Rearranged type encoding for crcs so they mostly fit in a single byte

I'm still not sure this is the best decision, since it may add some
complexity to tag parsing, but making most crcs one byte may be valuable
since these exist in every single commit.

This gives tags three high-level encodings:

  in-tree tags:
  iiiiiii iiiiitt ttTTTTT TTT00rv
              ^----^--------^--^^- 16-bit id
                   '--------|--||- 4-bit suptype
                            '--||- 8-bit subtype
                               '|- removed bit
                                '- valid bit
  lllllll lllllll lllllll lllllll
                                ^- n-bit length

  out-of-tree tags:
  ------- -----TT TTTTTTt ttt01pv
                       ^----^--^^- 8-bit subtype
                            '--||- 4-bit suptype
                               '|- perturb bit
                                '- valid bit
  lllllll lllllll lllllll lllllll
                                ^- n-bit length

  alt tags:
  wwwwwww wwwwwww wwwwwww www1dcv
                            ^-^^^- 28-bit weight
                              '||- direction bit
                               '|- color bit
                                '- valid bit
  jjjjjjj jjjjjjj jjjjjjj jjjjjjj
                                ^- n-bit jump

Having the location of the subtype flipped for crc tags vs tree tags is
unintuitive, but it makes more crc tags fit in a single byte, while
preserving expected tag ordering for tree tags.

The only case where crc tags don't fit in a single byte if is non-crc
checksums (sha256?) are added, at which point I expect the subtype to
indicate which checksum algorithm is in use.

lfs.c

@@ -466,7 +466,7 @@ enum lfsr_tag_type {
     LFSR_TAG_CRC     = 0x0002,
     LFSR_TAG_CRC0    = 0x0002,
     LFSR_TAG_CRC1    = 0x0003,
-    LFSR_TAG_FCRC    = 0x0802,
+    LFSR_TAG_FCRC    = 0x000a,
 
     LFSR_TAG_ALT     = 0x0004,
     LFSR_TAG_ALTBLT  = 0x0004,
@@ -586,10 +586,18 @@ static inline uint16_t lfsr_tag_suptype(lfsr_tag_t tag) {
     return tag & 0x7807;
 }
 
-static inline uint16_t lfsr_tag_subtype(lfsr_tag_t tag) {
+static inline uint8_t lfsr_tag_subtype(lfsr_tag_t tag) {
     return (tag >> 3) & 0xff;
 }
 
+static inline uint16_t lfsr_tag_xsuptype(lfsr_tag_t tag) {
+    return tag & 0x007e;
+}
+
+static inline uint8_t lfsr_tag_xsubtype(lfsr_tag_t tag) {
+    return (tag >> 7) & 0xff;
+}
+
 static inline uint16_t lfsr_tag_type(lfsr_tag_t tag) {
     return tag & 0x7fff;
 }
@@ -1179,7 +1187,7 @@ static int lfsr_rbyd_fetch(lfs_t *lfs, lfsr_rbyd_t *rbyd,
         }
 
         // not an end-of-commit crc
-        if ((lfsr_tag_suptype(tag) & ~0x1) != LFSR_TAG_CRC) {
+        if (lfsr_tag_xsuptype(tag) != LFSR_TAG_CRC) {
             // fcrc is only valid if the last tag was a crc
             hasfcrc = false;
 
@@ -1209,7 +1217,7 @@ static int lfsr_rbyd_fetch(lfs_t *lfs, lfsr_rbyd_t *rbyd,
                 // on a bad crc
                 count -= 1;
             // found an fcrc? save for later
-            } else if (lfsr_tag_suptype(tag) == LFSR_TAG_FCRC) {
+            } else if (lfsr_tag_xsuptype(tag) == LFSR_TAG_FCRC) {
                 err = lfs_bd_read(lfs,
                         NULL, &lfs->rcache, lfs->cfg->block_size,
                         block, off, &fcrc,
@@ -1963,14 +1971,14 @@ int lfsr_rbyd_commit(lfs_t *lfs, lfsr_rbyd_t *rbyd,
     //
     // this gets a bit complicated as we have two types of crcs:
     // - 7-word crc with fcrc to check following prog (middle of block)
-    //     2-byte fcrc tag
+    //     1-byte fcrc tag
     //   + 1-byte fcrc len (worst case)
     //   + 4-byte fcrc crc
     //   + 4-byte fcrc crc-len (worst case)
     //   + 1-byte crc tag
     //   + 4-byte crc+padding len (worst case)
     //   + 4-byte crc crc
-    //   = 19 bytes
+    //   = 18 bytes
     // - 3-word crc with no following prog (end of block)
     //     1-byte crc tag
     //   + 4-byte crc+padding len (worst case)
@@ -1978,7 +1986,7 @@ int lfsr_rbyd_commit(lfs_t *lfs, lfsr_rbyd_t *rbyd,
     //   = 9 bytes
     // 
     const lfs_off_t aligned = lfs_alignup(
-            rbyd_.off + 2+1+4+4 + 1+4+4,
+            rbyd_.off + 1+1+4+4 + 1+4+4,
             lfs->cfg->prog_size);
 
     // not even space for the crc?

scripts/dbgrbyd.py

@@ -54,6 +54,8 @@ def tagrepr(tag, size, off=None):
     type = tag & 0x7fff
     suptype = tag & 0x7807
     subtype = (tag >> 3) & 0xff
+    xsuptype = tag & 0x7e
+    xsubtype = (tag >> 7) & 0xff
     id = ((tag >> 15) & 0xffff) - 1
 
     if suptype == 0x0800:
@@ -73,14 +75,14 @@ def tagrepr(tag, size, off=None):
             subtype,
             ' id%d' % id if id != -1 else '',
             ' %d' % size if not tag & 0x1 else '')
-    elif (suptype & ~0x1) == 0x0002:
+    elif xsuptype == 0x0002:
         return 'crc%x%s %d' % (
-            suptype & 0x1,
-            ' 0x%02x' % subtype if subtype else '',
+            tag & 0x1,
+            ' 0x%02x' % xsubtype if xsubtype else '',
             size)
-    elif suptype == 0x0802:
+    elif xsuptype == 0x000a:
         return 'fcrc%s %d' % (
-            ' 0x%02x' % subtype if subtype else '',
+            ' 0x%02x' % xsubtype if xsubtype else '',
             size)
     elif suptype & 0x4:
         return 'alt%s%s 0x%x %s' % (
@@ -250,7 +252,7 @@ def show_log(block_size, data, rev, off, *,
         j_ += delta
 
         if not tag & 0x4:
-            if (tag & 0x7806) != 0x0002:
+            if (tag & 0x007e) != 0x0002:
                 crc = crc32c(data[j_:j_+size], crc)
             # found a crc?
             else:
@@ -599,7 +601,7 @@ def main(disk, block_size, block1, block2=None, *,
                 wastrunk = True
 
             if not tag & 0x4:
-                if (tag & 0x7806) != 0x0002:
+                if (tag & 0x007e) != 0x0002:
                     crc = crc32c(data[j_:j_+size], crc)
                     # keep track of id count
                     if (tag & 0x7807) == 0x0800: