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Based on kernel version 3.19. Page generated on 2015-02-13 21:16 EST.

1				Booting AArch64 Linux
2				=====================
3	
4	Author: Will Deacon <will.deacon@arm.com>
5	Date  : 07 September 2012
6	
7	This document is based on the ARM booting document by Russell King and
8	is relevant to all public releases of the AArch64 Linux kernel.
9	
10	The AArch64 exception model is made up of a number of exception levels
11	(EL0 - EL3), with EL0 and EL1 having a secure and a non-secure
12	counterpart.  EL2 is the hypervisor level and exists only in non-secure
13	mode. EL3 is the highest priority level and exists only in secure mode.
14	
15	For the purposes of this document, we will use the term `boot loader'
16	simply to define all software that executes on the CPU(s) before control
17	is passed to the Linux kernel.  This may include secure monitor and
18	hypervisor code, or it may just be a handful of instructions for
19	preparing a minimal boot environment.
20	
21	Essentially, the boot loader should provide (as a minimum) the
22	following:
23	
24	1. Setup and initialise the RAM
25	2. Setup the device tree
26	3. Decompress the kernel image
27	4. Call the kernel image
28	
29	
30	1. Setup and initialise RAM
31	---------------------------
32	
33	Requirement: MANDATORY
34	
35	The boot loader is expected to find and initialise all RAM that the
36	kernel will use for volatile data storage in the system.  It performs
37	this in a machine dependent manner.  (It may use internal algorithms
38	to automatically locate and size all RAM, or it may use knowledge of
39	the RAM in the machine, or any other method the boot loader designer
40	sees fit.)
41	
42	
43	2. Setup the device tree
44	-------------------------
45	
46	Requirement: MANDATORY
47	
48	The device tree blob (dtb) must be placed on an 8-byte boundary within
49	the first 512 megabytes from the start of the kernel image and must not
50	cross a 2-megabyte boundary. This is to allow the kernel to map the
51	blob using a single section mapping in the initial page tables.
52	
53	
54	3. Decompress the kernel image
55	------------------------------
56	
57	Requirement: OPTIONAL
58	
59	The AArch64 kernel does not currently provide a decompressor and
60	therefore requires decompression (gzip etc.) to be performed by the boot
61	loader if a compressed Image target (e.g. Image.gz) is used.  For
62	bootloaders that do not implement this requirement, the uncompressed
63	Image target is available instead.
64	
65	
66	4. Call the kernel image
67	------------------------
68	
69	Requirement: MANDATORY
70	
71	The decompressed kernel image contains a 64-byte header as follows:
72	
73	  u32 code0;			/* Executable code */
74	  u32 code1;			/* Executable code */
75	  u64 text_offset;		/* Image load offset, little endian */
76	  u64 image_size;		/* Effective Image size, little endian */
77	  u64 flags;			/* kernel flags, little endian */
78	  u64 res2	= 0;		/* reserved */
79	  u64 res3	= 0;		/* reserved */
80	  u64 res4	= 0;		/* reserved */
81	  u32 magic	= 0x644d5241;	/* Magic number, little endian, "ARM\x64" */
82	  u32 res5;      		/* reserved (used for PE COFF offset) */
83	
84	
85	Header notes:
86	
87	- As of v3.17, all fields are little endian unless stated otherwise.
88	
89	- code0/code1 are responsible for branching to stext.
90	
91	- when booting through EFI, code0/code1 are initially skipped.
92	  res5 is an offset to the PE header and the PE header has the EFI
93	  entry point (efi_stub_entry).  When the stub has done its work, it
94	  jumps to code0 to resume the normal boot process.
95	
96	- Prior to v3.17, the endianness of text_offset was not specified.  In
97	  these cases image_size is zero and text_offset is 0x80000 in the
98	  endianness of the kernel.  Where image_size is non-zero image_size is
99	  little-endian and must be respected.  Where image_size is zero,
100	  text_offset can be assumed to be 0x80000.
101	
102	- The flags field (introduced in v3.17) is a little-endian 64-bit field
103	  composed as follows:
104	  Bit 0: 	Kernel endianness.  1 if BE, 0 if LE.
105	  Bits 1-63:	Reserved.
106	
107	- When image_size is zero, a bootloader should attempt to keep as much
108	  memory as possible free for use by the kernel immediately after the
109	  end of the kernel image. The amount of space required will vary
110	  depending on selected features, and is effectively unbound.
111	
112	The Image must be placed text_offset bytes from a 2MB aligned base
113	address near the start of usable system RAM and called there. Memory
114	below that base address is currently unusable by Linux, and therefore it
115	is strongly recommended that this location is the start of system RAM.
116	At least image_size bytes from the start of the image must be free for
117	use by the kernel.
118	
119	Any memory described to the kernel (even that below the 2MB aligned base
120	address) which is not marked as reserved from the kernel e.g. with a
121	memreserve region in the device tree) will be considered as available to
122	the kernel.
123	
124	Before jumping into the kernel, the following conditions must be met:
125	
126	- Quiesce all DMA capable devices so that memory does not get
127	  corrupted by bogus network packets or disk data.  This will save
128	  you many hours of debug.
129	
130	- Primary CPU general-purpose register settings
131	  x0 = physical address of device tree blob (dtb) in system RAM.
132	  x1 = 0 (reserved for future use)
133	  x2 = 0 (reserved for future use)
134	  x3 = 0 (reserved for future use)
135	
136	- CPU mode
137	  All forms of interrupts must be masked in PSTATE.DAIF (Debug, SError,
138	  IRQ and FIQ).
139	  The CPU must be in either EL2 (RECOMMENDED in order to have access to
140	  the virtualisation extensions) or non-secure EL1.
141	
142	- Caches, MMUs
143	  The MMU must be off.
144	  Instruction cache may be on or off.
145	  The address range corresponding to the loaded kernel image must be
146	  cleaned to the PoC. In the presence of a system cache or other
147	  coherent masters with caches enabled, this will typically require
148	  cache maintenance by VA rather than set/way operations.
149	  System caches which respect the architected cache maintenance by VA
150	  operations must be configured and may be enabled.
151	  System caches which do not respect architected cache maintenance by VA
152	  operations (not recommended) must be configured and disabled.
153	
154	- Architected timers
155	  CNTFRQ must be programmed with the timer frequency and CNTVOFF must
156	  be programmed with a consistent value on all CPUs.  If entering the
157	  kernel at EL1, CNTHCTL_EL2 must have EL1PCTEN (bit 0) set where
158	  available.
159	
160	- Coherency
161	  All CPUs to be booted by the kernel must be part of the same coherency
162	  domain on entry to the kernel.  This may require IMPLEMENTATION DEFINED
163	  initialisation to enable the receiving of maintenance operations on
164	  each CPU.
165	
166	- System registers
167	  All writable architected system registers at the exception level where
168	  the kernel image will be entered must be initialised by software at a
169	  higher exception level to prevent execution in an UNKNOWN state.
170	
171	  For systems with a GICv3 interrupt controller:
172	  - If EL3 is present:
173	    ICC_SRE_EL3.Enable (bit 3) must be initialiased to 0b1.
174	    ICC_SRE_EL3.SRE (bit 0) must be initialised to 0b1.
175	  - If the kernel is entered at EL1:
176	    ICC.SRE_EL2.Enable (bit 3) must be initialised to 0b1
177	    ICC_SRE_EL2.SRE (bit 0) must be initialised to 0b1.
178	
179	The requirements described above for CPU mode, caches, MMUs, architected
180	timers, coherency and system registers apply to all CPUs.  All CPUs must
181	enter the kernel in the same exception level.
182	
183	The boot loader is expected to enter the kernel on each CPU in the
184	following manner:
185	
186	- The primary CPU must jump directly to the first instruction of the
187	  kernel image.  The device tree blob passed by this CPU must contain
188	  an 'enable-method' property for each cpu node.  The supported
189	  enable-methods are described below.
190	
191	  It is expected that the bootloader will generate these device tree
192	  properties and insert them into the blob prior to kernel entry.
193	
194	- CPUs with a "spin-table" enable-method must have a 'cpu-release-addr'
195	  property in their cpu node.  This property identifies a
196	  naturally-aligned 64-bit zero-initalised memory location.
197	
198	  These CPUs should spin outside of the kernel in a reserved area of
199	  memory (communicated to the kernel by a /memreserve/ region in the
200	  device tree) polling their cpu-release-addr location, which must be
201	  contained in the reserved region.  A wfe instruction may be inserted
202	  to reduce the overhead of the busy-loop and a sev will be issued by
203	  the primary CPU.  When a read of the location pointed to by the
204	  cpu-release-addr returns a non-zero value, the CPU must jump to this
205	  value.  The value will be written as a single 64-bit little-endian
206	  value, so CPUs must convert the read value to their native endianness
207	  before jumping to it.
208	
209	- CPUs with a "psci" enable method should remain outside of
210	  the kernel (i.e. outside of the regions of memory described to the
211	  kernel in the memory node, or in a reserved area of memory described
212	  to the kernel by a /memreserve/ region in the device tree).  The
213	  kernel will issue CPU_ON calls as described in ARM document number ARM
214	  DEN 0022A ("Power State Coordination Interface System Software on ARM
215	  processors") to bring CPUs into the kernel.
216	
217	  The device tree should contain a 'psci' node, as described in
218	  Documentation/devicetree/bindings/arm/psci.txt.
219	
220	- Secondary CPU general-purpose register settings
221	  x0 = 0 (reserved for future use)
222	  x1 = 0 (reserved for future use)
223	  x2 = 0 (reserved for future use)
224	  x3 = 0 (reserved for future use)
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