背景
littlefs是arm面向嵌入式设备推出的一款掉电安全的小型文件系统，具有抗掉电，动态磨损均衡，RAM/ROM需求少等特点，具体介绍可见 https://github.com/ARMmbed/littlefs
作为一款在嵌入式设备上使用的文件系统，出问题时，一般是需要将数据dump出来进行分析的。此时就需要PC端的调试工具了。
littlefs-fuse简介
这个项目提供了一个littlefs的FUSE封装，也就是你可以借助此项目，在PC上直接将littlefs镜像挂载起来，并正常进行一些文件系统的操作。
源码位于：https://github.com/ARMmbed/littlefs-fuse
结构
我们直接下载下来
$ git clone https://github.com/ARMmbed/littlefs-fuse.git
$ cd littlefs-fuse
先看下结构
$ tree -L 2
.
├── lfs_fuse_bd.c
├── lfs_fuse_bd.h
├── lfs_fuse.c
├── LICENSE.md
├── littlefs
│ ├── DESIGN.md
│ ├── emubd
│ ├── lfs.c
│ ├── lfs.h
│ ├── lfs_util.c
│ ├── lfs_util.h
│ ├── LICENSE.md
│ ├── Makefile
│ ├── README.md
│ ├── scripts
│ ├── SPEC.md
│ └── tests
├── Makefile
└── README.md
外层是封装，里面则直接包含了littlefs文件夹。当我们需要特定版本的littlefs，例如使用跟设备上同一版本的littlefs时，只需要替换掉里层的这个littlefs即可，非常方便。
编译
根据README，项目依赖 FUSE version 2.6及以上的版本，可以使用如下命令查看版本
fusermount -V
另外还需要安装下 libfuse-dev:
sudo apt-get install libfuse-dev
依赖项满足之后，直接make即可生成lfs应用程序
make
生成设备并挂载使用
首先需要造一个块设备出来
sudo chmod a+rw /dev/loop0 # make loop device user accessible
dd if=/dev/zero of=image bs=512 count=2048 # create a 1MB image
losetup /dev/loop0 image # attach the loop device
我们上一步是dd生成了一个空的image，所以需要先格式化，
./lfs --format /dev/loop0
再挂载
mkdir -p mount
./lfs /dev/loop0 mount
挂载成功后，执行mount可以看到挂载情况
$ mount | grep lfs
/home/zhuangqiubin/debug_littlefs/littlefs-fuse/lfs on /home/zhuangqiubin/debug_littlefs/littlefs-fuse/mount type fuse.lfs (rw,nosuid,nodev,relatime,user_id=1000,group_id=1000)
现在就可以在mount目录下，随意进行操作了，创建删除读出写入，都会被转换成littlefs的操作，最终作用到image上
过程中，可以随时dump出数据，就可以看到你的操作，最终存储到littlefs上时什么样的数据格式了，例如
$ echo "www.cnblogs.com/zqb-all/" >> mount/test_littlefs.txt
$ hexdump -C image
00000000 03 00 00 00 f0 0f ff f7 6c 69 74 74 6c 65 66 73 |........littlefs|
00000010 2f e0 00 10 00 00 02 00 00 02 00 00 00 08 00 00 |/...............|
00000020 ff 00 00 00 ff ff ff 7f fe 03 00 00 20 00 04 09 |............ ...|
00000030 74 65 73 74 5f 6c 69 74 74 6c 65 66 73 2e 74 78 |test_littlefs.tx|
00000040 74 20 00 00 11 70 0f f9 b7 ee c0 48 bb ff ff ff |t ...p.....H....|
00000050 ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|
*
00000200 04 00 00 00 f0 0f ff f7 6c 69 74 74 6c 65 66 73 |........littlefs|
00000210 2f e0 00 10 00 00 02 00 00 02 00 00 00 08 00 00 |/...............|
00000220 ff 00 00 00 ff ff ff 7f fe 03 00 00 20 00 04 09 |............ ...|
00000230 74 65 73 74 5f 6c 69 74 74 6c 65 66 73 2e 74 78 |test_littlefs.tx|
00000240 74 20 00 00 08 77 77 77 2e 63 6e 62 6c 6f 67 73 |t ...www.cnblogs|
00000250 2e 63 6f 6d 2f 7a 71 62 2d 61 6c 6c 2f 0a 70 0f |.com/zqb-all/.p.|
00000260 f9 87 46 fe c1 ad ff ff ff ff ff ff ff ff ff ff |..F.............|
00000270 ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|
*
00000400 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00100000
使用完毕之后，用以下命令卸载
umount mount
sudo losetup -d /dev/loop0
挂载一个指定镜像
以上是生成了空的镜像，格式化再挂载。如果要挂载一个现成的镜像，步骤也是差不多的，只是镜像换一下。假设现成的镜像叫littlefs.img，则
sudo chmod a+rw /dev/loop0 # make loop device user accessible
losetup /dev/loop0 littlefs.img # attach the loop device
mkdir -p mount
./lfs /dev/loop0 mount
卸载命令没有差别
umount mount
sudo losetup -d /dev/loop0
但从设备上dump出的littlefs镜像，其配置不一定会跟littlefs-fuse的默认配置匹配。
此时为了能正确挂载，以及准确地模拟littlefs在设备上运行的情况，我们需要将配置改成一样的。
解决方式一，直接修改源码，例如
diff --git a/lfs_fuse.c b/lfs_fuse.c
index 3c87dad..d1a99a2 100644
--- a/lfs_fuse.c
+++ b/lfs_fuse.c
@@ -26,7 +26,16 @@
// config and other state
-static struct lfs_config config = {0};
+/* static struct lfs_config config = {0}; */
+static struct lfs_config config = {
+ .read_size = 256,
+ .prog_size = 256,
+ .block_size = 4096,
+ .block_count = 1224,
+ .block_cycles = 512,
+ .cache_size = 256,
+ .lookahead_size = 32
+};
修改后重新make生成即可
解决方式二，在调用的时候，通过命令行参数指定，例如
./lfs --block_size=512 --format /dev/loop0
./lfs --block_size=512 /dev/loop0 mount
支持的选项，可以在help中找到
$lfs -h
usage: ./lfs [options] device mountpoint
general options:
-o opt,[opt...] FUSE options
-h --help print help
-V --version print version
littlefs options:
--format format instead of mounting
--migrate migrate previous version instead of mounting
-b --block_size logical block size, overrides the block device
--block_count block count, overrides the block device
--block_cycles number of erase cycles before eviction (512)
--read_size readable unit (block_size)
--prog_size programmable unit (block_size)
--cache_size size of caches (block_size)
--lookahead_size size of lookahead buffer (8192)
--name_max max size of file names (255)
--file_max max size of file contents (2147483647)
--attr_max max size of custom attributes (1022)
FUSE options:
-d -o debug enable debug output (implies -f)
-f foreground operation
-s disable multi-threaded operation
-o allow_other allow access to other users
-o allow_root allow access to root
-o auto_unmount auto unmount on process termination
-o nonempty allow mounts over non-empty file/dir
-o default_permissions enable permission checking by kernel
-o fsname=NAME set filesystem name
-o subtype=NAME set filesystem type
-o large_read issue large read requests (2.4 only)
-o max_read=N set maximum size of read requests
-o hard_remove immediate removal (don't hide files)
-o use_ino let filesystem set inode numbers
-o readdir_ino try to fill in d_ino in readdir
-o direct_io use direct I/O
-o kernel_cache cache files in kernel
-o [no]auto_cache enable caching based on modification times (off)
-o umask=M set file permissions (octal)
-o uid=N set file owner
-o gid=N set file group
-o entry_timeout=T cache timeout for names (1.0s)
-o negative_timeout=T cache timeout for deleted names (0.0s)
-o attr_timeout=T cache timeout for attributes (1.0s)
-o ac_attr_timeout=T auto cache timeout for attributes (attr_timeout)
-o noforget never forget cached inodes
-o remember=T remember cached inodes for T seconds (0s)
-o nopath don't supply path if not necessary
-o intr allow requests to be interrupted
-o intr_signal=NUM signal to send on interrupt (10)
-o modules=M1[:M2...] names of modules to push onto filesystem stack
-o max_write=N set maximum size of write requests
-o max_readahead=N set maximum readahead
-o max_background=N set number of maximum background requests
-o congestion_threshold=N set kernel's congestion threshold
-o async_read perform reads asynchronously (default)
-o sync_read perform reads synchronously
-o atomic_o_trunc enable atomic open+truncate support
-o big_writes enable larger than 4kB writes
-o no_remote_lock disable remote file locking
-o no_remote_flock disable remote file locking (BSD)
-o no_remote_posix_lock disable remove file locking (POSIX)
-o [no_]splice_write use splice to write to the fuse device
-o [no_]splice_move move data while splicing to the fuse device
-o [no_]splice_read use splice to read from the fuse device
Module options:
[iconv]
-o from_code=CHARSET original encoding of file names (default: UTF-8)
-o to_code=CHARSET new encoding of the file names (default: UTF-8)
[subdir]
-o subdir=DIR prepend this directory to all paths (mandatory)
-o [no]rellinks transform absolute symlinks to relative
调试
既然可以在PC上模拟挂载和读写，那首先最直观的，就是可以在littlefs的源码中按需添加调试代码，再重新挂载dump出来的镜像，通过打印来分析问题。
其次，必要的时候，我们还可以上gdb
make DEBUG=1 clean all # build with debug info
gdb --args ./lfs -d /dev/loop0 mount # run with gdb