extmod/vfs_lfs: Add mtime support to littlefs files.

This commit adds support for modification time of files on littlefs v2
filesystems, using file attributes.  For some background see issue #6114.

Features/properties of this implementation:
- Only supported on littlefs2 (not littlefs1).
- Uses littlefs2's general file attributes to store the timestamp.
- The timestamp is 64-bits and stores nanoseconds since 1970/1/1 (if the
  range to the year 2554 is not enough then additional bits can be added to
  this timestamp by adding another file attribute).
- mtime is enabled by default but can be disabled in the constructor, eg:
  uos.mount(uos.VfsLfs2(bdev, mtime=False), '/flash')
- It's fully backwards compatible, existing littlefs2 filesystems will work
  without reformatting and timestamps will be added transparently to
  existing files (once they are opened for writing).
- Files without timestamps will open correctly, and stat will just return 0
  for their timestamp.
- mtime can be disabled or enabled each mount time and timestamps will only
  be updated if mtime is enabled (otherwise they will be untouched).

Signed-off-by: Damien George <damien@micropython.org>

1 files changed, 12 insertions, 4 deletions

diff --git a/docs/library/uos.rst b/docs/library/uos.rst
index 051247b71..edc94556b 100644
--- a/docs/library/uos.rst
+++ b/docs/library/uos.rst
@@ -178,7 +178,7 @@ represented by VFS classes.
 
         Build a FAT filesystem on *block_dev*.
 
-.. class:: VfsLfs1(block_dev)
+.. class:: VfsLfs1(block_dev, readsize=32, progsize=32, lookahead=32)
 
     Create a filesystem object that uses the `littlefs v1 filesystem format`_.
     Storage of the littlefs filesystem is provided by *block_dev*, which must
@@ -187,23 +187,31 @@ represented by VFS classes.
 
     See :ref:`filesystem` for more information.
 
-    .. staticmethod:: mkfs(block_dev)
+    .. staticmethod:: mkfs(block_dev, readsize=32, progsize=32, lookahead=32)
 
         Build a Lfs1 filesystem on *block_dev*.
 
     .. note:: There are reports of littlefs v1 failing in certain situations,
               for details see `littlefs issue 347`_.
 
-.. class:: VfsLfs2(block_dev)
+.. class:: VfsLfs2(block_dev, readsize=32, progsize=32, lookahead=32, mtime=True)
 
     Create a filesystem object that uses the `littlefs v2 filesystem format`_.
     Storage of the littlefs filesystem is provided by *block_dev*, which must
     support the :ref:`extended interface <block-device-interface>`.
     Objects created by this constructor can be mounted using :func:`mount`.
 
+    The *mtime* argument enables modification timestamps for files, stored using
+    littlefs attributes.  This option can be disabled or enabled differently each
+    mount time and timestamps will only be added or updated if *mtime* is enabled,
+    otherwise the timestamps will remain untouched.  Littlefs v2 filesystems without
+    timestamps will work without reformatting and timestamps will be added
+    transparently to existing files once they are opened for writing.  When *mtime*
+    is enabled `uos.stat` on files without timestamps will return 0 for the timestamp.
+
     See :ref:`filesystem` for more information.
 
-    .. staticmethod:: mkfs(block_dev)
+    .. staticmethod:: mkfs(block_dev, readsize=32, progsize=32, lookahead=32)
 
         Build a Lfs2 filesystem on *block_dev*.