mimxrt: Spend a few words about littlfs file system and MSC access.

robert-hh · robert-hh · commit 48d46bbdcff7 · 2022-07-07T14:59:39.000+02:00
Since the board may have been formatted with littlfs, the FS device
different when attached to a PC.

And: fix typos.
diff --git a/docs/mimxrt/quickref.rst b/docs/mimxrt/quickref.rst
@@ -544,7 +544,7 @@ External drive mode
 On some boards a mode is enabled, that mounts the board's internal file system as
 external USB drive, called MSC support. Data of that drive can be accessed and changed by the PC.
 In that state, access to the internal drive by the MicroPython is limited to
-readonly mode, avoiding file corruption. Changes made by the PC to the file system may not be visible
+read-only mode, avoiding file corruption. Changes made by the PC to the file system may not be visible
 at the board until the drive is ejected by the PC or a soft reset of the board
 is made.
 
@@ -566,15 +566,40 @@ The external drive mode can be enabled of disabled by calling mimxrt.usb_mode().
 These command must be placed into boot.py. They get effective after the next hard reset or
 power cycle.
 
-The readonly state of the local file system access can be told by an IOCTL call of the file
-system's block device. ::
+The read-only state of the local file system access can be told by an IOCTL call of the file
+system's block device.::
 
     import mimxrt
 
     bdev = mimxrt.Flash()
     readonly = bdev.ioctl(7, 0)
 
-If the drive is in readonly state, bdev.ioctl(7, 0) returns `True`.
+If the drive is in read-only state, bdev.ioctl(7, 0) returns `True`.
+
+The file system of the board has to be of FAT type for mounting and using with standard PC
+tools. But FAT is not enforced at the board. If the board's file system is littlefs, MSC
+mode is disable by default. If enabled in boot.py, the file system will be attached to the
+PC and will be accessible as a drive (e.g. /dev/sdc usign Linux), but by default
+there is no file access. In that case, the files are locally still writable.
+Changing the board's file system to FAT can be done then by formatting it from the PC.
+Alternatively, you can erase the root sector. Then, the FAT file system will be
+created at the next power-up. For erasing the root sector, write::
+
+    from mimxrt import Flash
+    Flash().ioctl(6, 0)
+
+Using littlefs-fuse for Linux you can mount the board's littlefs file system to the PC.
+See: https://github.com/littlefs-project/littlefs-fuse
+The block_size if 4096, the block_count depends on the size of the filesystem. e.g.::
+
+    # mounting the lfs2 file system of a Teensy 4.1 board at the PC
+    mkdir mount
+    sudo ./lfs --block_size=4096 --block_count=1791 -o allow_other -o nonempty /dev/sdc mount
+    cd mount
+    ls -l
+
+In that case, the exclusive write access is NOT enforced. So be careful to only write
+to the file system by the PC.
 
 
 Transferring files
