doc/README.gpt

   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
   9 Glossary:
  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
  19 Introduction:
  20 =============
  21 This document describes the GPT partition table format and usage of
  22 the gpt command in u-boot.
  23
  24 UUID introduction:
  25 ====================
  26
  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)
  33
  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
  36
  37 Historically there are 5 methods to generate this number. The oldest one is
  38 combining machine's MAC address and timer (epoch) value.
  39
  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).
  43
  44 GPT 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
  80 For a legacy reasons, GPT's LBA 0 sector has a MBR structure. It is called
  81 "protective MBR".
  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.
  84
  85 It 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
  90 Primary/Backup GPT header:
  91 ----------------------------
  92 Offset  Size    Description
  93
  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
  98                 during calculation
  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
 103                 LBA + 1)
 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)
 111
 112 TOTAL: 512 B
 113
 114
 115 IMPORTANT:
 116
 117 GPT headers and partition entries are protected by CRC32 (the POSIX CRC32).
 118
 119 Primary GPT header and Backup GPT header have swapped values of "Current LBA"
 120 and "Backup LBA" and therefore different CRC32 check-sum.
 121
 122 CRC32 for GPT headers (field "CRC of header") are calculated up till
 123 "Header size" (92), NOT 512 bytes.
 124
 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)))
 128
 129 Observe, how Backup GPT is placed in the memory. It is NOT a mirror reflect
 130 of 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
 149 Creating GPT partitions in U-Boot:
 150 ==============
 151
 152 To restore GUID partition table one needs to:
 153 1. 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
 169 2. Define 'CONFIG_EFI_PARTITION' and 'CONFIG_CMD_GPT'
 170
 171 3. From u-boot prompt type:
 172    gpt write mmc 0 $partitions
 173
 174 Checking (validating) GPT partitions in U-Boot:
 175 ===============================================
 176
 177 Procedure is the same as above. The only change is at point 3.
 178
 179 At u-boot prompt one needs to write:
 180    gpt verify mmc 0 [$partitions]
 181
 182 where [$partitions] is an optional parameter.
 183
 184 When it is not provided, only basic checks based on CRC32 calculation for GPT
 185 header and PTEs are performed.
 186 When provided, additionally partition data - name, size and starting
 187 offset (last two in LBA) - are compared with data defined in '$partitions'
 188 environment variable.
 189
 190 After running this command, return code is set to 0 if no errors found in
 191 on non-volatile medium stored GPT.
 192
 193 Following line can be used to assess if GPT verification has succeed:
 194
 195 U-BOOT> gpt verify mmc 0 $partitions
 196 U-BOOT> if test $? = 0; then echo "GPT OK"; else echo "GPT ERR"; fi
 197
 198
 199 Partition type GUID:
 200 ====================
 201
 202 For created partition, the used partition type GUID is
 203 PARTITION_BASIC_DATA_GUID (EBD0A0A2-B9E5-4433-87C0-68B6B72699C7).
 204
 205 If you define 'CONFIG_PARTITION_TYPE_GUID', a optionnal parameter 'type'
 206 can 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
 212 Some 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
 233 They are also used to display the type of partition in "part list" command.
 234
 235
 236 Useful info:
 237 ============
 238
 239 Two programs, namely: 'gdisk' and 'parted' are recommended to work with GPT
 240 recovery. Both are able to handle GUID partitions.
 241 Please, 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
 245 passed to u-boot environment variables.
 246 If optional CONFIG_RANDOM_UUID is defined then for any partition which environment
 247 uuid is unset, uuid is randomly generated and stored in correspond environment
 248 variable.
 249
 250 note:
 251 Each string block of UUID generated by program "uuid" is in big endian and it is
 252 also stored in big endian in disk GPT.
 253 Partitions layout can be printed by typing "mmc part". Note that each partition
 254 GUID has different byte order than UUID generated before, this is because first
 255 three blocks of GUID string are in Little Endian.