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gpt:doc: GPT (GUID Partition Table) documentation
Documentation of the GPT format. Signed-off-by: Lukasz Majewski <l.majewski@samsung.com> Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
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Lukasz Majewski
Tom Rini
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doc/README.gpt
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#
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# Copyright (C) 2012 Samsung Electronics
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#
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# Lukasz Majewski <l.majewski@samsung.com>
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#
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#
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# See file CREDITS for list of people who contributed to this
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# project.
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#
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# This program is free software; you can redistribute it and/or
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# modify it under the terms of the GNU General Public License as
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# published by the Free Software Foundation; either version 2 of
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# the License, or (at your option) any later version.
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#
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# This program is distributed in the hope that it will be useful,
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# but WITHOUT ANY WARRANTY; without even the implied warranty of
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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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# GNU General Public License for more details.
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#
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# You should have received a copy of the GNU General Public License
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# along with this program; if not, write to the Free Software
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# Foundation, Inc., 59 Temple Place, Suite 330, Boston,
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# MA 02111-1307 USA
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Glossary:
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========
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- UUID -(Universally Unique Identifier)
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- GUID - (Globally Unique ID)
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- EFI - (Extensible Firmware Interface)
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- UEFI - (Unified EFI) - EFI evolution
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- GPT (GUID Partition Table) - it is the EFI standard part
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- partitions - lists of available partitions (defined at u-boot):
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./include/configs/{target}.h
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Introduction:
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=============
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This document describes the GPT partition table format and usage of
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the gpt command in u-boot.
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UUID introduction:
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====================
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GPT for marking disks/partitions is using the UUID. It is supposed to be a
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globally unique value. A UUID is a 16-byte (128-bit) number. The number of
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theoretically possible UUIDs is therefore about 3 x 10^38.
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More often UUID is displayed as 32 hexadecimal digits, in 5 groups,
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separated by hyphens, in the form 8-4-4-4-12 for a total of 36 characters
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(32 digits and 4 hyphens)
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For instance, GUID of Linux data partition: EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
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Historically there are 5 methods to generate this number. The oldest one is
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combining machine's MAC address and timer (epoch) value.
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Successive versions are using MD5 hash, random numbers and SHA-1 hash. All major
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OSes and programming languages are providing libraries to compute UUID (e.g.
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uuid command line tool).
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GPT brief explanation:
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======================
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Layout:
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-------
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--------------------------------------------------
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LBA 0 |Protective MBR |
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----------------------------------------------------------
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LBA 1 |Primary GPT Header | Primary
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-------------------------------------------------- GPT
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LBA 2 |Entry 1|Entry 2| Entry 3| Entry 4|
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--------------------------------------------------
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LBA 3 |Entries 5 - 128 |
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----------------------------------------------------------
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LBA 34 |Partition 1 |
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-----------------------------------
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|Partition 2 |
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-----------------------------------
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|Partition n |
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----------------------------------------------------------
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LBA -34 |Entry 1|Entry 2| Entry 3| Entry 4| Secondary
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-------------------------------------------------- (bkp)
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LBA -33 |Entries 5 - 128 | GPT
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LBA -2 | |
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--------------------------------------------------
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LBA -1 |Secondary GPT Header |
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----------------------------------------------------------
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For a legacy reasons, GPT's LBA 0 sector has a MBR structure. It is called
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"protective MBR".
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Its first partition entry ID has 0xEE value, and disk software, which is not
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handling the GPT sees it as a storage device without free space.
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It is possible to define 128 linearly placed partition entries.
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"LBA -1" means the last addressable block (in the mmc subsystem:
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"dev_desc->lba - 1")
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Primary/Secondary GPT header:
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----------------------------
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Offset Size Description
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0 8 B Signature ("EFI PART", 45 46 49 20 50 41 52 54)
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8 4 B Revision (For version 1.0, the value is 00 00 01 00)
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12 4 B Header size (in bytes, usually 5C 00 00 00 meaning 92 bytes)
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16 4 B CRC32 of header (0 to header size), with this field zeroed
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during calculation
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20 4 B Reserved (ZERO);
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24 8 B Current LBA (location of this header copy)
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32 8 B Backup LBA (location of the other header copy)
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40 8 B First usable LBA for partitions (primary partition table last
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LBA + 1)
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48 8 B Last usable LBA (secondary partition table first LBA - 1)
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56 16 B Disk GUID (also referred as UUID on UNIXes)
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72 8 B Partition entries starting LBA (always 2 in primary copy)
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80 4 B Number of partition entries
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84 4 B Size of a partition entry (usually 128)
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88 4 B CRC32 of partition array
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92 * Reserved; must be ZERO (420 bytes for a 512-byte LBA)
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TOTAL: 512 B
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IMPORTANT:
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GPT headers and partition entries are protected by CRC32 (the POSIX CRC32).
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Primary GPT header and Secondary GPT header have swapped values of "Current LBA"
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and "Backup LBA" and therefore different CRC32 check-sum.
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CRC32 for GPT headers (field "CRC of header") are calculated up till
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"Header size" (92), NOT 512 bytes.
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CRC32 for partition entries (field "CRC32 of partition array") is calculated for
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the whole array entry ( Number_of_partition_entries *
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sizeof(partition_entry_size (usually 128)))
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Observe, how Secondary GPT is placed in the memory. It is NOT a mirror reflect
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of the Primary.
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Partition Entry Format:
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----------------------
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Offset Size Description
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0 16 B Partition type GUID
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16 16 B Unique partition GUID
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32 8 B First LBA (Little Endian)
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40 8 B Last LBA (inclusive)
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48 8 B Attribute flags [+]
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56 72 B Partition name (text)
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Attribute flags:
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Bit 0 - System partition
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Bit 60 - Read-only
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Bit 62 - Hidden
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Bit 63 - Not mount
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Creating GPT partitions in U-Boot:
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==============
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To restore GUID partition table one needs to:
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1. Define partition layout in the environment.
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Format of partitions layout:
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"partitions=uuid_disk=...;name=u-boot,size=60MiB,uuid=...;
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name=kernel,size=60MiB,uuid=...;"
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or
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"partitions=uuid_disk=${uuid_gpt_disk};name=${uboot_name},
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size=${uboot_size},uuid=${uboot_uuid};"
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Fields 'name', 'size' and 'uuid' are mandatory for every partition.
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The field 'start' is optional.
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2. Define 'CONFIG_EFI_PARTITION' and 'CONFIG_CMD_GPT'
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2. From u-boot prompt type:
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gpt write mmc 0 $partitions
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Useful info:
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============
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Two programs, namely: 'fdisk' and 'parted' are recommended to work with GPT
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recovery. Parted is able to handle GUID partitions. Unfortunately the 'fdisk'
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hasn't got such ability.
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Please, pay attention at -l switch for parted.
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"uuid" program is recommended to generate UUID string. Moreover it can decode
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(-d switch) passed in UUID string. It can be used to generate partitions UUID
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passed to u-boot environment variables.
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