| # |
| # 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 Linux data partition: EBD0A0A2-B9E5-4433-87C0-68B6B72699C7 |
| |
| 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 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};" |
| |
| Fields 'name', 'size' and 'uuid' are mandatory for every partition. |
| The field 'start' is optional. |
| |
| option: CONFIG_RANDOM_UUID |
| If any partition "UUID" no exists then it is randomly generated. |
| |
| 2. Define 'CONFIG_EFI_PARTITION' and 'CONFIG_CMD_GPT' |
| |
| 2. From u-boot prompt type: |
| gpt write mmc 0 $partitions |
| |
| 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. |