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https://github.com/littlefs-project/littlefs.git
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Adopted a simpler rbyd perturb scheme
The previous cksum + parity scheme worked, but needing to calculate both
cksum + parity on slightly different sets of metadata felt overly
complicated. After taking a step back, I've realized the problem is that
we're trying to force perturb effects to be implicit via the parity. If we
instead actually implement perturb effects explicitly, things get quite
a bit simpler...
This does add a bit more logic to the read path, but I don't think it's
worse than the mess we needed to parse separate cksum + parity.
Now, the perturb bit has the explicit behavior of inverting all tag
valid bits in the following commit. Which is conveniently the same as
xoring the crc32c with 00000080 before parsing each tag:
.---+---+---+---. . . .---+---+---+---. \ \ \ \
|v| tag | |v| tag | | | | |
+---+---+---+---+ +---+---+---+---+ | | | |
| commit | | commit | | | | |
| | | | +-. | | |
+---+---+---+---+ +---+---+---+---+ / | | | |
|v|p--------------. |v|p| tag | | . . .
+---+---+---+---+ | +---+---+---+---+ | . . .
| cksum | | | cksum | | . . .
+---+---+---+---+ | +---+---+---+---+ | . . .
| padding | | | padding | | . . .
| | | | | | . . .
+---+---+---+---+ | . +---+---+---+---+ | | | |
| erased | +-> |v------------------' | | |
| | | +---+---+---+---+ | | |
. . | | commit | +-. | +- rbyd
. . | | | | | | | cksum
| +---+---+---+---+ / | +-. /
'-> |v----------------------' | |
+---+---+---+---+ / |
| cksum ----------------'
+---+---+---+---+
| padding |
| |
+---+---+---+---+
| erased |
| |
. .
. .
With this scheme, we don't need to calculate a separate parity, because
each valid bit effectively validates the current state of the perturb
bit.
We also don't need extra logic to omit valid bits from the cksum,
because flipping all valid bits effectively makes perturb=0 the
canonical metadata encoding and cksum.
---
I also considered only inverting the first valid bit, which would have
the additional benefit of allowing entire commits to be crc32ced at
once, but since we don't actually track when we've started a commit
this turned out to be quite a bit more complicated than I thought.
We need someway to validate the first valid bit, otherwise it could be
flipped by a failed prog and we'd never notice. This is fine, we can
store a copy of the previous perturb bit in the next cksum tag, but it
does mean we need to track the perturb bit for the duration of the
commit. So we'd end up needing to track both start-of-commit and the
perturb bit state, which starts getting difficult to fit into our rbyd
struct...
It's easier and simpler to just flip every valid bit. As a plus this
means every valid bit contributes to validating the perturb bit.
---
Also renamed LFSR_TAG_PERTURB -> LFSR_TAG_NOISE just to avoid confusion.
Though not sure if this tag should stick around...
The end result is a nice bit of code/stack savings, which is what we'd
expect with a simpler scheme:
code stack
before: 33746 2600
after: 33570 (-0.5%) 2592 (-0.3%)
This commit is contained in:
Christopher Haster
@@ -38,12 +38,16 @@ TAG_BRANCH = 0x032c
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TAG_UATTR = 0x0400
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TAG_SATTR = 0x0600
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TAG_SHRUB = 0x1000
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TAG_CKSUM = 0x3000
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TAG_PERTURB = 0x3100
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TAG_ECKSUM = 0x3200
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TAG_ALT = 0x4000
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TAG_B = 0x0000
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TAG_R = 0x2000
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TAG_LE = 0x0000
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TAG_GT = 0x1000
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TAG_CKSUM = 0x3000
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TAG_Q = 0x0000
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TAG_P = 0x0001
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TAG_NOISE = 0x3100
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TAG_ECKSUM = 0x3200
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# some ways of block geometry representations
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@@ -236,21 +240,6 @@ def tagrepr(tag, w=None, size=None, off=None):
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((tag & 0x100) >> 1) | (tag & 0xff),
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' w%d' % w if w else '',
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' %s' % size if size is not None else '')
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elif (tag & 0x7f00) == TAG_CKSUM:
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return 'cksum 0x%02x%s%s' % (
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tag & 0xff,
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' w%d' % w if w else '',
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' %s' % size if size is not None else '')
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elif (tag & 0x7f00) == TAG_PERTURB:
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return 'perturb%s%s%s' % (
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' 0x%02x' % (tag & 0xff) if tag & 0xff else '',
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' w%d' % w if w else '',
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' %s' % size if size is not None else '')
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elif (tag & 0x7f00) == TAG_ECKSUM:
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return 'ecksum%s%s%s' % (
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' 0x%02x' % (tag & 0xff) if tag & 0xff else '',
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' w%d' % w if w else '',
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' %s' % size if size is not None else '')
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elif tag & TAG_ALT:
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return 'alt%s%s%s%s%s' % (
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'r' if tag & TAG_R else 'b',
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@@ -264,6 +253,23 @@ def tagrepr(tag, w=None, size=None, off=None):
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if size and off is not None
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else ' -%d' % size if size
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else '')
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elif (tag & 0x7f00) == TAG_CKSUM:
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return 'cksum%s%s%s' % (
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'p' if tag & 0xff == TAG_P
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else 'q' if tag & 0xff == TAG_Q
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else ' 0x%02x' % (tag & 0xff),
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' w%d' % w if w else '',
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' %s' % size if size is not None else '')
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elif (tag & 0x7f00) == TAG_NOISE:
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return 'noise%s%s%s' % (
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' 0x%02x' % (tag & 0xff) if tag & 0xff else '',
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' w%d' % w if w else '',
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' %s' % size if size is not None else '')
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elif (tag & 0x7f00) == TAG_ECKSUM:
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return 'ecksum%s%s%s' % (
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' 0x%02x' % (tag & 0xff) if tag & 0xff else '',
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' w%d' % w if w else '',
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' %s' % size if size is not None else '')
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else:
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return '0x%04x%s%s' % (
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tag,
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@@ -335,7 +341,7 @@ class Rbyd:
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cksum = 0
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cksum_ = crc32c(data[0:4])
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cksum__ = cksum_
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parity__ = parity(cksum_)
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perturb = False
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eoff = 0
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eoff_ = None
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j_ = 4
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@@ -346,13 +352,15 @@ class Rbyd:
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weight_ = 0
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weight__ = 0
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while j_ < len(data) and (not trunk or eoff <= trunk):
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# perturb?
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if perturb:
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cksum__ ^= 0x00000080
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# read next tag
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v, tag, w, size, d = fromtag(data[j_:])
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if v != parity__:
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if v != parity(cksum__):
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break
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parity__ ^= parity(cksum__)
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cksum__ = crc32c([data[j_] & ~0x80], cksum__)
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cksum__ = crc32c(data[j_+1:j_+d], cksum__)
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parity__ ^= parity(cksum__)
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cksum__ = crc32c(data[j_:j_+d], cksum__)
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j_ += d
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if not tag & TAG_ALT and j_ + size > len(data):
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break
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@@ -360,9 +368,7 @@ class Rbyd:
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# take care of cksums
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if not tag & TAG_ALT:
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if (tag & 0xff00) != TAG_CKSUM:
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parity__ ^= parity(cksum__)
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cksum__ = crc32c(data[j_:j_+size], cksum__)
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parity__ ^= parity(cksum__)
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# found a cksum?
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else:
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cksum___ = fromle32(data[j_:j_+4])
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@@ -373,6 +379,8 @@ class Rbyd:
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cksum = cksum_
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trunk_ = trunk__
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weight = weight_
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# update perturb bit
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perturb = tag & TAG_P
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# revert to data cksum
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cksum__ = cksum_
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