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
10
11 In order to boot ARM Linux, you require a boot
12 program that runs before the main kernel. The
13 to initialise various devices, and eventually
14 passing information to the kernel.
15
16 Essentially, the boot loader should provide (a
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 initia
36 kernel will use for volatile data storage in t
37 this in a machine dependent manner. (It may u
38 to automatically locate and size all RAM, or i
39 the RAM in the machine, or any other method th
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 o
52 target. This allows the kernel serial driver
53 which serial port it should use for the kernel
54 used for debugging purposes, or communication
55
56 As an alternative, the boot loader can pass th
57 option to the kernel via the tagged lists spec
58 serial format options as described in
59
60 Documentation/admin-guide/kernel-parame
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
72 method. Whether this is a hard coded value or
73 looks at the connected hardware is beyond the
74 The boot loader must ultimately be able to pro
75 value to the kernel. (see linux/arch/arm/tools
76 should be passed to the kernel in register r1.
77
78 For DT-only platforms, the machine type will b
79 tree. set the machine type to all ones (~0).
80 necessary, but assures that it will not match
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 l
91 passing configuration data to the kernel. The
92 boot data is passed to the kernel in register
93
94 4a. Setup the kernel tagged list
95 --------------------------------
96
97 The boot loader must create and initialise the
98 A valid tagged list starts with ATAG_CORE and
99 The ATAG_CORE tag may or may not be empty. An
100 has the size field set to '2' (0x00000002). T
101 the size field to zero.
102
103 Any number of tags can be placed in the list.
104 whether a repeated tag appends to the informat
105 previous tag, or whether it replaces the infor
106 entirety; some tags behave as the former, othe
107
108 The boot loader must pass at a minimum the siz
109 the system memory, and root filesystem locatio
110 minimum tagged list should look::
111
112 +-----------+
113 base -> | ATAG_CORE | |
114 +-----------+ |
115 | ATAG_MEM | | increasing ad
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
123 the kernel decompressor nor initrd 'bootp' pro
124 it. The recommended placement is in the first
125
126 4b. Setup the device tree
127 -------------------------
128
129 The boot loader must load a device tree image
130 at a 64bit aligned address and initialize it w
131 dtb format is documented at https://www.device
132 The kernel will look for the dtb magic value o
133 physical address to determine if a dtb has bee
134 tagged list.
135
136 The boot loader must pass at a minimum the siz
137 system memory, and the root filesystem locatio
138 placed in a region of memory where the kernel
139 overwrite it, while remaining within the regio
140 by the kernel's low-memory mapping.
141
142 A safe location is just above the 128MiB bound
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 dt
153 a region of memory where the kernel decompress
154 while also with the region which will be cover
155 low-memory mapping.
156
157 A safe location is just above the device tree
158 be loaded just above the 128MiB boundary from
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 z
170 is stored in flash, and is linked correctly to
171 then it is legal for the boot loader to call t
172 directly.
173
174 The zImage may also be placed in system RAM an
175 kernel should be placed in the first 128MiB of
176 that it is loaded above 32MiB in order to avoi
177 prior to decompression, which will make the bo
178 faster.
179
180 When booting a raw (non-zImage) kernel the con
181 In this case the kernel must be loaded at an o
182 to TEXT_OFFSET - PAGE_OFFSET.
183
184 In any case, the following conditions must be
185
186 - Quiesce all DMA capable devices so that memo
187 corrupted by bogus network packets or disk d
188 you many hours of debug.
189
190 - CPU register settings
191
192 - r0 = 0,
193 - r1 = machine type number discovered in (3)
194 - r2 = physical address of tagged list in sy
195 physical address of device tree block (dtb
196
197 - CPU mode
198
199 All forms of interrupts must be disabled (IR
200
201 For CPUs which do not include the ARM virtua
202 CPU must be in SVC mode. (A special excepti
203
204 CPUs which include support for the virtualiz
205 entered in HYP mode in order to enable the k
206 these extensions. This is the recommended b
207 unless the virtualisations are already in us
208 hypervisor.
209
210 If the kernel is not entered in HYP mode for
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 ab
222 the HYP mode configuration in addition to th
223 kernel modes) configuration. In addition, a
224 hypervisor must be disabled, and PL1 access
225 peripherals and CPU resources for which this
226 possible. Except for entering in HYP mode,
227 should be such that a kernel which does not
228 virtualization extensions can boot correctly
229
230 - The boot loader is expected to call the kern
231 directly to the first instruction of the ker
232
233 On CPUs supporting the ARM instruction set,
234 made in ARM state, even for a Thumb-2 kernel
235
236 On CPUs supporting only the Thumb instructio
237 Cortex-M class CPUs, the entry must be made
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