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Linus Torvalds1da177e2005-04-16 15:20:36 -07001 Booting ARM Linux
2 =================
3
4Author: Russell King
5Date : 18 May 2002
6
7The following documentation is relevant to 2.4.18-rmk6 and beyond.
8
9In order to boot ARM Linux, you require a boot loader, which is a small
10program that runs before the main kernel. The boot loader is expected
11to initialise various devices, and eventually call the Linux kernel,
12passing information to the kernel.
13
14Essentially, the boot loader should provide (as a minimum) the
15following:
16
171. Setup and initialise the RAM.
182. Initialise one serial port.
193. Detect the machine type.
204. Setup the kernel tagged list.
Ian Campbell83d26d12013-08-21 11:41:42 +0100215. Load initramfs.
226. Call the kernel image.
Linus Torvalds1da177e2005-04-16 15:20:36 -070023
24
251. Setup and initialise RAM
26---------------------------
27
28Existing boot loaders: MANDATORY
29New boot loaders: MANDATORY
30
31The boot loader is expected to find and initialise all RAM that the
32kernel will use for volatile data storage in the system. It performs
33this in a machine dependent manner. (It may use internal algorithms
34to automatically locate and size all RAM, or it may use knowledge of
35the RAM in the machine, or any other method the boot loader designer
36sees fit.)
37
38
392. Initialise one serial port
40-----------------------------
41
42Existing boot loaders: OPTIONAL, RECOMMENDED
43New boot loaders: OPTIONAL, RECOMMENDED
44
45The boot loader should initialise and enable one serial port on the
46target. This allows the kernel serial driver to automatically detect
47which serial port it should use for the kernel console (generally
48used for debugging purposes, or communication with the target.)
49
50As an alternative, the boot loader can pass the relevant 'console='
51option to the kernel via the tagged lists specifying the port, and
52serial format options as described in
53
54 Documentation/kernel-parameters.txt.
55
56
573. Detect the machine type
58--------------------------
59
60Existing boot loaders: OPTIONAL
61New boot loaders: MANDATORY
62
63The boot loader should detect the machine type its running on by some
64method. Whether this is a hard coded value or some algorithm that
65looks at the connected hardware is beyond the scope of this document.
66The boot loader must ultimately be able to provide a MACH_TYPE_xxx
67value to the kernel. (see linux/arch/arm/tools/mach-types).
68
Grant Likelyede338f2011-04-28 14:27:23 -0600694. Setup boot data
70------------------
Linus Torvalds1da177e2005-04-16 15:20:36 -070071
72Existing boot loaders: OPTIONAL, HIGHLY RECOMMENDED
73New boot loaders: MANDATORY
74
Grant Likelyede338f2011-04-28 14:27:23 -060075The boot loader must provide either a tagged list or a dtb image for
76passing configuration data to the kernel. The physical address of the
77boot data is passed to the kernel in register r2.
78
794a. Setup the kernel tagged list
80--------------------------------
81
Linus Torvalds1da177e2005-04-16 15:20:36 -070082The boot loader must create and initialise the kernel tagged list.
83A valid tagged list starts with ATAG_CORE and ends with ATAG_NONE.
84The ATAG_CORE tag may or may not be empty. An empty ATAG_CORE tag
85has the size field set to '2' (0x00000002). The ATAG_NONE must set
86the size field to zero.
87
88Any number of tags can be placed in the list. It is undefined
89whether a repeated tag appends to the information carried by the
90previous tag, or whether it replaces the information in its
91entirety; some tags behave as the former, others the latter.
92
93The boot loader must pass at a minimum the size and location of
94the system memory, and root filesystem location. Therefore, the
95minimum tagged list should look:
96
97 +-----------+
98base -> | ATAG_CORE | |
99 +-----------+ |
100 | ATAG_MEM | | increasing address
101 +-----------+ |
102 | ATAG_NONE | |
103 +-----------+ v
104
105The tagged list should be stored in system RAM.
106
107The tagged list must be placed in a region of memory where neither
108the kernel decompressor nor initrd 'bootp' program will overwrite
109it. The recommended placement is in the first 16KiB of RAM.
110
Grant Likelyede338f2011-04-28 14:27:23 -06001114b. Setup the device tree
112-------------------------
113
114The boot loader must load a device tree image (dtb) into system ram
115at a 64bit aligned address and initialize it with the boot data. The
116dtb format is documented in Documentation/devicetree/booting-without-of.txt.
117The kernel will look for the dtb magic value of 0xd00dfeed at the dtb
118physical address to determine if a dtb has been passed instead of a
119tagged list.
120
121The boot loader must pass at a minimum the size and location of the
122system memory, and the root filesystem location. The dtb must be
123placed in a region of memory where the kernel decompressor will not
Ian Campbell83d26d12013-08-21 11:41:42 +0100124overwrite it, whilst remaining within the region which will be covered
125by the kernel's low-memory mapping.
Grant Likelyede338f2011-04-28 14:27:23 -0600126
Ian Campbell83d26d12013-08-21 11:41:42 +0100127A safe location is just above the 128MiB boundary from start of RAM.
128
1295. Load initramfs.
130------------------
131
132Existing boot loaders: OPTIONAL
133New boot loaders: OPTIONAL
134
135If an initramfs is in use then, as with the dtb, it must be placed in
136a region of memory where the kernel decompressor will not overwrite it
137while also with the region which will be covered by the kernel's
138low-memory mapping.
139
140A safe location is just above the device tree blob which itself will
141be loaded just above the 128MiB boundary from the start of RAM as
142recommended above.
143
1446. Calling the kernel image
Linus Torvalds1da177e2005-04-16 15:20:36 -0700145---------------------------
146
147Existing boot loaders: MANDATORY
148New boot loaders: MANDATORY
149
150There are two options for calling the kernel zImage. If the zImage
151is stored in flash, and is linked correctly to be run from flash,
152then it is legal for the boot loader to call the zImage in flash
153directly.
154
Ian Campbell83d26d12013-08-21 11:41:42 +0100155The zImage may also be placed in system RAM and called there. The
156kernel should be placed in the first 128MiB of RAM. It is recommended
157that it is loaded above 32MiB in order to avoid the need to relocate
158prior to decompression, which will make the boot process slightly
159faster.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700160
Ian Campbell83d26d12013-08-21 11:41:42 +0100161When booting a raw (non-zImage) kernel the constraints are tighter.
162In this case the kernel must be loaded at an offset into system equal
163to TEXT_OFFSET - PAGE_OFFSET.
164
165In any case, the following conditions must be met:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700166
Andrzej Zaborowski13fce802006-03-24 18:13:37 +0100167- Quiesce all DMA capable devices so that memory does not get
Linus Torvalds1da177e2005-04-16 15:20:36 -0700168 corrupted by bogus network packets or disk data. This will save
169 you many hours of debug.
170
171- CPU register settings
172 r0 = 0,
173 r1 = machine type number discovered in (3) above.
Grant Likelyede338f2011-04-28 14:27:23 -0600174 r2 = physical address of tagged list in system RAM, or
175 physical address of device tree block (dtb) in system RAM
Linus Torvalds1da177e2005-04-16 15:20:36 -0700176
177- CPU mode
178 All forms of interrupts must be disabled (IRQs and FIQs)
Dave Martin6a6d55c2012-02-10 18:07:07 -0800179
180 For CPUs which do not include the ARM virtualization extensions, the
181 CPU must be in SVC mode. (A special exception exists for Angel)
182
183 CPUs which include support for the virtualization extensions can be
184 entered in HYP mode in order to enable the kernel to make full use of
185 these extensions. This is the recommended boot method for such CPUs,
186 unless the virtualisations are already in use by a pre-installed
187 hypervisor.
188
189 If the kernel is not entered in HYP mode for any reason, it must be
190 entered in SVC mode.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700191
192- Caches, MMUs
193 The MMU must be off.
194 Instruction cache may be on or off.
195 Data cache must be off.
196
Dave Martin6a6d55c2012-02-10 18:07:07 -0800197 If the kernel is entered in HYP mode, the above requirements apply to
198 the HYP mode configuration in addition to the ordinary PL1 (privileged
199 kernel modes) configuration. In addition, all traps into the
200 hypervisor must be disabled, and PL1 access must be granted for all
201 peripherals and CPU resources for which this is architecturally
202 possible. Except for entering in HYP mode, the system configuration
203 should be such that a kernel which does not include support for the
204 virtualization extensions can boot correctly without extra help.
205
Linus Torvalds1da177e2005-04-16 15:20:36 -0700206- The boot loader is expected to call the kernel image by jumping
207 directly to the first instruction of the kernel image.
208
Dave Martin540b5732011-07-13 15:53:30 +0100209 On CPUs supporting the ARM instruction set, the entry must be
210 made in ARM state, even for a Thumb-2 kernel.
211
212 On CPUs supporting only the Thumb instruction set such as
213 Cortex-M class CPUs, the entry must be made in Thumb state.