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1da177e4 LT |
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 | ||
13fce806 | 121 | - Quiesce all DMA capable devices so that memory does not get |
1da177e4 LT |
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. | |
141 |