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Moved revision count noise behind ifdef LFS_NOISY

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littlefs is intentionally designed to not rely on noise, even with cksum
collisions (hello, perturb bit!). So it makes sense for this to be an
optional feature, even if it's a small one.

Disabling revision count noise by default also helps with testing. The
whole point of revision count noise is to make cksum collisions less
likely, which is a bit counterproductive when that's something we want
to test!

This doesn't really change the revision count encoding:

  vvvvrrrr rrrrrrnn nnnnnnnn nnnnnnnn
  '-.''----.----''---------.--------'
    '------|---------------|---------- 4-bit relocation revision
           '---------------|---------- recycle-bits recycle counter
                           '---------- pseudorandom noise (optional)

I considered moving the recycle-bits down when we're not adding noise,
but the extra logic just isn't worth making the revision count a bit
more human-readable.

---

This saves a small bit of code in the default build, at the cost of some
code for the runtime checks in the LFS_NOISY build. Though I'm hoping
future config work will let users opt-out of these runtime checks:

                    code          stack          ctx
  before:          38548           2624          640
  default after:   38508 (-0.1%)   2624 (+0.0%)  640 (+0.0%)
  LFS_NOISY after: 38568 (+0.1%)   2624 (+0.0%)  640 (+0.0%)

Honestly the thing I'm more worried about is using one of our precious
mount flags for this... There's not that many bits left!
This commit is contained in:
Christopher Haster
2025-01-29 13:44:00 -06:00
parent 0aef3cbcde
commit 415e6325d1
6 changed files with 193 additions and 138 deletions
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lfs.c
+26 -10
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@@ -6888,6 +6888,12 @@ static inline bool lfsr_m_isrdonly(uint32_t flags) {
return flags & LFS_M_RDONLY;
}
#ifdef LFS_NOISY
static inline bool lfsr_m_isnoisy(uint32_t flags) {
return flags & LFS_M_NOISY;
}
#endif
#ifdef LFS_CKPROGS
static inline bool lfsr_m_isckprogs(uint32_t flags) {
return flags & LFS_M_CKPROGS;
@@ -7198,15 +7204,20 @@ static int lfsr_fs_consumegdelta(lfs_t *lfs, const lfsr_mdir_t *mdir) {
// '-.''----.----''---------.--------'
// '------|---------------|---------- 4-bit relocation revision
// '---------------|---------- recycle-bits recycle counter
// '---------- pseudorandom nonce
// '---------- pseudorandom noise (optional)
static inline uint32_t lfsr_rev_init(lfs_t *lfs, uint32_t rev) {
(void)lfs;
// we really only care about the top revision bits here
rev &= ~((1 << 28)-1);
// increment revision
rev += 1 << 28;
// xor in a pseudorandom nonce
rev ^= ((1 << (28-lfs_smax(lfs->recycle_bits, 0)))-1) & lfs->gcksum;
// xor in pseudorandom noise
#ifdef LFS_NOISY
if (lfsr_m_isnoisy(lfs->flags)) {
rev ^= ((1 << (28-lfs_smax(lfs->recycle_bits, 0)))-1) & lfs->gcksum;
}
#endif
return rev;
}
@@ -7223,8 +7234,12 @@ static inline bool lfsr_rev_needsrelocation(lfs_t *lfs, uint32_t rev) {
static inline uint32_t lfsr_rev_inc(lfs_t *lfs, uint32_t rev) {
// increment recycle counter/revision
rev += 1 << (28-lfs_smax(lfs->recycle_bits, 0));
// xor in a pseudorandom nonce
rev ^= ((1 << (28-lfs_smax(lfs->recycle_bits, 0)))-1) & lfs->gcksum;
// xor in pseudorandom noise
#ifdef LFS_NOISY
if (lfsr_m_isnoisy(lfs->flags)) {
rev ^= ((1 << (28-lfs_smax(lfs->recycle_bits, 0)))-1) & lfs->gcksum;
}
#endif
return rev;
}
@@ -13421,6 +13436,7 @@ static int lfs_init(lfs_t *lfs, uint32_t flags,
| LFS_M_RDONLY
| LFS_M_FLUSH
| LFS_M_SYNC
| LFS_IFDEF_NOISY(LFS_M_NOISY, 0)
| LFS_IFDEF_CKPROGS(LFS_M_CKPROGS, 0)
| LFS_IFDEF_CKFETCHES(LFS_M_CKFETCHES, 0)
| LFS_IFDEF_CKPARITY(LFS_M_CKPARITY, 0)
@@ -14249,6 +14265,7 @@ int lfsr_mount(lfs_t *lfs, uint32_t flags,
| LFS_M_RDONLY
| LFS_M_FLUSH
| LFS_M_SYNC
| LFS_IFDEF_NOISY(LFS_M_NOISY, 0)
| LFS_IFDEF_CKPROGS(LFS_M_CKPROGS, 0)
| LFS_IFDEF_CKFETCHES(LFS_M_CKFETCHES, 0)
| LFS_IFDEF_CKPARITY(LFS_M_CKPARITY, 0)
@@ -14262,9 +14279,6 @@ int lfsr_mount(lfs_t *lfs, uint32_t flags,
LFS_ASSERT(!lfsr_m_isrdonly(flags) || !lfsr_t_ismkconsistent(flags));
LFS_ASSERT(!lfsr_m_isrdonly(flags) || !lfsr_t_islookahead(flags));
LFS_ASSERT(!lfsr_m_isrdonly(flags) || !lfsr_t_iscompact(flags));
// some flags don't make sense when only traversing the mtree
LFS_ASSERT(!lfsr_t_ismtreeonly(flags) || !lfsr_t_islookahead(flags));
LFS_ASSERT(!lfsr_t_ismtreeonly(flags) || !lfsr_t_isckdata(flags));
int err = lfs_init(lfs,
flags & (
@@ -14272,6 +14286,7 @@ int lfsr_mount(lfs_t *lfs, uint32_t flags,
| LFS_M_RDONLY
| LFS_M_FLUSH
| LFS_M_SYNC
| LFS_IFDEF_NOISY(LFS_M_NOISY, 0)
| LFS_IFDEF_CKPROGS(LFS_M_CKPROGS, 0)
| LFS_IFDEF_CKFETCHES(LFS_M_CKFETCHES, 0)
| LFS_IFDEF_CKPARITY(LFS_M_CKPARITY, 0)
@@ -14441,18 +14456,18 @@ int lfsr_format(lfs_t *lfs, uint32_t flags,
// unknown flags?
LFS_ASSERT((flags & ~(
LFS_F_RDWR
| LFS_IFDEF_NOISY(LFS_F_NOISY, 0)
| LFS_IFDEF_CKPROGS(LFS_F_CKPROGS, 0)
| LFS_IFDEF_CKFETCHES(LFS_F_CKFETCHES, 0)
| LFS_IFDEF_CKPARITY(LFS_F_CKPARITY, 0)
| LFS_IFDEF_CKDATACKSUMS(LFS_F_CKDATACKSUMS, 0)
| LFS_F_CKMETA
| LFS_F_CKDATA)) == 0);
// some flags don't make sense when only traversing the mtree
LFS_ASSERT(!lfsr_t_ismtreeonly(flags) || !lfsr_t_isckdata(flags));
int err = lfs_init(lfs,
flags & (
LFS_F_RDWR
| LFS_IFDEF_NOISY(LFS_F_NOISY, 0)
| LFS_IFDEF_CKPROGS(LFS_F_CKPROGS, 0)
| LFS_IFDEF_CKFETCHES(LFS_F_CKFETCHES, 0)
| LFS_IFDEF_CKPARITY(LFS_F_CKPARITY, 0)
@@ -14512,6 +14527,7 @@ int lfsr_fs_stat(lfs_t *lfs, struct lfs_fsinfo *fsinfo) {
LFS_I_RDONLY
| LFS_I_FLUSH
| LFS_I_SYNC
| LFS_IFDEF_NOISY(LFS_I_NOISY, 0)
| LFS_IFDEF_CKPROGS(LFS_I_CKPROGS, 0)
| LFS_IFDEF_CKFETCHES(LFS_I_CKFETCHES, 0)
| LFS_IFDEF_CKPARITY(LFS_I_CKPARITY, 0)