2 # Copyright (C) 2012 Samsung Electronics
4 # Lukasz Majewski <l.majewski@samsung.com>
7 # SPDX-License-Identifier: GPL-2.0+
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
21 This document describes the GPT partition table format and usage of
22 the gpt command in u-boot.
27 GPT for marking disks/partitions is using the UUID. It is supposed to be a
28 globally unique value. A UUID is a 16-byte (128-bit) number. The number of
29 theoretically possible UUIDs is therefore about 3 x 10^38.
30 More often UUID is displayed as 32 hexadecimal digits, in 5 groups,
31 separated by hyphens, in the form 8-4-4-4-12 for a total of 36 characters
32 (32 digits and 4 hyphens)
34 For instance, GUID of Basic data partition: EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
35 and GUID of Linux filesystem data: 0FC63DAF-8483-4772-8E79-3D69D8477DE4
37 Historically there are 5 methods to generate this number. The oldest one is
38 combining machine's MAC address and timer (epoch) value.
40 Successive versions are using MD5 hash, random numbers and SHA-1 hash. All major
41 OSes and programming languages are providing libraries to compute UUID (e.g.
42 uuid command line tool).
44 GPT brief explanation:
45 ======================
50 --------------------------------------------------
51 LBA 0 |Protective MBR |
52 ----------------------------------------------------------
53 LBA 1 |Primary GPT Header | Primary
54 -------------------------------------------------- GPT
55 LBA 2 |Entry 1|Entry 2| Entry 3| Entry 4|
56 --------------------------------------------------
57 LBA 3 |Entries 5 - 128 |
60 ----------------------------------------------------------
63 -----------------------------------
66 -----------------------------------
69 ----------------------------------------------------------
70 LBA -34 |Entry 1|Entry 2| Entry 3| Entry 4| Backup
71 -------------------------------------------------- GPT
72 LBA -33 |Entries 5 - 128 |
76 --------------------------------------------------
77 LBA -1 |Backup GPT Header |
78 ----------------------------------------------------------
80 For a legacy reasons, GPT's LBA 0 sector has a MBR structure. It is called
82 Its first partition entry ID has 0xEE value, and disk software, which is not
83 handling the GPT sees it as a storage device without free space.
85 It is possible to define 128 linearly placed partition entries.
87 "LBA -1" means the last addressable block (in the mmc subsystem:
90 Primary/Backup GPT header:
91 ----------------------------
92 Offset Size Description
94 0 8 B Signature ("EFI PART", 45 46 49 20 50 41 52 54)
95 8 4 B Revision (For version 1.0, the value is 00 00 01 00)
96 12 4 B Header size (in bytes, usually 5C 00 00 00 meaning 92 bytes)
97 16 4 B CRC32 of header (0 to header size), with this field zeroed
99 20 4 B Reserved (ZERO);
100 24 8 B Current LBA (location of this header copy)
101 32 8 B Backup LBA (location of the other header copy)
102 40 8 B First usable LBA for partitions (primary partition table last
104 48 8 B Last usable LBA (secondary partition table first LBA - 1)
105 56 16 B Disk GUID (also referred as UUID on UNIXes)
106 72 8 B Partition entries starting LBA (always 2 in primary copy)
107 80 4 B Number of partition entries
108 84 4 B Size of a partition entry (usually 128)
109 88 4 B CRC32 of partition array
110 92 * Reserved; must be ZERO (420 bytes for a 512-byte LBA)
117 GPT headers and partition entries are protected by CRC32 (the POSIX CRC32).
119 Primary GPT header and Backup GPT header have swapped values of "Current LBA"
120 and "Backup LBA" and therefore different CRC32 check-sum.
122 CRC32 for GPT headers (field "CRC of header") are calculated up till
123 "Header size" (92), NOT 512 bytes.
125 CRC32 for partition entries (field "CRC32 of partition array") is calculated for
126 the whole array entry ( Number_of_partition_entries *
127 sizeof(partition_entry_size (usually 128)))
129 Observe, how Backup GPT is placed in the memory. It is NOT a mirror reflect
132 Partition Entry Format:
133 ----------------------
134 Offset Size Description
136 0 16 B Partition type GUID (Big Endian)
137 16 16 B Unique partition GUID in (Big Endian)
138 32 8 B First LBA (Little Endian)
139 40 8 B Last LBA (inclusive)
140 48 8 B Attribute flags [+]
141 56 72 B Partition name (text)
144 Bit 0 - System partition
145 Bit 1 - Hide from EFI
146 Bit 2 - Legacy BIOS bootable
147 Bit 48-63 - Defined and used by the individual partition type
148 For Basic data partition :
153 Creating GPT partitions in U-Boot:
156 To restore GUID partition table one needs to:
157 1. Define partition layout in the environment.
158 Format of partitions layout:
159 "partitions=uuid_disk=...;name=u-boot,size=60MiB,uuid=...;
160 name=kernel,size=60MiB,uuid=...;"
162 "partitions=uuid_disk=${uuid_gpt_disk};name=${uboot_name},
163 size=${uboot_size},uuid=${uboot_uuid};"
165 The fields 'name' and 'size' are mandatory for every partition.
166 The field 'start' is optional.
168 If field 'size' of the last partition is 0, the partiton is extended
169 up to the end of the device.
171 The fields 'uuid' and 'uuid_disk' are optional if CONFIG_RANDOM_UUID is
172 enabled. A random uuid will be used if omitted or they point to an empty/
173 non-existent environment variable. The environment variable will be set to
176 The field 'bootable' is optional, it is used to mark the GPT partition
177 bootable (set attribute flags "Legacy BIOS bootable").
178 "name=u-boot,size=60MiB;name=boot,size=60Mib,bootable;name=rootfs,size=0"
179 It can be used to locate bootable disks with command
180 "part list <interface> <dev> -bootable <varname>",
181 please check out doc/README.distro for use.
183 2. Define 'CONFIG_EFI_PARTITION' and 'CONFIG_CMD_GPT'
185 3. From u-boot prompt type:
186 gpt write mmc 0 $partitions
188 Checking (validating) GPT partitions in U-Boot:
189 ===============================================
191 Procedure is the same as above. The only change is at point 3.
193 At u-boot prompt one needs to write:
194 gpt verify mmc 0 [$partitions]
196 where [$partitions] is an optional parameter.
198 When it is not provided, only basic checks based on CRC32 calculation for GPT
199 header and PTEs are performed.
200 When provided, additionally partition data - name, size and starting
201 offset (last two in LBA) - are compared with data defined in '$partitions'
202 environment variable.
204 After running this command, return code is set to 0 if no errors found in
205 on non-volatile medium stored GPT.
207 Following line can be used to assess if GPT verification has succeed:
209 U-BOOT> gpt verify mmc 0 $partitions
210 U-BOOT> if test $? = 0; then echo "GPT OK"; else echo "GPT ERR"; fi
216 For created partition, the used partition type GUID is
217 PARTITION_BASIC_DATA_GUID (EBD0A0A2-B9E5-4433-87C0-68B6B72699C7).
219 If you define 'CONFIG_PARTITION_TYPE_GUID', a optionnal parameter 'type'
220 can specify a other partition type guid:
222 "partitions=uuid_disk=...;name=u-boot,size=60MiB,uuid=...;
223 name=kernel,size=60MiB,uuid=...,
224 type=0FC63DAF-8483-4772-8E79-3D69D8477DE4;"
226 Some strings can be also used at the place of known GUID :
227 "system" = PARTITION_SYSTEM_GUID
228 (C12A7328-F81F-11D2-BA4B-00A0C93EC93B)
229 "mbr" = LEGACY_MBR_PARTITION_GUID
230 (024DEE41-33E7-11D3-9D69-0008C781F39F)
231 "msft" = PARTITION_MSFT_RESERVED_GUID
232 (E3C9E316-0B5C-4DB8-817D-F92DF00215AE)
233 "data" = PARTITION_BASIC_DATA_GUID
234 (EBD0A0A2-B9E5-4433-87C0-68B6B72699C7)
235 "linux" = PARTITION_LINUX_FILE_SYSTEM_DATA_GUID
236 (0FC63DAF-8483-4772-8E79-3D69D8477DE4)
237 "raid" = PARTITION_LINUX_RAID_GUID
238 (A19D880F-05FC-4D3B-A006-743F0F84911E)
239 "swap" = PARTITION_LINUX_SWAP_GUID
240 (0657FD6D-A4AB-43C4-84E5-0933C84B4F4F)
241 "lvm" = PARTITION_LINUX_LVM_GUID
242 (E6D6D379-F507-44C2-A23C-238F2A3DF928)
244 "partitions=uuid_disk=...;name=u-boot,size=60MiB,uuid=...;
245 name=kernel,size=60MiB,uuid=...,type=linux;"
247 They are also used to display the type of partition in "part list" command.
253 Two programs, namely: 'gdisk' and 'parted' are recommended to work with GPT
254 recovery. Both are able to handle GUID partitions.
255 Please, pay attention at -l switch for parted.
257 "uuid" program is recommended to generate UUID string. Moreover it can decode
258 (-d switch) passed in UUID string. It can be used to generate partitions UUID
259 passed to u-boot environment variables.
260 If optional CONFIG_RANDOM_UUID is defined then for any partition which environment
261 uuid is unset, uuid is randomly generated and stored in correspond environment
265 Each string block of UUID generated by program "uuid" is in big endian and it is
266 also stored in big endian in disk GPT.
267 Partitions layout can be printed by typing "mmc part". Note that each partition
268 GUID has different byte order than UUID generated before, this is because first
269 three blocks of GUID string are in Little Endian.