[U-Boot] [PATCH 1/6] gpt:doc: GPT (GUID Partition Table) documentation

Lukasz Majewski
2012-08-24T08:13:10Z

Documentation of the GPT table format.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
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+#
+# 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.
--
1.7.2.3