xref: /openbmc/u-boot/doc/README.gpt (revision c2cde27d)
1#
2#  Copyright (C) 2012 Samsung Electronics
3#
4#  Lukasz Majewski <l.majewski@samsung.com>
5#
6#
7# SPDX-License-Identifier:	GPL-2.0+
8
9Glossary:
10========
11- UUID -(Universally Unique Identifier)
12- GUID - (Globally Unique ID)
13- EFI - (Extensible Firmware Interface)
14- UEFI - (Unified EFI) - EFI evolution
15- GPT (GUID Partition Table) - it is the EFI standard part
16- partitions - lists of available partitions (defined at u-boot):
17  ./include/configs/{target}.h
18
19Introduction:
20=============
21This document describes the GPT partition table format and usage of
22the gpt command in u-boot.
23
24UUID introduction:
25====================
26
27GPT for marking disks/partitions is using the UUID. It is supposed to be a
28globally unique value. A UUID is a 16-byte (128-bit) number. The number of
29theoretically possible UUIDs is therefore about 3 x 10^38.
30More often UUID is displayed as 32 hexadecimal digits, in 5 groups,
31separated by hyphens, in the form 8-4-4-4-12 for a total of 36 characters
32(32 digits and 4 hyphens)
33
34For instance, GUID of Linux data partition: EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
35
36Historically there are 5 methods to generate this number. The oldest one is
37combining machine's MAC address and timer (epoch) value.
38
39Successive versions are using MD5 hash, random numbers and SHA-1 hash. All major
40OSes and programming languages are providing libraries to compute UUID (e.g.
41uuid command line tool).
42
43GPT brief explanation:
44======================
45
46	Layout:
47	-------
48
49	--------------------------------------------------
50	LBA 0          |Protective MBR                   |
51	----------------------------------------------------------
52	LBA 1          |Primary GPT Header               | Primary
53	-------------------------------------------------- GPT
54	LBA 2          |Entry 1|Entry 2| Entry 3| Entry 4|
55	--------------------------------------------------
56	LBA 3          |Entries 5 - 128                  |
57		       |                                 |
58		       |                                 |
59	----------------------------------------------------------
60	LBA 34         |Partition 1                      |
61		       |                                 |
62		       -----------------------------------
63		       |Partition 2                      |
64		       |                                 |
65		       -----------------------------------
66		       |Partition n                      |
67		       |                                 |
68	----------------------------------------------------------
69	LBA -34        |Entry 1|Entry 2| Entry 3| Entry 4| Secondary
70	-------------------------------------------------- (bkp)
71	LBA -33        |Entries 5 - 128                  | GPT
72		       |                                 |
73		       |                                 |
74	LBA -2         |                                 |
75	--------------------------------------------------
76	LBA -1         |Secondary GPT Header             |
77	----------------------------------------------------------
78
79For a legacy reasons, GPT's LBA 0 sector has a MBR structure. It is called
80"protective MBR".
81Its first partition entry ID has 0xEE value, and disk software, which is not
82handling the GPT sees it as a storage device without free space.
83
84It is possible to define 128 linearly placed partition entries.
85
86"LBA -1" means the last addressable block (in the mmc subsystem:
87"dev_desc->lba - 1")
88
89Primary/Secondary GPT header:
90----------------------------
91Offset  Size    Description
92
930       8 B     Signature ("EFI PART", 45 46 49 20 50 41 52 54)
948       4 B     Revision (For version 1.0, the value is 00 00 01 00)
9512      4 B     Header size (in bytes, usually 5C 00 00 00 meaning 92 bytes)
9616      4 B     CRC32 of header (0 to header size), with this field zeroed
97		during calculation
9820      4 B     Reserved (ZERO);
9924      8 B     Current LBA (location of this header copy)
10032      8 B     Backup LBA (location of the other header copy)
10140      8 B     First usable LBA for partitions (primary partition table last
102		LBA + 1)
10348      8 B     Last usable LBA (secondary partition table first LBA - 1)
10456      16 B    Disk GUID (also referred as UUID on UNIXes)
10572      8 B     Partition entries starting LBA (always 2 in primary copy)
10680      4 B     Number of partition entries
10784      4 B     Size of a partition entry (usually 128)
10888      4 B     CRC32 of partition array
10992      *       Reserved; must be ZERO (420 bytes for a 512-byte LBA)
110
111TOTAL: 512 B
112
113
114IMPORTANT:
115
116GPT headers and partition entries are protected by CRC32 (the POSIX CRC32).
117
118Primary GPT header and Secondary GPT header have swapped values of "Current LBA"
119and "Backup LBA" and therefore different CRC32 check-sum.
120
121CRC32 for GPT headers (field "CRC of header") are calculated up till
122"Header size" (92), NOT 512 bytes.
123
124CRC32 for partition entries (field "CRC32 of partition array") is calculated for
125the whole array entry ( Number_of_partition_entries *
126sizeof(partition_entry_size (usually 128)))
127
128Observe, how Secondary GPT is placed in the memory. It is NOT a mirror reflect
129of the Primary.
130
131	   Partition Entry Format:
132	   ----------------------
133	   Offset  Size    Description
134
135	   0       16 B    Partition type GUID
136	   16      16 B    Unique partition GUID
137	   32      8  B    First LBA (Little Endian)
138	   40      8  B    Last LBA (inclusive)
139	   48      8  B    Attribute flags [+]
140	   56      72 B    Partition name (text)
141
142	   Attribute flags:
143	   Bit 0  - System partition
144	   Bit 60 - Read-only
145	   Bit 62 - Hidden
146	   Bit 63 - Not mount
147
148Creating GPT partitions in U-Boot:
149==============
150
151To restore GUID partition table one needs to:
1521. Define partition layout in the environment.
153   Format of partitions layout:
154     "partitions=uuid_disk=...;name=u-boot,size=60MiB,uuid=...;
155	name=kernel,size=60MiB,uuid=...;"
156     or
157     "partitions=uuid_disk=${uuid_gpt_disk};name=${uboot_name},
158	size=${uboot_size},uuid=${uboot_uuid};"
159
160   Fields 'name', 'size' and 'uuid' are mandatory for every partition.
161   The field 'start' is optional.
162
1632. Define 'CONFIG_EFI_PARTITION' and 'CONFIG_CMD_GPT'
164
1652. From u-boot prompt type:
166   gpt write mmc 0 $partitions
167
168Useful info:
169============
170
171Two programs, namely: 'fdisk' and 'parted' are recommended to work with GPT
172recovery. Parted is able to handle GUID partitions. Unfortunately the 'fdisk'
173hasn't got such ability.
174Please, pay attention at -l switch for parted.
175
176"uuid" program is recommended to generate UUID string. Moreover it can decode
177(-d switch) passed in UUID string. It can be used to generate partitions UUID
178passed to u-boot environment variables.
179