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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 | ||
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68 | 4. Setup boot data |
69 | ------------------ | |
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70 | |
71 | Existing boot loaders: OPTIONAL, HIGHLY RECOMMENDED | |
72 | New boot loaders: MANDATORY | |
73 | ||
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74 | The boot loader must provide either a tagged list or a dtb image for |
75 | passing configuration data to the kernel. The physical address of the | |
76 | boot data is passed to the kernel in register r2. | |
77 | ||
78 | 4a. Setup the kernel tagged list | |
79 | -------------------------------- | |
80 | ||
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81 | The boot loader must create and initialise the kernel tagged list. |
82 | A valid tagged list starts with ATAG_CORE and ends with ATAG_NONE. | |
83 | The ATAG_CORE tag may or may not be empty. An empty ATAG_CORE tag | |
84 | has the size field set to '2' (0x00000002). The ATAG_NONE must set | |
85 | the size field to zero. | |
86 | ||
87 | Any number of tags can be placed in the list. It is undefined | |
88 | whether a repeated tag appends to the information carried by the | |
89 | previous tag, or whether it replaces the information in its | |
90 | entirety; some tags behave as the former, others the latter. | |
91 | ||
92 | The boot loader must pass at a minimum the size and location of | |
93 | the system memory, and root filesystem location. Therefore, the | |
94 | minimum tagged list should look: | |
95 | ||
96 | +-----------+ | |
97 | base -> | ATAG_CORE | | | |
98 | +-----------+ | | |
99 | | ATAG_MEM | | increasing address | |
100 | +-----------+ | | |
101 | | ATAG_NONE | | | |
102 | +-----------+ v | |
103 | ||
104 | The tagged list should be stored in system RAM. | |
105 | ||
106 | The tagged list must be placed in a region of memory where neither | |
107 | the kernel decompressor nor initrd 'bootp' program will overwrite | |
108 | it. The recommended placement is in the first 16KiB of RAM. | |
109 | ||
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110 | 4b. Setup the device tree |
111 | ------------------------- | |
112 | ||
113 | The boot loader must load a device tree image (dtb) into system ram | |
114 | at a 64bit aligned address and initialize it with the boot data. The | |
115 | dtb format is documented in Documentation/devicetree/booting-without-of.txt. | |
116 | The kernel will look for the dtb magic value of 0xd00dfeed at the dtb | |
117 | physical address to determine if a dtb has been passed instead of a | |
118 | tagged list. | |
119 | ||
120 | The boot loader must pass at a minimum the size and location of the | |
121 | system memory, and the root filesystem location. The dtb must be | |
122 | placed in a region of memory where the kernel decompressor will not | |
123 | overwrite it. The recommended placement is in the first 16KiB of RAM | |
124 | with the caveat that it may not be located at physical address 0 since | |
125 | the kernel interprets a value of 0 in r2 to mean neither a tagged list | |
126 | nor a dtb were passed. | |
127 | ||
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128 | 5. Calling the kernel image |
129 | --------------------------- | |
130 | ||
131 | Existing boot loaders: MANDATORY | |
132 | New boot loaders: MANDATORY | |
133 | ||
134 | There are two options for calling the kernel zImage. If the zImage | |
135 | is stored in flash, and is linked correctly to be run from flash, | |
136 | then it is legal for the boot loader to call the zImage in flash | |
137 | directly. | |
138 | ||
139 | The zImage may also be placed in system RAM (at any location) and | |
140 | called there. Note that the kernel uses 16K of RAM below the image | |
141 | to store page tables. The recommended placement is 32KiB into RAM. | |
142 | ||
143 | In either case, the following conditions must be met: | |
144 | ||
13fce806 | 145 | - Quiesce all DMA capable devices so that memory does not get |
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146 | corrupted by bogus network packets or disk data. This will save |
147 | you many hours of debug. | |
148 | ||
149 | - CPU register settings | |
150 | r0 = 0, | |
151 | r1 = machine type number discovered in (3) above. | |
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152 | r2 = physical address of tagged list in system RAM, or |
153 | physical address of device tree block (dtb) in system RAM | |
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154 | |
155 | - CPU mode | |
156 | All forms of interrupts must be disabled (IRQs and FIQs) | |
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157 | |
158 | For CPUs which do not include the ARM virtualization extensions, the | |
159 | CPU must be in SVC mode. (A special exception exists for Angel) | |
160 | ||
161 | CPUs which include support for the virtualization extensions can be | |
162 | entered in HYP mode in order to enable the kernel to make full use of | |
163 | these extensions. This is the recommended boot method for such CPUs, | |
164 | unless the virtualisations are already in use by a pre-installed | |
165 | hypervisor. | |
166 | ||
167 | If the kernel is not entered in HYP mode for any reason, it must be | |
168 | entered in SVC mode. | |
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169 | |
170 | - Caches, MMUs | |
171 | The MMU must be off. | |
172 | Instruction cache may be on or off. | |
173 | Data cache must be off. | |
174 | ||
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175 | If the kernel is entered in HYP mode, the above requirements apply to |
176 | the HYP mode configuration in addition to the ordinary PL1 (privileged | |
177 | kernel modes) configuration. In addition, all traps into the | |
178 | hypervisor must be disabled, and PL1 access must be granted for all | |
179 | peripherals and CPU resources for which this is architecturally | |
180 | possible. Except for entering in HYP mode, the system configuration | |
181 | should be such that a kernel which does not include support for the | |
182 | virtualization extensions can boot correctly without extra help. | |
183 | ||
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184 | - The boot loader is expected to call the kernel image by jumping |
185 | directly to the first instruction of the kernel image. | |
186 | ||
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187 | On CPUs supporting the ARM instruction set, the entry must be |
188 | made in ARM state, even for a Thumb-2 kernel. | |
189 | ||
190 | On CPUs supporting only the Thumb instruction set such as | |
191 | Cortex-M class CPUs, the entry must be made in Thumb state. |