137 Data cache must be off. 138 139 - The boot loader is expected to call the kernel image by jumping 140 directly to the first instruction of the kernel image.