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Edited tag structure to balance size vs id count

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This is a minor tweak that resulted from looking at some other use cases
for the littlefs data-structure on disk. Consider an implementation that
does not need to buffer inline-files in RAM. In this case we should have
as large a tag size field as possible. Unfortunately, we don't have much
space to work with in the 32-bit tag struct, so we have to make some
compromises. These limitations could be removed with a 64-bit tag
struct, at the cost of code size.

32-bit tag structure:
[---       32       ---]
[1|- 9 -|- 9 -|-- 13 --]
 ^   ^     ^      ^- entry length
 |   |     \-------- file id
 |   \-------------- tag type
 \------------------ valid bit
This commit is contained in:
Christopher Haster
2018-09-15 02:40:53 -05:00
parent cafe6ab466
commit d7e4abad0b
4 changed files with 45 additions and 45 deletions
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2
tests/corrupt.py
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@@ -19,7 +19,7 @@ def corrupt(block):
break
tag ^= ntag
size = (tag & 0xfff) if (tag & 0xfff) != 0xfff else 0
size = (tag & 0x1fff) if (tag & 0x1fff) != 0x1fff else 0
file.seek(size, os.SEEK_CUR)
# lob off last 3 bytes

12
tests/debug.py
View File

@@ -76,11 +76,11 @@ def main(*blocks):
off += 4
type = (tag & 0x7fc00000) >> 22
id = (tag & 0x003ff000) >> 12
size = (tag & 0x00000fff) >> 0
id = (tag & 0x003fe000) >> 13
size = (tag & 0x00001fff) >> 0
iscrc = (type & 0x1f0) == 0x0f0
data = file.read(size if size != 0xfff else 0)
data = file.read(size if size != 0x1fff else 0)
if iscrc:
crc = binascii.crc32(data[:4], crc)
else:
@@ -89,12 +89,12 @@ def main(*blocks):
print '%04x: %08x %-14s %3s %3s %-23s %-8s' % (
off, tag,
typeof(type) + (' bad!' if iscrc and ~crc else ''),
id if id != 0x3ff else '.',
size if size != 0xfff else 'x',
id if id != 0x1ff else '.',
size if size != 0x1fff else 'x',
' '.join('%02x' % ord(c) for c in data[:8]),
''.join(c if c >= ' ' and c <= '~' else '.' for c in data[:8]))
off += size if size != 0xfff else 0
off += size if size != 0x1fff else 0
if iscrc:
crc = 0
tag ^= (type & 1) << 31