About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / arm / Booting

Based on kernel version 2.6.26. Page generated on 2008-07-16 21:12 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	
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	
121	- Quiesce all DMA capable devices so that memory does not get
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.
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.