Commit 02e43537b322 ("part_efi: support padding between the GPT header and partition entries") added support for deviating from the typical GPT layout.
Explicitly state deviations are allowed/possible, and mention when (SoC SPL) and when not (compatibility) deviations are useful. Also mention support for non-standard layouts in gdisk 1.0.3.
Signed-off-by: Stefan Brüns stefan.bruens@rwth-aachen.de
---
doc/README.gpt | 33 +++++++++++++++++++++++++++++---- 1 file changed, 29 insertions(+), 4 deletions(-)
diff --git a/doc/README.gpt b/doc/README.gpt index d3db8bce1c..7134f3b3cf 100644 --- a/doc/README.gpt +++ b/doc/README.gpt @@ -44,8 +44,8 @@ uuid command line tool). GPT brief explanation: ======================
- Layout: - ------- + Default layout: + ---------------
-------------------------------------------------- LBA 0 |Protective MBR | @@ -82,7 +82,29 @@ For a legacy reasons, GPT's LBA 0 sector has a MBR structure. It is called 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 linearly placed partition entries. +By default, the first partition entry of the primary GPT is stored in LBA 2, +although this is not explicitly mandated by the UEFI specification. The +start LBAs of the partition entries are given in the corresponding GPT +headers (Primary/Backup) (offset 72 bytes). + +The UEFI specification mandates at least 128 contigously stored partition +entries, the number is specified in the GPT headers (offset 80). + +As several SoCs require the SPL to be located at a fixed position, often +below LBA 34 (17 kByte for 512 byte blocks), it is possible to deviate +from the standard layout: + +1. Lower the number of partition entries. This violates the UEFI/GPT + specification, but usually works. +2. Insert a gap between Primary GPT Header and partition entries. This + is in line with the specification, but may cause problems with tools + or operating systems hardcoding the partition entries LBA to 2. + +There is limited support for both variants in U-Boot - reading is fully +supported, while the "gpt write" command always creates a GPT with 128 +entries. A gap is created when CONFIG_EFI_PARTITION_ENTRIES_OFF is set +or when the device tree "/config" node contains a property +"u-boot,efi-partition-entries-offset".
"LBA -1" means the last addressable block (in the mmc subsystem: "dev_desc->lba - 1") @@ -103,7 +125,7 @@ Offset Size Description 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) +72 8 B Partition entries starting LBA (usually 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 @@ -283,6 +305,9 @@ Two programs, namely: 'gdisk' and 'parted' are recommended to work with GPT recovery. Both are able to handle GUID partitions. Please, pay attention at -l switch for parted.
+'gdisk' as of version 1.0.3 is able to create tables with a number of partition +entries different to 128, and insert padding after the GPT header. + "uuid" program is recommended to generate UUID string. Moreover it can decode (-d switch) passed in UUID string. It can be used to generate partitions UUID passed to u-boot environment variables.