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Clarified what is accessible at specific superblock offsets in SPEC.md

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It used to be the case that the entire superblock entry could be found
at specific offsets, but this was only possible while the superblock
entry was immutable. Now that the superblock entry is very mutable
(block-count changes, lfs2.0 -> lfs2.1 version bumps, etc), the correct
superblock entry may end up later in the metadata log.

At the very least, the "littlefs" magic string is still immutable and at
the specific offset offset=8. This is arguably the most useful
fixed-offset item.
This commit is contained in:
Christopher Haster 2024-03-19 00:20:32 -05:00
parent a60a986c9c
commit 25ee90fdf1
1 changed files with 4 additions and 3 deletions
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7
SPEC.md
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@ -441,9 +441,10 @@ Superblock fields:
7. **Attr max (32-bits)** - Maximum size of file attributes in bytes.
The superblock must always be the first entry (id 0) in a metadata pair as well
as be the first entry written to the block. This means that the superblock
entry can be read from a device using offsets alone.
The superblock must always be the first entry (id 0) in the metadata pair, and
the name tag must always be the first tag in the metadata pair. This makes it
so that the magic string "littlefs" will always reside at offset=8 in a valid
littlefs superblock.
---
#### `0x2xx` LFS_TYPE_STRUCT