Documentation of the GPT table format.
Signed-off-by: Lukasz Majewski l.majewski@samsung.com Signed-off-by: Kyungmin Park kyungmin.park@samsung.com --- doc/README.gpt | 199 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 files changed, 199 insertions(+), 0 deletions(-) create mode 100644 doc/README.gpt
diff --git a/doc/README.gpt b/doc/README.gpt new file mode 100644 index 0000000..0bdacf3 --- /dev/null +++ b/doc/README.gpt @@ -0,0 +1,199 @@ +# +# Copyright (C) 2012 Samsung Electronics +# +# Lukasz Majewski l.majewski@samsung.com +# +# +# See file CREDITS for list of people who contributed to this +# project. +# +# This program is free software; you can redistribute it and/or +# modify it under the terms of the GNU General Public License as +# published by the Free Software Foundation; either version 2 of +# the License, or (at your option) any later version. +# +# This program is distributed in the hope that it will be useful, +# but WITHOUT ANY WARRANTY; without even the implied warranty of +# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the +# GNU General Public License for more details. +# +# You should have received a copy of the GNU General Public License +# along with this program; if not, write to the Free Software +# Foundation, Inc., 59 Temple Place, Suite 330, Boston, +# MA 02111-1307 USA + + +Glossary: +======== +- UUID -(Universally Unique Identifier) +- GUID - (Globally Unique ID) +- EFI - (Extensible Firmware Interface) +- UEFI - (Unified EFI) - EFI evolution +- GPT (GUID Page Table) - it is the EFI standard part +- partitions - lists of availavle partitions (defined at u-boot): + ./include/configs/{target}.h + +Introduction: +============= +This document describes the GPT partition table format when used with u-boot. + + +UUID introduction[5]: +==================== + +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 × 10^38. +More often UUID is stored as 32 hexadecimal digits, displayed 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 +For u-boot GPT hyphens are omitted. + +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. + +However it costs in terms of the computational power and memory footprint. +Therefore u-boot uses the crc32 with reading random block (512B) from MMC +storage device to generate UUID/GUID. + + +GUID 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| Secondary + --------------------------------------------------- (bkp) + LBA 34 |Partition 1 | + | | + ----------------------------------- + |Partition 2 | + | | + ----------------------------------- + |Partition n | + | | + ----------------------------------------------------------- + LBA -34 |Entry 1|Entry 2| Entry 3| Entry 4| Secondary + --------------------------------------------------- (bkp) + LBA -33 |Entries 5 - 128 | GPT + | | + | | + LBA -2 | | + --------------------------------------------------- + LBA -1 |Secondary 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 lineary placed partition entries. + +"LBA -1" means the last addressable block (in the mmc subsystem: +"dev_desc->lba - 1") + +Primary/Secondary 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 Secondary 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 Secondary 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 + 16 16 B Unique partition GUID + 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 (Don't used at u-boot): + Bit 0 - System partition + Bit 60 - Read-only + Bit 62 - Hidden + Bit 63 - Not mount + + + +Useful info: +============ + +Two programs, namely: 'fdisk' and 'parted' are recommended to work with GPT +recovery. Parted is able to handle GUID partitions. Unfortunately the 'fdisk' +hasn't got such ability. +Please, pay attention at -l switch for parted. + + +Problems with current implementation: +==================================== + +1. Entropy of the u-boot's current GUID generation is only sufficient for an +emergency situations.