This turned out to be tricky.
At littlefs's core, we have the lfsr_rbyd_t struct. It is really
important this is as small as possible since littlefs creates many rbyd
copies in order to track state of metadata on disk.
Wrapping rbyd, we have the lfsr_btree_t struct, which can alternatively
contain a single inlined entry, accomplished by overlapping the width
field in both cases. And the lfsr_mdir_t struct, which tracks any redundant
blocks, and would be nice if the blocks lined up as neighbors so all blocks
involved in the mdir could be passed around as an array. Both of these
wrappers attempt to overlap fields of the lfsr_rbyd_t struct, which presents
a bit of a problem.
The solution here is to put the rbyd block field at the beginning of the
lfsr_rbyd_t struct, and use exactly 32-bits of padding in lfsr_btree_t
to overlap the width field even though it is not at the beginning of the
struct. To avoid inflating the lfsr_btree_t size, we sneak the inlined
size and tag into the overlapping padding. This will need special
handling if the size of these fields change, but saves a decent amount
of RAM:
lfsr_rbyd_t lfsr_btree_t lfsr_mdir_t
8b 8b 8b 8b
.----+----+----+----.
| mid.bid | mid.rid |
|----+----+----+----|
8b 8b 8b 8b 8b 8b 8b 8b | blocks |
.----+----+----+----. .----+----+----+----. | |
| block |..| tag |size|padd|.>| |
|----+----+----+----| |----+----+----+----| |----+----+----+----|
| weight |.>| weight | | weight |
|----+----+----+----| |----+----+----+----| |----+----+----+----|
| trunk | | inlined data | | trunk |
|----+----+----+----| | | | |----+----+----+----|
| off | | v | | off |
|----+----+----+----| | | |----+----+----+----|
| crc | | | | crc |
'----+----+----+----' '----+----+----+----' '----+----+----+----'
Also tried to reduce the amount of mdir usage in lfsr_mdir_commit by
better using only the arrays of relevant mdir blocks, to limited success.
Christopher Haster
fe941ef443