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Based on kernel version 3.9. Page generated on 2013-05-02 22:54 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	4. Setup boot data
69	------------------
70	
71	Existing boot loaders:		OPTIONAL, HIGHLY RECOMMENDED
72	New boot loaders:		MANDATORY
73	
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	
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	
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	
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	
145	- Quiesce all DMA capable devices so that memory does not get
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.
152	  r2 = physical address of tagged list in system RAM, or
153	       physical address of device tree block (dtb) in system RAM
154	
155	- CPU mode
156	  All forms of interrupts must be disabled (IRQs and FIQs)
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.
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	
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	
184	- The boot loader is expected to call the kernel image by jumping
185	  directly to the first instruction of the kernel image.
186	
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.
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