1# 2# Copyright (C) 2012 Samsung Electronics 3# 4# Lukasz Majewski <l.majewski@samsung.com> 5# 6# 7# See file CREDITS for list of people who contributed to this 8# project. 9# 10# This program is free software; you can redistribute it and/or 11# modify it under the terms of the GNU General Public License as 12# published by the Free Software Foundation; either version 2 of 13# the License, or (at your option) any later version. 14# 15# This program is distributed in the hope that it will be useful, 16# but WITHOUT ANY WARRANTY; without even the implied warranty of 17# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 18# GNU General Public License for more details. 19# 20# You should have received a copy of the GNU General Public License 21# along with this program; if not, write to the Free Software 22# Foundation, Inc., 59 Temple Place, Suite 330, Boston, 23# MA 02111-1307 USA 24 25 26Glossary: 27======== 28- UUID -(Universally Unique Identifier) 29- GUID - (Globally Unique ID) 30- EFI - (Extensible Firmware Interface) 31- UEFI - (Unified EFI) - EFI evolution 32- GPT (GUID Partition Table) - it is the EFI standard part 33- partitions - lists of available partitions (defined at u-boot): 34 ./include/configs/{target}.h 35 36Introduction: 37============= 38This document describes the GPT partition table format and usage of 39the gpt command in u-boot. 40 41 42UUID introduction: 43==================== 44 45GPT for marking disks/partitions is using the UUID. It is supposed to be a 46globally unique value. A UUID is a 16-byte (128-bit) number. The number of 47theoretically possible UUIDs is therefore about 3 x 10^38. 48More often UUID is displayed as 32 hexadecimal digits, in 5 groups, 49separated by hyphens, in the form 8-4-4-4-12 for a total of 36 characters 50(32 digits and 4 hyphens) 51 52For instance, GUID of Linux data partition: EBD0A0A2-B9E5-4433-87C0-68B6B72699C7 53 54Historically there are 5 methods to generate this number. The oldest one is 55combining machine's MAC address and timer (epoch) value. 56 57Successive versions are using MD5 hash, random numbers and SHA-1 hash. All major 58OSes and programming languages are providing libraries to compute UUID (e.g. 59uuid command line tool). 60 61GPT brief explanation: 62====================== 63 64 Layout: 65 ------- 66 67 -------------------------------------------------- 68 LBA 0 |Protective MBR | 69 ---------------------------------------------------------- 70 LBA 1 |Primary GPT Header | Primary 71 -------------------------------------------------- GPT 72 LBA 2 |Entry 1|Entry 2| Entry 3| Entry 4| 73 -------------------------------------------------- 74 LBA 3 |Entries 5 - 128 | 75 | | 76 | | 77 ---------------------------------------------------------- 78 LBA 34 |Partition 1 | 79 | | 80 ----------------------------------- 81 |Partition 2 | 82 | | 83 ----------------------------------- 84 |Partition n | 85 | | 86 ---------------------------------------------------------- 87 LBA -34 |Entry 1|Entry 2| Entry 3| Entry 4| Secondary 88 -------------------------------------------------- (bkp) 89 LBA -33 |Entries 5 - 128 | GPT 90 | | 91 | | 92 LBA -2 | | 93 -------------------------------------------------- 94 LBA -1 |Secondary GPT Header | 95 ---------------------------------------------------------- 96 97 98For a legacy reasons, GPT's LBA 0 sector has a MBR structure. It is called 99"protective MBR". 100Its first partition entry ID has 0xEE value, and disk software, which is not 101handling the GPT sees it as a storage device without free space. 102 103It is possible to define 128 linearly placed partition entries. 104 105"LBA -1" means the last addressable block (in the mmc subsystem: 106"dev_desc->lba - 1") 107 108Primary/Secondary GPT header: 109---------------------------- 110Offset Size Description 111 1120 8 B Signature ("EFI PART", 45 46 49 20 50 41 52 54) 1138 4 B Revision (For version 1.0, the value is 00 00 01 00) 11412 4 B Header size (in bytes, usually 5C 00 00 00 meaning 92 bytes) 11516 4 B CRC32 of header (0 to header size), with this field zeroed 116 during calculation 11720 4 B Reserved (ZERO); 11824 8 B Current LBA (location of this header copy) 11932 8 B Backup LBA (location of the other header copy) 12040 8 B First usable LBA for partitions (primary partition table last 121 LBA + 1) 12248 8 B Last usable LBA (secondary partition table first LBA - 1) 12356 16 B Disk GUID (also referred as UUID on UNIXes) 12472 8 B Partition entries starting LBA (always 2 in primary copy) 12580 4 B Number of partition entries 12684 4 B Size of a partition entry (usually 128) 12788 4 B CRC32 of partition array 12892 * Reserved; must be ZERO (420 bytes for a 512-byte LBA) 129 130TOTAL: 512 B 131 132 133 134IMPORTANT: 135 136GPT headers and partition entries are protected by CRC32 (the POSIX CRC32). 137 138Primary GPT header and Secondary GPT header have swapped values of "Current LBA" 139and "Backup LBA" and therefore different CRC32 check-sum. 140 141CRC32 for GPT headers (field "CRC of header") are calculated up till 142"Header size" (92), NOT 512 bytes. 143 144CRC32 for partition entries (field "CRC32 of partition array") is calculated for 145the whole array entry ( Number_of_partition_entries * 146sizeof(partition_entry_size (usually 128))) 147 148Observe, how Secondary GPT is placed in the memory. It is NOT a mirror reflect 149of the Primary. 150 151 152 Partition Entry Format: 153 ---------------------- 154 Offset Size Description 155 156 0 16 B Partition type GUID 157 16 16 B Unique partition GUID 158 32 8 B First LBA (Little Endian) 159 40 8 B Last LBA (inclusive) 160 48 8 B Attribute flags [+] 161 56 72 B Partition name (text) 162 163 Attribute flags: 164 Bit 0 - System partition 165 Bit 60 - Read-only 166 Bit 62 - Hidden 167 Bit 63 - Not mount 168 169 170Creating GPT partitions in U-Boot: 171============== 172 173To restore GUID partition table one needs to: 1741. Define partition layout in the environment. 175 Format of partitions layout: 176 "partitions=uuid_disk=...;name=u-boot,size=60MiB,uuid=...; 177 name=kernel,size=60MiB,uuid=...;" 178 or 179 "partitions=uuid_disk=${uuid_gpt_disk};name=${uboot_name}, 180 size=${uboot_size},uuid=${uboot_uuid};" 181 182 Fields 'name', 'size' and 'uuid' are mandatory for every partition. 183 The field 'start' is optional. 184 1852. Define 'CONFIG_EFI_PARTITION' and 'CONFIG_CMD_GPT' 186 1872. From u-boot prompt type: 188 gpt write mmc 0 $partitions 189 190 191Useful info: 192============ 193 194Two programs, namely: 'fdisk' and 'parted' are recommended to work with GPT 195recovery. Parted is able to handle GUID partitions. Unfortunately the 'fdisk' 196hasn't got such ability. 197Please, pay attention at -l switch for parted. 198 199"uuid" program is recommended to generate UUID string. Moreover it can decode 200(-d switch) passed in UUID string. It can be used to generate partitions UUID 201passed to u-boot environment variables. 202