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 10Glossary: 11======== 12- UUID -(Universally Unique Identifier) 13- GUID - (Globally Unique ID) 14- EFI - (Extensible Firmware Interface) 15- UEFI - (Unified EFI) - EFI evolution 16- GPT (GUID Partition Table) - it is the EFI standard part 17- partitions - lists of available partitions (defined at u-boot): 18 ./include/configs/{target}.h 19 20Introduction: 21============= 22This document describes the GPT partition table format and usage of 23the gpt command in u-boot. 24 25 26UUID introduction: 27==================== 28 29GPT for marking disks/partitions is using the UUID. It is supposed to be a 30globally unique value. A UUID is a 16-byte (128-bit) number. The number of 31theoretically possible UUIDs is therefore about 3 x 10^38. 32More often UUID is displayed as 32 hexadecimal digits, in 5 groups, 33separated by hyphens, in the form 8-4-4-4-12 for a total of 36 characters 34(32 digits and 4 hyphens) 35 36For instance, GUID of Linux data partition: EBD0A0A2-B9E5-4433-87C0-68B6B72699C7 37 38Historically there are 5 methods to generate this number. The oldest one is 39combining machine's MAC address and timer (epoch) value. 40 41Successive versions are using MD5 hash, random numbers and SHA-1 hash. All major 42OSes and programming languages are providing libraries to compute UUID (e.g. 43uuid command line tool). 44 45GPT brief explanation: 46====================== 47 48 Layout: 49 ------- 50 51 -------------------------------------------------- 52 LBA 0 |Protective MBR | 53 ---------------------------------------------------------- 54 LBA 1 |Primary GPT Header | Primary 55 -------------------------------------------------- GPT 56 LBA 2 |Entry 1|Entry 2| Entry 3| Entry 4| 57 -------------------------------------------------- 58 LBA 3 |Entries 5 - 128 | 59 | | 60 | | 61 ---------------------------------------------------------- 62 LBA 34 |Partition 1 | 63 | | 64 ----------------------------------- 65 |Partition 2 | 66 | | 67 ----------------------------------- 68 |Partition n | 69 | | 70 ---------------------------------------------------------- 71 LBA -34 |Entry 1|Entry 2| Entry 3| Entry 4| Secondary 72 -------------------------------------------------- (bkp) 73 LBA -33 |Entries 5 - 128 | GPT 74 | | 75 | | 76 LBA -2 | | 77 -------------------------------------------------- 78 LBA -1 |Secondary GPT Header | 79 ---------------------------------------------------------- 80 81 82For a legacy reasons, GPT's LBA 0 sector has a MBR structure. It is called 83"protective MBR". 84Its first partition entry ID has 0xEE value, and disk software, which is not 85handling the GPT sees it as a storage device without free space. 86 87It is possible to define 128 linearly placed partition entries. 88 89"LBA -1" means the last addressable block (in the mmc subsystem: 90"dev_desc->lba - 1") 91 92Primary/Secondary GPT header: 93---------------------------- 94Offset Size Description 95 960 8 B Signature ("EFI PART", 45 46 49 20 50 41 52 54) 978 4 B Revision (For version 1.0, the value is 00 00 01 00) 9812 4 B Header size (in bytes, usually 5C 00 00 00 meaning 92 bytes) 9916 4 B CRC32 of header (0 to header size), with this field zeroed 100 during calculation 10120 4 B Reserved (ZERO); 10224 8 B Current LBA (location of this header copy) 10332 8 B Backup LBA (location of the other header copy) 10440 8 B First usable LBA for partitions (primary partition table last 105 LBA + 1) 10648 8 B Last usable LBA (secondary partition table first LBA - 1) 10756 16 B Disk GUID (also referred as UUID on UNIXes) 10872 8 B Partition entries starting LBA (always 2 in primary copy) 10980 4 B Number of partition entries 11084 4 B Size of a partition entry (usually 128) 11188 4 B CRC32 of partition array 11292 * Reserved; must be ZERO (420 bytes for a 512-byte LBA) 113 114TOTAL: 512 B 115 116 117 118IMPORTANT: 119 120GPT headers and partition entries are protected by CRC32 (the POSIX CRC32). 121 122Primary GPT header and Secondary GPT header have swapped values of "Current LBA" 123and "Backup LBA" and therefore different CRC32 check-sum. 124 125CRC32 for GPT headers (field "CRC of header") are calculated up till 126"Header size" (92), NOT 512 bytes. 127 128CRC32 for partition entries (field "CRC32 of partition array") is calculated for 129the whole array entry ( Number_of_partition_entries * 130sizeof(partition_entry_size (usually 128))) 131 132Observe, how Secondary GPT is placed in the memory. It is NOT a mirror reflect 133of the Primary. 134 135 136 Partition Entry Format: 137 ---------------------- 138 Offset Size Description 139 140 0 16 B Partition type GUID 141 16 16 B Unique partition GUID 142 32 8 B First LBA (Little Endian) 143 40 8 B Last LBA (inclusive) 144 48 8 B Attribute flags [+] 145 56 72 B Partition name (text) 146 147 Attribute flags: 148 Bit 0 - System partition 149 Bit 60 - Read-only 150 Bit 62 - Hidden 151 Bit 63 - Not mount 152 153 154Creating GPT partitions in U-Boot: 155============== 156 157To restore GUID partition table one needs to: 1581. Define partition layout in the environment. 159 Format of partitions layout: 160 "partitions=uuid_disk=...;name=u-boot,size=60MiB,uuid=...; 161 name=kernel,size=60MiB,uuid=...;" 162 or 163 "partitions=uuid_disk=${uuid_gpt_disk};name=${uboot_name}, 164 size=${uboot_size},uuid=${uboot_uuid};" 165 166 Fields 'name', 'size' and 'uuid' are mandatory for every partition. 167 The field 'start' is optional. 168 1692. Define 'CONFIG_EFI_PARTITION' and 'CONFIG_CMD_GPT' 170 1712. From u-boot prompt type: 172 gpt write mmc 0 $partitions 173 174 175Useful info: 176============ 177 178Two programs, namely: 'fdisk' and 'parted' are recommended to work with GPT 179recovery. Parted is able to handle GUID partitions. Unfortunately the 'fdisk' 180hasn't got such ability. 181Please, pay attention at -l switch for parted. 182 183"uuid" program is recommended to generate UUID string. Moreover it can decode 184(-d switch) passed in UUID string. It can be used to generate partitions UUID 185passed to u-boot environment variables. 186