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Based on kernel version 3.15.4. Page generated on 2014-07-07 08:59 EST.

1				Booting AArch64 Linux
2				=====================
4	Author: Will Deacon <will.deacon@arm.com>
5	Date  : 07 September 2012
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.
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.
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.
21	Essentially, the boot loader should provide (as a minimum) the
22	following:
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
30	1. Setup and initialise RAM
31	---------------------------
33	Requirement: MANDATORY
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.)
43	2. Setup the device tree
44	-------------------------
46	Requirement: MANDATORY
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.
54	3. Decompress the kernel image
55	------------------------------
57	Requirement: OPTIONAL
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.
66	4. Call the kernel image
67	------------------------
69	Requirement: MANDATORY
71	The decompressed kernel image contains a 64-byte header as follows:
73	  u32 code0;			/* Executable code */
74	  u32 code1;			/* Executable code */
75	  u64 text_offset;		/* Image load offset */
76	  u64 res0	= 0;		/* reserved */
77	  u64 res1	= 0;		/* reserved */
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 = 0;      		/* reserved */
85	Header notes:
87	- code0/code1 are responsible for branching to stext.
89	The image must be placed at the specified offset (currently 0x80000)
90	from the start of the system RAM and called there. The start of the
91	system RAM must be aligned to 2MB.
93	Before jumping into the kernel, the following conditions must be met:
95	- Quiesce all DMA capable devices so that memory does not get
96	  corrupted by bogus network packets or disk data.  This will save
97	  you many hours of debug.
99	- Primary CPU general-purpose register settings
100	  x0 = physical address of device tree blob (dtb) in system RAM.
101	  x1 = 0 (reserved for future use)
102	  x2 = 0 (reserved for future use)
103	  x3 = 0 (reserved for future use)
105	- CPU mode
106	  All forms of interrupts must be masked in PSTATE.DAIF (Debug, SError,
107	  IRQ and FIQ).
108	  The CPU must be in either EL2 (RECOMMENDED in order to have access to
109	  the virtualisation extensions) or non-secure EL1.
111	- Caches, MMUs
112	  The MMU must be off.
113	  Instruction cache may be on or off.
114	  The address range corresponding to the loaded kernel image must be
115	  cleaned to the PoC. In the presence of a system cache or other
116	  coherent masters with caches enabled, this will typically require
117	  cache maintenance by VA rather than set/way operations.
118	  System caches which respect the architected cache maintenance by VA
119	  operations must be configured and may be enabled.
120	  System caches which do not respect architected cache maintenance by VA
121	  operations (not recommended) must be configured and disabled.
123	- Architected timers
124	  CNTFRQ must be programmed with the timer frequency and CNTVOFF must
125	  be programmed with a consistent value on all CPUs.  If entering the
126	  kernel at EL1, CNTHCTL_EL2 must have EL1PCTEN (bit 0) set where
127	  available.
129	- Coherency
130	  All CPUs to be booted by the kernel must be part of the same coherency
131	  domain on entry to the kernel.  This may require IMPLEMENTATION DEFINED
132	  initialisation to enable the receiving of maintenance operations on
133	  each CPU.
135	- System registers
136	  All writable architected system registers at the exception level where
137	  the kernel image will be entered must be initialised by software at a
138	  higher exception level to prevent execution in an UNKNOWN state.
140	The requirements described above for CPU mode, caches, MMUs, architected
141	timers, coherency and system registers apply to all CPUs.  All CPUs must
142	enter the kernel in the same exception level.
144	The boot loader is expected to enter the kernel on each CPU in the
145	following manner:
147	- The primary CPU must jump directly to the first instruction of the
148	  kernel image.  The device tree blob passed by this CPU must contain
149	  an 'enable-method' property for each cpu node.  The supported
150	  enable-methods are described below.
152	  It is expected that the bootloader will generate these device tree
153	  properties and insert them into the blob prior to kernel entry.
155	- CPUs with a "spin-table" enable-method must have a 'cpu-release-addr'
156	  property in their cpu node.  This property identifies a
157	  naturally-aligned 64-bit zero-initalised memory location.
159	  These CPUs should spin outside of the kernel in a reserved area of
160	  memory (communicated to the kernel by a /memreserve/ region in the
161	  device tree) polling their cpu-release-addr location, which must be
162	  contained in the reserved region.  A wfe instruction may be inserted
163	  to reduce the overhead of the busy-loop and a sev will be issued by
164	  the primary CPU.  When a read of the location pointed to by the
165	  cpu-release-addr returns a non-zero value, the CPU must jump to this
166	  value.  The value will be written as a single 64-bit little-endian
167	  value, so CPUs must convert the read value to their native endianness
168	  before jumping to it.
170	- CPUs with a "psci" enable method should remain outside of
171	  the kernel (i.e. outside of the regions of memory described to the
172	  kernel in the memory node, or in a reserved area of memory described
173	  to the kernel by a /memreserve/ region in the device tree).  The
174	  kernel will issue CPU_ON calls as described in ARM document number ARM
175	  DEN 0022A ("Power State Coordination Interface System Software on ARM
176	  processors") to bring CPUs into the kernel.
178	  The device tree should contain a 'psci' node, as described in
179	  Documentation/devicetree/bindings/arm/psci.txt.
181	- Secondary CPU general-purpose register settings
182	  x0 = 0 (reserved for future use)
183	  x1 = 0 (reserved for future use)
184	  x2 = 0 (reserved for future use)
185	  x3 = 0 (reserved for future use)
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