It was a bit tricky to figure out what this should look like.
Traditionally, filesystems tend to fallback to readonly if they detect
unsupported wcompat (ro_compat) flags or similar config mismatch.
We could do something similar in littlefs, but since we default to
asserting on writes to readonly objects for smaller code size, this
would be really weird and hard to use from a users perspective...
Instead, lfsr_mount returns LFS_ERR_NOTSUP on encountering wcompat-
mismatch in RDWR mode, but _not_ RDONLY mode. This allows the common
rdonly-fallback pattern to be implemented on the user's side of things,
similar to the common format-fallback pattern:
int err = lfsr_mount(&lfs, LFS_M_RDWR, &cfg);
if (err && err != LFS_ERR_NOTSUP) {
return err;
}
if (err == LFS_ERR_NOTSUP) {
err = lfsr_mount(&lfs, LFS_M_RDONLY, &cfg);
if (err) {
return err;
}
}
Note that lfsr_mount may still return LFS_ERR_NOTSUP if it encounters
rcompat-flags, even with RDONLY. Detecting this state will likely need
two lfsr_mount calls with the current API, but I don't think that will
be a big deal.
The main benefit of this scheme is that it is quite cheap thanks to
pushing the fallback logic on the user:
code stack
before: 36356 2664
after: 36396 (+0.1%) 2664 (+0.0%)
One missing puzzle piece here is how do you upgrade the filesystem? But I
think the lesson from the on-disk v2.0 -> v2.1 version bump is that this
should really be an explicit function (lfsr_fs_upgrade?). If explicit
and stand-alone, like lfsr_format, we shouldn't need a weird pseudo-
rdonly mode at all.
Christopher Haster
0ab0406d53