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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.

1				Booting ARM Linux
2				=================
4	Author:	Russell King
5	Date  : 18 May 2002
7	The following documentation is relevant to 2.4.18-rmk6 and beyond.
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.
14	Essentially, the boot loader should provide (as a minimum) the
15	following:
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. Load initramfs.
22	6. Call the kernel image.
25	1. Setup and initialise RAM
26	---------------------------
28	Existing boot loaders:		MANDATORY
29	New boot loaders:		MANDATORY
31	The boot loader is expected to find and initialise all RAM that the
32	kernel will use for volatile data storage in the system.  It performs
33	this in a machine dependent manner.  (It may use internal algorithms
34	to automatically locate and size all RAM, or it may use knowledge of
35	the RAM in the machine, or any other method the boot loader designer
36	sees fit.)
39	2. Initialise one serial port
40	-----------------------------
42	Existing boot loaders:		OPTIONAL, RECOMMENDED
43	New boot loaders:		OPTIONAL, RECOMMENDED
45	The boot loader should initialise and enable one serial port on the
46	target.  This allows the kernel serial driver to automatically detect
47	which serial port it should use for the kernel console (generally
48	used for debugging purposes, or communication with the target.)
50	As an alternative, the boot loader can pass the relevant 'console='
51	option to the kernel via the tagged lists specifying the port, and
52	serial format options as described in
54	       Documentation/admin-guide/kernel-parameters.rst.
57	3. Detect the machine type
58	--------------------------
60	Existing boot loaders:		OPTIONAL
61	New boot loaders:		MANDATORY except for DT-only platforms
63	The boot loader should detect the machine type its running on by some
64	method.  Whether this is a hard coded value or some algorithm that
65	looks at the connected hardware is beyond the scope of this document.
66	The boot loader must ultimately be able to provide a MACH_TYPE_xxx
67	value to the kernel. (see linux/arch/arm/tools/mach-types).  This
68	should be passed to the kernel in register r1.
70	For DT-only platforms, the machine type will be determined by device
71	tree.  set the machine type to all ones (~0).  This is not strictly
72	necessary, but assures that it will not match any existing types.
74	4. Setup boot data
75	------------------
77	Existing boot loaders:		OPTIONAL, HIGHLY RECOMMENDED
78	New boot loaders:		MANDATORY
80	The boot loader must provide either a tagged list or a dtb image for
81	passing configuration data to the kernel.  The physical address of the
82	boot data is passed to the kernel in register r2.
84	4a. Setup the kernel tagged list
85	--------------------------------
87	The boot loader must create and initialise the kernel tagged list.
88	A valid tagged list starts with ATAG_CORE and ends with ATAG_NONE.
89	The ATAG_CORE tag may or may not be empty.  An empty ATAG_CORE tag
90	has the size field set to '2' (0x00000002).  The ATAG_NONE must set
91	the size field to zero.
93	Any number of tags can be placed in the list.  It is undefined
94	whether a repeated tag appends to the information carried by the
95	previous tag, or whether it replaces the information in its
96	entirety; some tags behave as the former, others the latter.
98	The boot loader must pass at a minimum the size and location of
99	the system memory, and root filesystem location.  Therefore, the
100	minimum tagged list should look:
102		+-----------+
103	base ->	| ATAG_CORE |  |
104		+-----------+  |
105		| ATAG_MEM  |  | increasing address
106		+-----------+  |
107		| ATAG_NONE |  |
108		+-----------+  v
110	The tagged list should be stored in system RAM.
112	The tagged list must be placed in a region of memory where neither
113	the kernel decompressor nor initrd 'bootp' program will overwrite
114	it.  The recommended placement is in the first 16KiB of RAM.
116	4b. Setup the device tree
117	-------------------------
119	The boot loader must load a device tree image (dtb) into system ram
120	at a 64bit aligned address and initialize it with the boot data.  The
121	dtb format is documented in Documentation/devicetree/booting-without-of.txt.
122	The kernel will look for the dtb magic value of 0xd00dfeed at the dtb
123	physical address to determine if a dtb has been passed instead of a
124	tagged list.
126	The boot loader must pass at a minimum the size and location of the
127	system memory, and the root filesystem location.  The dtb must be
128	placed in a region of memory where the kernel decompressor will not
129	overwrite it, whilst remaining within the region which will be covered
130	by the kernel's low-memory mapping.
132	A safe location is just above the 128MiB boundary from start of RAM.
134	5. Load initramfs.
135	------------------
137	Existing boot loaders:		OPTIONAL
138	New boot loaders:		OPTIONAL
140	If an initramfs is in use then, as with the dtb, it must be placed in
141	a region of memory where the kernel decompressor will not overwrite it
142	while also with the region which will be covered by the kernel's
143	low-memory mapping.
145	A safe location is just above the device tree blob which itself will
146	be loaded just above the 128MiB boundary from the start of RAM as
147	recommended above.
149	6. Calling the kernel image
150	---------------------------
152	Existing boot loaders:		MANDATORY
153	New boot loaders:		MANDATORY
155	There are two options for calling the kernel zImage.  If the zImage
156	is stored in flash, and is linked correctly to be run from flash,
157	then it is legal for the boot loader to call the zImage in flash
158	directly.
160	The zImage may also be placed in system RAM and called there.  The
161	kernel should be placed in the first 128MiB of RAM.  It is recommended
162	that it is loaded above 32MiB in order to avoid the need to relocate
163	prior to decompression, which will make the boot process slightly
164	faster.
166	When booting a raw (non-zImage) kernel the constraints are tighter.
167	In this case the kernel must be loaded at an offset into system equal
170	In any case, the following conditions must be met:
172	- Quiesce all DMA capable devices so that memory does not get
173	  corrupted by bogus network packets or disk data. This will save
174	  you many hours of debug.
176	- CPU register settings
177	  r0 = 0,
178	  r1 = machine type number discovered in (3) above.
179	  r2 = physical address of tagged list in system RAM, or
180	       physical address of device tree block (dtb) in system RAM
182	- CPU mode
183	  All forms of interrupts must be disabled (IRQs and FIQs)
185	  For CPUs which do not include the ARM virtualization extensions, the
186	  CPU must be in SVC mode.  (A special exception exists for Angel)
188	  CPUs which include support for the virtualization extensions can be
189	  entered in HYP mode in order to enable the kernel to make full use of
190	  these extensions.  This is the recommended boot method for such CPUs,
191	  unless the virtualisations are already in use by a pre-installed
192	  hypervisor.
194	  If the kernel is not entered in HYP mode for any reason, it must be
195	  entered in SVC mode.
197	- Caches, MMUs
198	  The MMU must be off.
199	  Instruction cache may be on or off.
200	  Data cache must be off.
202	  If the kernel is entered in HYP mode, the above requirements apply to
203	  the HYP mode configuration in addition to the ordinary PL1 (privileged
204	  kernel modes) configuration.  In addition, all traps into the
205	  hypervisor must be disabled, and PL1 access must be granted for all
206	  peripherals and CPU resources for which this is architecturally
207	  possible.  Except for entering in HYP mode, the system configuration
208	  should be such that a kernel which does not include support for the
209	  virtualization extensions can boot correctly without extra help.
211	- The boot loader is expected to call the kernel image by jumping
212	  directly to the first instruction of the kernel image.
214	  On CPUs supporting the ARM instruction set, the entry must be
215	  made in ARM state, even for a Thumb-2 kernel.
217	  On CPUs supporting only the Thumb instruction set such as
218	  Cortex-M class CPUs, the entry must be made in Thumb state.
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