| # | |
| # Copyright (C) 2012 Samsung Electronics | |
| # | |
| # Lukasz Majewski <l.majewski@samsung.com> | |
| # | |
| # | |
| # SPDX-License-Identifier: GPL-2.0+ | |
| Glossary: | |
| ======== | |
| - UUID -(Universally Unique Identifier) | |
| - GUID - (Globally Unique ID) | |
| - EFI - (Extensible Firmware Interface) | |
| - UEFI - (Unified EFI) - EFI evolution | |
| - GPT (GUID Partition Table) - it is the EFI standard part | |
| - partitions - lists of available partitions (defined at u-boot): | |
| ./include/configs/{target}.h | |
| Introduction: | |
| ============= | |
| This document describes the GPT partition table format and usage of | |
| the gpt command in u-boot. | |
| UUID introduction: | |
| ==================== | |
| GPT for marking disks/partitions is using the UUID. It is supposed to be a | |
| globally unique value. A UUID is a 16-byte (128-bit) number. The number of | |
| theoretically possible UUIDs is therefore about 3 x 10^38. | |
| More often UUID is displayed as 32 hexadecimal digits, in 5 groups, | |
| separated by hyphens, in the form 8-4-4-4-12 for a total of 36 characters | |
| (32 digits and 4 hyphens) | |
| For instance, GUID of Basic data partition: EBD0A0A2-B9E5-4433-87C0-68B6B72699C7 | |
| and GUID of Linux filesystem data: 0FC63DAF-8483-4772-8E79-3D69D8477DE4 | |
| Historically there are 5 methods to generate this number. The oldest one is | |
| combining machine's MAC address and timer (epoch) value. | |
| Successive versions are using MD5 hash, random numbers and SHA-1 hash. All major | |
| OSes and programming languages are providing libraries to compute UUID (e.g. | |
| uuid command line tool). | |
| GPT brief explanation: | |
| ====================== | |
| Layout: | |
| ------- | |
| -------------------------------------------------- | |
| LBA 0 |Protective MBR | | |
| ---------------------------------------------------------- | |
| LBA 1 |Primary GPT Header | Primary | |
| -------------------------------------------------- GPT | |
| LBA 2 |Entry 1|Entry 2| Entry 3| Entry 4| | |
| -------------------------------------------------- | |
| LBA 3 |Entries 5 - 128 | | |
| | | | |
| | | | |
| ---------------------------------------------------------- | |
| LBA 34 |Partition 1 | | |
| | | | |
| ----------------------------------- | |
| |Partition 2 | | |
| | | | |
| ----------------------------------- | |
| |Partition n | | |
| | | | |
| ---------------------------------------------------------- | |
| LBA -34 |Entry 1|Entry 2| Entry 3| Entry 4| Backup | |
| -------------------------------------------------- GPT | |
| LBA -33 |Entries 5 - 128 | | |
| | | | |
| | | | |
| LBA -2 | | | |
| -------------------------------------------------- | |
| LBA -1 |Backup GPT Header | | |
| ---------------------------------------------------------- | |
| For a legacy reasons, GPT's LBA 0 sector has a MBR structure. It is called | |
| "protective MBR". | |
| Its first partition entry ID has 0xEE value, and disk software, which is not | |
| handling the GPT sees it as a storage device without free space. | |
| It is possible to define 128 linearly placed partition entries. | |
| "LBA -1" means the last addressable block (in the mmc subsystem: | |
| "dev_desc->lba - 1") | |
| Primary/Backup GPT header: | |
| ---------------------------- | |
| Offset Size Description | |
| 0 8 B Signature ("EFI PART", 45 46 49 20 50 41 52 54) | |
| 8 4 B Revision (For version 1.0, the value is 00 00 01 00) | |
| 12 4 B Header size (in bytes, usually 5C 00 00 00 meaning 92 bytes) | |
| 16 4 B CRC32 of header (0 to header size), with this field zeroed | |
| during calculation | |
| 20 4 B Reserved (ZERO); | |
| 24 8 B Current LBA (location of this header copy) | |
| 32 8 B Backup LBA (location of the other header copy) | |
| 40 8 B First usable LBA for partitions (primary partition table last | |
| LBA + 1) | |
| 48 8 B Last usable LBA (secondary partition table first LBA - 1) | |
| 56 16 B Disk GUID (also referred as UUID on UNIXes) | |
| 72 8 B Partition entries starting LBA (always 2 in primary copy) | |
| 80 4 B Number of partition entries | |
| 84 4 B Size of a partition entry (usually 128) | |
| 88 4 B CRC32 of partition array | |
| 92 * Reserved; must be ZERO (420 bytes for a 512-byte LBA) | |
| TOTAL: 512 B | |
| IMPORTANT: | |
| GPT headers and partition entries are protected by CRC32 (the POSIX CRC32). | |
| Primary GPT header and Backup GPT header have swapped values of "Current LBA" | |
| and "Backup LBA" and therefore different CRC32 check-sum. | |
| CRC32 for GPT headers (field "CRC of header") are calculated up till | |
| "Header size" (92), NOT 512 bytes. | |
| CRC32 for partition entries (field "CRC32 of partition array") is calculated for | |
| the whole array entry ( Number_of_partition_entries * | |
| sizeof(partition_entry_size (usually 128))) | |
| Observe, how Backup GPT is placed in the memory. It is NOT a mirror reflect | |
| of the Primary. | |
| Partition Entry Format: | |
| ---------------------- | |
| Offset Size Description | |
| 0 16 B Partition type GUID (Big Endian) | |
| 16 16 B Unique partition GUID in (Big Endian) | |
| 32 8 B First LBA (Little Endian) | |
| 40 8 B Last LBA (inclusive) | |
| 48 8 B Attribute flags [+] | |
| 56 72 B Partition name (text) | |
| Attribute flags: | |
| Bit 0 - System partition | |
| Bit 1 - Hide from EFI | |
| Bit 2 - Legacy BIOS bootable | |
| Bit 48-63 - Defined and used by the individual partition type | |
| For Basic data partition : | |
| Bit 60 - Read-only | |
| Bit 62 - Hidden | |
| Bit 63 - Not mount | |
| Creating GPT partitions in U-Boot: | |
| ============== | |
| To restore GUID partition table one needs to: | |
| 1. Define partition layout in the environment. | |
| Format of partitions layout: | |
| "partitions=uuid_disk=...;name=u-boot,size=60MiB,uuid=...; | |
| name=kernel,size=60MiB,uuid=...;" | |
| or | |
| "partitions=uuid_disk=${uuid_gpt_disk};name=${uboot_name}, | |
| size=${uboot_size},uuid=${uboot_uuid};" | |
| The fields 'name' and 'size' are mandatory for every partition. | |
| The field 'start' is optional. | |
| If field 'size' of the last partition is 0, the partiton is extended | |
| up to the end of the device. | |
| The fields 'uuid' and 'uuid_disk' are optional if CONFIG_RANDOM_UUID is | |
| enabled. A random uuid will be used if omitted or they point to an empty/ | |
| non-existent environment variable. The environment variable will be set to | |
| the generated UUID. | |
| The field 'bootable' is optional, it is used to mark the GPT partition | |
| bootable (set attribute flags "Legacy BIOS bootable"). | |
| "name=u-boot,size=60MiB;name=boot,size=60Mib,bootable;name=rootfs,size=0" | |
| It can be used to locate bootable disks with command | |
| "part list <interface> <dev> -bootable <varname>", | |
| please check out doc/README.distro for use. | |
| 2. Define 'CONFIG_EFI_PARTITION' and 'CONFIG_CMD_GPT' | |
| 3. From u-boot prompt type: | |
| gpt write mmc 0 $partitions | |
| Checking (validating) GPT partitions in U-Boot: | |
| =============================================== | |
| Procedure is the same as above. The only change is at point 3. | |
| At u-boot prompt one needs to write: | |
| gpt verify mmc 0 [$partitions] | |
| where [$partitions] is an optional parameter. | |
| When it is not provided, only basic checks based on CRC32 calculation for GPT | |
| header and PTEs are performed. | |
| When provided, additionally partition data - name, size and starting | |
| offset (last two in LBA) - are compared with data defined in '$partitions' | |
| environment variable. | |
| After running this command, return code is set to 0 if no errors found in | |
| on non-volatile medium stored GPT. | |
| Following line can be used to assess if GPT verification has succeed: | |
| U-BOOT> gpt verify mmc 0 $partitions | |
| U-BOOT> if test $? = 0; then echo "GPT OK"; else echo "GPT ERR"; fi | |
| Partition type GUID: | |
| ==================== | |
| For created partition, the used partition type GUID is | |
| PARTITION_BASIC_DATA_GUID (EBD0A0A2-B9E5-4433-87C0-68B6B72699C7). | |
| If you define 'CONFIG_PARTITION_TYPE_GUID', a optionnal parameter 'type' | |
| can specify a other partition type guid: | |
| "partitions=uuid_disk=...;name=u-boot,size=60MiB,uuid=...; | |
| name=kernel,size=60MiB,uuid=..., | |
| type=0FC63DAF-8483-4772-8E79-3D69D8477DE4;" | |
| Some strings can be also used at the place of known GUID : | |
| "system" = PARTITION_SYSTEM_GUID | |
| (C12A7328-F81F-11D2-BA4B-00A0C93EC93B) | |
| "mbr" = LEGACY_MBR_PARTITION_GUID | |
| (024DEE41-33E7-11D3-9D69-0008C781F39F) | |
| "msft" = PARTITION_MSFT_RESERVED_GUID | |
| (E3C9E316-0B5C-4DB8-817D-F92DF00215AE) | |
| "data" = PARTITION_BASIC_DATA_GUID | |
| (EBD0A0A2-B9E5-4433-87C0-68B6B72699C7) | |
| "linux" = PARTITION_LINUX_FILE_SYSTEM_DATA_GUID | |
| (0FC63DAF-8483-4772-8E79-3D69D8477DE4) | |
| "raid" = PARTITION_LINUX_RAID_GUID | |
| (A19D880F-05FC-4D3B-A006-743F0F84911E) | |
| "swap" = PARTITION_LINUX_SWAP_GUID | |
| (0657FD6D-A4AB-43C4-84E5-0933C84B4F4F) | |
| "lvm" = PARTITION_LINUX_LVM_GUID | |
| (E6D6D379-F507-44C2-A23C-238F2A3DF928) | |
| "partitions=uuid_disk=...;name=u-boot,size=60MiB,uuid=...; | |
| name=kernel,size=60MiB,uuid=...,type=linux;" | |
| They are also used to display the type of partition in "part list" command. | |
| Useful info: | |
| ============ | |
| Two programs, namely: 'gdisk' and 'parted' are recommended to work with GPT | |
| recovery. Both are able to handle GUID partitions. | |
| Please, pay attention at -l switch for parted. | |
| "uuid" program is recommended to generate UUID string. Moreover it can decode | |
| (-d switch) passed in UUID string. It can be used to generate partitions UUID | |
| passed to u-boot environment variables. | |
| If optional CONFIG_RANDOM_UUID is defined then for any partition which environment | |
| uuid is unset, uuid is randomly generated and stored in correspond environment | |
| variable. | |
| note: | |
| Each string block of UUID generated by program "uuid" is in big endian and it is | |
| also stored in big endian in disk GPT. | |
| Partitions layout can be printed by typing "mmc part". Note that each partition | |
| GUID has different byte order than UUID generated before, this is because first | |
| three blocks of GUID string are in Little Endian. |