About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / arm64 / booting.txt

Custom Search

Based on kernel version 3.13. Page generated on 2014-01-20 22:00 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	  Data cache must be off and invalidated.
115	  External caches (if present) must be configured and disabled.
117	- Architected timers
118	  CNTFRQ must be programmed with the timer frequency and CNTVOFF must
119	  be programmed with a consistent value on all CPUs.  If entering the
120	  kernel at EL1, CNTHCTL_EL2 must have EL1PCTEN (bit 0) set where
121	  available.
123	- Coherency
124	  All CPUs to be booted by the kernel must be part of the same coherency
125	  domain on entry to the kernel.  This may require IMPLEMENTATION DEFINED
126	  initialisation to enable the receiving of maintenance operations on
127	  each CPU.
129	- System registers
130	  All writable architected system registers at the exception level where
131	  the kernel image will be entered must be initialised by software at a
132	  higher exception level to prevent execution in an UNKNOWN state.
134	The requirements described above for CPU mode, caches, MMUs, architected
135	timers, coherency and system registers apply to all CPUs.  All CPUs must
136	enter the kernel in the same exception level.
138	The boot loader is expected to enter the kernel on each CPU in the
139	following manner:
141	- The primary CPU must jump directly to the first instruction of the
142	  kernel image.  The device tree blob passed by this CPU must contain
143	  an 'enable-method' property for each cpu node.  The supported
144	  enable-methods are described below.
146	  It is expected that the bootloader will generate these device tree
147	  properties and insert them into the blob prior to kernel entry.
149	- CPUs with a "spin-table" enable-method must have a 'cpu-release-addr'
150	  property in their cpu node.  This property identifies a
151	  naturally-aligned 64-bit zero-initalised memory location.
153	  These CPUs should spin outside of the kernel in a reserved area of
154	  memory (communicated to the kernel by a /memreserve/ region in the
155	  device tree) polling their cpu-release-addr location, which must be
156	  contained in the reserved region.  A wfe instruction may be inserted
157	  to reduce the overhead of the busy-loop and a sev will be issued by
158	  the primary CPU.  When a read of the location pointed to by the
159	  cpu-release-addr returns a non-zero value, the CPU must jump to this
160	  value.  The value will be written as a single 64-bit little-endian
161	  value, so CPUs must convert the read value to their native endianness
162	  before jumping to it.
164	- CPUs with a "psci" enable method should remain outside of
165	  the kernel (i.e. outside of the regions of memory described to the
166	  kernel in the memory node, or in a reserved area of memory described
167	  to the kernel by a /memreserve/ region in the device tree).  The
168	  kernel will issue CPU_ON calls as described in ARM document number ARM
169	  DEN 0022A ("Power State Coordination Interface System Software on ARM
170	  processors") to bring CPUs into the kernel.
172	  The device tree should contain a 'psci' node, as described in
173	  Documentation/devicetree/bindings/arm/psci.txt.
175	- Secondary CPU general-purpose register settings
176	  x0 = 0 (reserved for future use)
177	  x1 = 0 (reserved for future use)
178	  x2 = 0 (reserved for future use)
179	  x3 = 0 (reserved for future use)
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.