We don't need the staging shrub if we never stage shrubs!
The only hangup was reuse of the staging shrub to load bshrubs/btrees in
lfs3_file_fetch (we need to be able to fallback to the previous shrub if
we error in lfs3_file_resync), but this can be handled with a stack
allocated shrub.
If btree-leaf-caches make a return, we would need to stack allocate this
anyways due to the lopsided cost of the main/staging btrees/bshrubs
introduced to avoid wasting space on the useless
staging-shrub-leaf-cache.
This saves some code in LFS3_RDONLY, and apparently an instruction or
two in the default build (I guess stack loads/stores are cheaper?):
code stack ctx
rdonly before: 10680 840 524
rdonly after: 10640 (-0.4%) 816 (+0.0%) 524 (+0.0%)
default before: 37324 2280 636
default after: 37320 (-0.0%) 2280 (+0.0%) 636 (+0.0%)
It's not apparent in ctx because lfs3_info.name dominates (guh), but
this does save some RAM in lfs3_file_t:
rdonly ctx
lfs3_file_t before: 136
lfs3_file_t after: 112 (-17.6%)
It does add some stack cost to lfs3_file_fetch, but because this isn't
on the stack hot-path in either build, we don't really care:
default code stack ctx
lfs3_file_fetch before: 372 416 0
lfs3_file_fetch after: 368 (-1.1%) 440 (+5.8%) 0 (+0.0%)
This is the new readonly flag, to be consistent with LFS3_M_RDONLY and
friends.
Note this overlaps with LFS3_YES_RDONLY in a weird way, where
LFS3_YES_RDONLY is basically just an alias for LFS3_RDONLY. For most
flags, LFS3_THING enables the _option_ of using LFS3_M_THING, with
LFS3_YES_THING implying LFS3_M_THING in all mount calls. But
LFS3_RDONLY _disables_ the option of using LFS3_M_RDWR, so it's a bit
different...
Do we really need two flags for the same thing? Not sure. But most users
probably expect LFS3_RDONLY coming from other filesystems.
Worst case this can be revisited in the planned config API rework.
---
As for the readonly code size, this is just the first draft and limited
to mostly ifdefing out all prog/write logic paths. There's some TODOs in
the code that may save a bit more (rbyd.eoff, file.b.shrub_ for
example). But the results are looking ok:
code stack ctx
v2.11.0 rdonly: 6270 448 580
v3-alpha rdonly: 10776 (+71.9%) 840 (+87.5%) 524 (-9.7%)
It's interesting to note most of the additional code/stack cost come
from filesystem traversal. In v2, the threaded linked-list made rdonly
traversal _incredibly_ cheap. But the extra rdwr baggage of turning
littlefs into a fully connected graph made it something to be avoided
in v3.
This hits v3 with the double whammy of:
1. Filesystem traversal is more complicated since we need to keep track
of which btree and where in the btree we are
2. Everything needs to be tracked explicitly due to the new inverted
state-machine driven API (no callbacks)
Note that even if we disabled the traversal APIs, lfs3_fs_usage, cksum
checking, etc, we'd still need to traverse to rebuild gstate. Otherwise
we risk showing grmed files after a powerloss.
---
This did affect the default build a little bit, due to moving things
around for nicer ifdef groupings:
code stack ctx
default before: 37300 2280 636
default after: 37304 (+0.0%) 2280 (+0.0%) 636 (+0.0%)
Christopher Haster