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 Basic data partition: EBD0A0A2-B9E5-4433-87C0-68B6B72699C7 35and GUID of Linux filesystem data: 0FC63DAF-8483-4772-8E79-3D69D8477DE4 36 37Historically there are 5 methods to generate this number. The oldest one is 38combining machine's MAC address and timer (epoch) value. 39 40Successive versions are using MD5 hash, random numbers and SHA-1 hash. All major 41OSes and programming languages are providing libraries to compute UUID (e.g. 42uuid command line tool). 43 44GPT brief explanation: 45====================== 46 47 Layout: 48 ------- 49 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 | 58 | | 59 | | 60 ---------------------------------------------------------- 61 LBA 34 |Partition 1 | 62 | | 63 ----------------------------------- 64 |Partition 2 | 65 | | 66 ----------------------------------- 67 |Partition n | 68 | | 69 ---------------------------------------------------------- 70 LBA -34 |Entry 1|Entry 2| Entry 3| Entry 4| Backup 71 -------------------------------------------------- GPT 72 LBA -33 |Entries 5 - 128 | 73 | | 74 | | 75 LBA -2 | | 76 -------------------------------------------------- 77 LBA -1 |Backup GPT Header | 78 ---------------------------------------------------------- 79 80For a legacy reasons, GPT's LBA 0 sector has a MBR structure. It is called 81"protective MBR". 82Its first partition entry ID has 0xEE value, and disk software, which is not 83handling the GPT sees it as a storage device without free space. 84 85It is possible to define 128 linearly placed partition entries. 86 87"LBA -1" means the last addressable block (in the mmc subsystem: 88"dev_desc->lba - 1") 89 90Primary/Backup GPT header: 91---------------------------- 92Offset Size Description 93 940 8 B Signature ("EFI PART", 45 46 49 20 50 41 52 54) 958 4 B Revision (For version 1.0, the value is 00 00 01 00) 9612 4 B Header size (in bytes, usually 5C 00 00 00 meaning 92 bytes) 9716 4 B CRC32 of header (0 to header size), with this field zeroed 98 during calculation 9920 4 B Reserved (ZERO); 10024 8 B Current LBA (location of this header copy) 10132 8 B Backup LBA (location of the other header copy) 10240 8 B First usable LBA for partitions (primary partition table last 103 LBA + 1) 10448 8 B Last usable LBA (secondary partition table first LBA - 1) 10556 16 B Disk GUID (also referred as UUID on UNIXes) 10672 8 B Partition entries starting LBA (always 2 in primary copy) 10780 4 B Number of partition entries 10884 4 B Size of a partition entry (usually 128) 10988 4 B CRC32 of partition array 11092 * Reserved; must be ZERO (420 bytes for a 512-byte LBA) 111 112TOTAL: 512 B 113 114 115IMPORTANT: 116 117GPT headers and partition entries are protected by CRC32 (the POSIX CRC32). 118 119Primary GPT header and Backup GPT header have swapped values of "Current LBA" 120and "Backup LBA" and therefore different CRC32 check-sum. 121 122CRC32 for GPT headers (field "CRC of header") are calculated up till 123"Header size" (92), NOT 512 bytes. 124 125CRC32 for partition entries (field "CRC32 of partition array") is calculated for 126the whole array entry ( Number_of_partition_entries * 127sizeof(partition_entry_size (usually 128))) 128 129Observe, how Backup GPT is placed in the memory. It is NOT a mirror reflect 130of the Primary. 131 132 Partition Entry Format: 133 ---------------------- 134 Offset Size Description 135 136 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) 142 143 Attribute flags: 144 Bit 0 - System partition 145 Bit 60 - Read-only 146 Bit 62 - Hidden 147 Bit 63 - Not mount 148 149Creating GPT partitions in U-Boot: 150============== 151 152To restore GUID partition table one needs to: 1531. Define partition layout in the environment. 154 Format of partitions layout: 155 "partitions=uuid_disk=...;name=u-boot,size=60MiB,uuid=...; 156 name=kernel,size=60MiB,uuid=...;" 157 or 158 "partitions=uuid_disk=${uuid_gpt_disk};name=${uboot_name}, 159 size=${uboot_size},uuid=${uboot_uuid};" 160 161 The fields 'name' and 'size' are mandatory for every partition. 162 The field 'start' is optional. 163 164 The fields 'uuid' and 'uuid_disk' are optional if CONFIG_RANDOM_UUID is 165 enabled. A random uuid will be used if omitted or they point to an empty/ 166 non-existent environment variable. The environment variable will be set to 167 the generated UUID. 168 1692. Define 'CONFIG_EFI_PARTITION' and 'CONFIG_CMD_GPT' 170 1713. From u-boot prompt type: 172 gpt write mmc 0 $partitions 173 174Checking (validating) GPT partitions in U-Boot: 175=============================================== 176 177Procedure is the same as above. The only change is at point 3. 178 179At u-boot prompt one needs to write: 180 gpt verify mmc 0 [$partitions] 181 182where [$partitions] is an optional parameter. 183 184When it is not provided, only basic checks based on CRC32 calculation for GPT 185header and PTEs are performed. 186When provided, additionally partition data - name, size and starting 187offset (last two in LBA) - are compared with data defined in '$partitions' 188environment variable. 189 190After running this command, return code is set to 0 if no errors found in 191on non-volatile medium stored GPT. 192 193Following line can be used to assess if GPT verification has succeed: 194 195U-BOOT> gpt verify mmc 0 $partitions 196U-BOOT> if test $? = 0; then echo "GPT OK"; else echo "GPT ERR"; fi 197 198 199Partition type GUID: 200==================== 201 202For created partition, the used partition type GUID is 203PARTITION_BASIC_DATA_GUID (EBD0A0A2-B9E5-4433-87C0-68B6B72699C7). 204 205If you define 'CONFIG_PARTITION_TYPE_GUID', a optionnal parameter 'type' 206can specify a other partition type guid: 207 208 "partitions=uuid_disk=...;name=u-boot,size=60MiB,uuid=...; 209 name=kernel,size=60MiB,uuid=..., 210 type=0FC63DAF-8483-4772-8E79-3D69D8477DE4;" 211 212Some strings can be also used at the place of known GUID : 213 "system" = PARTITION_SYSTEM_GUID 214 (C12A7328-F81F-11D2-BA4B-00A0C93EC93B) 215 "mbr" = LEGACY_MBR_PARTITION_GUID 216 (024DEE41-33E7-11D3-9D69-0008C781F39F) 217 "msft" = PARTITION_MSFT_RESERVED_GUID 218 (E3C9E316-0B5C-4DB8-817D-F92DF00215AE) 219 "data" = PARTITION_BASIC_DATA_GUID 220 (EBD0A0A2-B9E5-4433-87C0-68B6B72699C7) 221 "linux" = PARTITION_LINUX_FILE_SYSTEM_DATA_GUID 222 (0FC63DAF-8483-4772-8E79-3D69D8477DE4) 223 "raid" = PARTITION_LINUX_RAID_GUID 224 (A19D880F-05FC-4D3B-A006-743F0F84911E) 225 "swap" = PARTITION_LINUX_SWAP_GUID 226 (0657FD6D-A4AB-43C4-84E5-0933C84B4F4F) 227 "lvm" = PARTITION_LINUX_LVM_GUID 228 (E6D6D379-F507-44C2-A23C-238F2A3DF928) 229 230 "partitions=uuid_disk=...;name=u-boot,size=60MiB,uuid=...; 231 name=kernel,size=60MiB,uuid=...,type=linux;" 232 233They are also used to display the type of partition in "part list" command. 234 235 236Useful info: 237============ 238 239Two programs, namely: 'gdisk' and 'parted' are recommended to work with GPT 240recovery. Both are able to handle GUID partitions. 241Please, pay attention at -l switch for parted. 242 243"uuid" program is recommended to generate UUID string. Moreover it can decode 244(-d switch) passed in UUID string. It can be used to generate partitions UUID 245passed to u-boot environment variables. 246If optional CONFIG_RANDOM_UUID is defined then for any partition which environment 247uuid is unset, uuid is randomly generated and stored in correspond environment 248variable. 249 250note: 251Each string block of UUID generated by program "uuid" is in big endian and it is 252also stored in big endian in disk GPT. 253Partitions layout can be printed by typing "mmc part". Note that each partition 254GUID has different byte order than UUID generated before, this is because first 255three blocks of GUID string are in Little Endian. 256