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Based on kernel version 4.8. Page generated on 2016-10-06 23:10 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 and must
49	not exceed 2 megabytes in size. Since the dtb will be mapped cacheable
50	using blocks of up to 2 megabytes in size, it must not be placed within
51	any 2M region which must be mapped with any specific attributes.
53	NOTE: versions prior to v4.2 also require that the DTB be placed within
54	the 512 MB region starting at text_offset bytes below the kernel Image.
56	3. Decompress the kernel image
57	------------------------------
59	Requirement: OPTIONAL
61	The AArch64 kernel does not currently provide a decompressor and
62	therefore requires decompression (gzip etc.) to be performed by the boot
63	loader if a compressed Image target (e.g. Image.gz) is used.  For
64	bootloaders that do not implement this requirement, the uncompressed
65	Image target is available instead.
68	4. Call the kernel image
69	------------------------
71	Requirement: MANDATORY
73	The decompressed kernel image contains a 64-byte header as follows:
75	  u32 code0;			/* Executable code */
76	  u32 code1;			/* Executable code */
77	  u64 text_offset;		/* Image load offset, little endian */
78	  u64 image_size;		/* Effective Image size, little endian */
79	  u64 flags;			/* kernel flags, little endian */
80	  u64 res2	= 0;		/* reserved */
81	  u64 res3	= 0;		/* reserved */
82	  u64 res4	= 0;		/* reserved */
83	  u32 magic	= 0x644d5241;	/* Magic number, little endian, "ARM\x64" */
84	  u32 res5;			/* reserved (used for PE COFF offset) */
87	Header notes:
89	- As of v3.17, all fields are little endian unless stated otherwise.
91	- code0/code1 are responsible for branching to stext.
93	- when booting through EFI, code0/code1 are initially skipped.
94	  res5 is an offset to the PE header and the PE header has the EFI
95	  entry point (efi_stub_entry).  When the stub has done its work, it
96	  jumps to code0 to resume the normal boot process.
98	- Prior to v3.17, the endianness of text_offset was not specified.  In
99	  these cases image_size is zero and text_offset is 0x80000 in the
100	  endianness of the kernel.  Where image_size is non-zero image_size is
101	  little-endian and must be respected.  Where image_size is zero,
102	  text_offset can be assumed to be 0x80000.
104	- The flags field (introduced in v3.17) is a little-endian 64-bit field
105	  composed as follows:
106	  Bit 0:	Kernel endianness.  1 if BE, 0 if LE.
107	  Bit 1-2:	Kernel Page size.
108				0 - Unspecified.
109				1 - 4K
110				2 - 16K
111				3 - 64K
112	  Bit 3:	Kernel physical placement
113				0 - 2MB aligned base should be as close as possible
114				    to the base of DRAM, since memory below it is not
115				    accessible via the linear mapping
116				1 - 2MB aligned base may be anywhere in physical
117				    memory
118	  Bits 4-63:	Reserved.
120	- When image_size is zero, a bootloader should attempt to keep as much
121	  memory as possible free for use by the kernel immediately after the
122	  end of the kernel image. The amount of space required will vary
123	  depending on selected features, and is effectively unbound.
125	The Image must be placed text_offset bytes from a 2MB aligned base
126	address anywhere in usable system RAM and called there. The region
127	between the 2 MB aligned base address and the start of the image has no
128	special significance to the kernel, and may be used for other purposes.
129	At least image_size bytes from the start of the image must be free for
130	use by the kernel.
131	NOTE: versions prior to v4.6 cannot make use of memory below the
132	physical offset of the Image so it is recommended that the Image be
133	placed as close as possible to the start of system RAM.
135	If an initrd/initramfs is passed to the kernel at boot, it must reside
136	entirely within a 1 GB aligned physical memory window of up to 32 GB in
137	size that fully covers the kernel Image as well.
139	Any memory described to the kernel (even that below the start of the
140	image) which is not marked as reserved from the kernel (e.g., with a
141	memreserve region in the device tree) will be considered as available to
142	the kernel.
144	Before jumping into the kernel, the following conditions must be met:
146	- Quiesce all DMA capable devices so that memory does not get
147	  corrupted by bogus network packets or disk data.  This will save
148	  you many hours of debug.
150	- Primary CPU general-purpose register settings
151	  x0 = physical address of device tree blob (dtb) in system RAM.
152	  x1 = 0 (reserved for future use)
153	  x2 = 0 (reserved for future use)
154	  x3 = 0 (reserved for future use)
156	- CPU mode
157	  All forms of interrupts must be masked in PSTATE.DAIF (Debug, SError,
158	  IRQ and FIQ).
159	  The CPU must be in either EL2 (RECOMMENDED in order to have access to
160	  the virtualisation extensions) or non-secure EL1.
162	- Caches, MMUs
163	  The MMU must be off.
164	  Instruction cache may be on or off.
165	  The address range corresponding to the loaded kernel image must be
166	  cleaned to the PoC. In the presence of a system cache or other
167	  coherent masters with caches enabled, this will typically require
168	  cache maintenance by VA rather than set/way operations.
169	  System caches which respect the architected cache maintenance by VA
170	  operations must be configured and may be enabled.
171	  System caches which do not respect architected cache maintenance by VA
172	  operations (not recommended) must be configured and disabled.
174	- Architected timers
175	  CNTFRQ must be programmed with the timer frequency and CNTVOFF must
176	  be programmed with a consistent value on all CPUs.  If entering the
177	  kernel at EL1, CNTHCTL_EL2 must have EL1PCTEN (bit 0) set where
178	  available.
180	- Coherency
181	  All CPUs to be booted by the kernel must be part of the same coherency
182	  domain on entry to the kernel.  This may require IMPLEMENTATION DEFINED
183	  initialisation to enable the receiving of maintenance operations on
184	  each CPU.
186	- System registers
187	  All writable architected system registers at the exception level where
188	  the kernel image will be entered must be initialised by software at a
189	  higher exception level to prevent execution in an UNKNOWN state.
191	  For systems with a GICv3 interrupt controller to be used in v3 mode:
192	  - If EL3 is present:
193	    ICC_SRE_EL3.Enable (bit 3) must be initialiased to 0b1.
194	    ICC_SRE_EL3.SRE (bit 0) must be initialised to 0b1.
195	  - If the kernel is entered at EL1:
196	    ICC.SRE_EL2.Enable (bit 3) must be initialised to 0b1
197	    ICC_SRE_EL2.SRE (bit 0) must be initialised to 0b1.
198	  - The DT or ACPI tables must describe a GICv3 interrupt controller.
200	  For systems with a GICv3 interrupt controller to be used in
201	  compatibility (v2) mode:
202	  - If EL3 is present:
203	    ICC_SRE_EL3.SRE (bit 0) must be initialised to 0b0.
204	  - If the kernel is entered at EL1:
205	    ICC_SRE_EL2.SRE (bit 0) must be initialised to 0b0.
206	  - The DT or ACPI tables must describe a GICv2 interrupt controller.
208	The requirements described above for CPU mode, caches, MMUs, architected
209	timers, coherency and system registers apply to all CPUs.  All CPUs must
210	enter the kernel in the same exception level.
212	The boot loader is expected to enter the kernel on each CPU in the
213	following manner:
215	- The primary CPU must jump directly to the first instruction of the
216	  kernel image.  The device tree blob passed by this CPU must contain
217	  an 'enable-method' property for each cpu node.  The supported
218	  enable-methods are described below.
220	  It is expected that the bootloader will generate these device tree
221	  properties and insert them into the blob prior to kernel entry.
223	- CPUs with a "spin-table" enable-method must have a 'cpu-release-addr'
224	  property in their cpu node.  This property identifies a
225	  naturally-aligned 64-bit zero-initalised memory location.
227	  These CPUs should spin outside of the kernel in a reserved area of
228	  memory (communicated to the kernel by a /memreserve/ region in the
229	  device tree) polling their cpu-release-addr location, which must be
230	  contained in the reserved region.  A wfe instruction may be inserted
231	  to reduce the overhead of the busy-loop and a sev will be issued by
232	  the primary CPU.  When a read of the location pointed to by the
233	  cpu-release-addr returns a non-zero value, the CPU must jump to this
234	  value.  The value will be written as a single 64-bit little-endian
235	  value, so CPUs must convert the read value to their native endianness
236	  before jumping to it.
238	- CPUs with a "psci" enable method should remain outside of
239	  the kernel (i.e. outside of the regions of memory described to the
240	  kernel in the memory node, or in a reserved area of memory described
241	  to the kernel by a /memreserve/ region in the device tree).  The
242	  kernel will issue CPU_ON calls as described in ARM document number ARM
243	  DEN 0022A ("Power State Coordination Interface System Software on ARM
244	  processors") to bring CPUs into the kernel.
246	  The device tree should contain a 'psci' node, as described in
247	  Documentation/devicetree/bindings/arm/psci.txt.
249	- Secondary CPU general-purpose register settings
250	  x0 = 0 (reserved for future use)
251	  x1 = 0 (reserved for future use)
252	  x2 = 0 (reserved for future use)
253	  x3 = 0 (reserved for future use)
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