1Copyright 2009 Jonathan Corbet <corbet@lwn.net>
2
3Debugfs exists as a simple way for kernel developers to make information
4available to user space.  Unlike /proc, which is only meant for information
5about a process, or sysfs, which has strict one-value-per-file rules,
6debugfs has no rules at all.  Developers can put any information they want
7there.  The debugfs filesystem is also intended to not serve as a stable
8ABI to user space; in theory, there are no stability constraints placed on
9files exported there.  The real world is not always so simple, though [1];
10even debugfs interfaces are best designed with the idea that they will need
11to be maintained forever.
12
13Debugfs is typically mounted with a command like:
14
15    mount -t debugfs none /sys/kernel/debug
16
17(Or an equivalent /etc/fstab line).
18
19Note that the debugfs API is exported GPL-only to modules.
20
21Code using debugfs should include <linux/debugfs.h>.  Then, the first order
22of business will be to create at least one directory to hold a set of
23debugfs files:
24
25    struct dentry *debugfs_create_dir(const char *name, struct dentry *parent);
26
27This call, if successful, will make a directory called name underneath the
28indicated parent directory.  If parent is NULL, the directory will be
29created in the debugfs root.  On success, the return value is a struct
30dentry pointer which can be used to create files in the directory (and to
31clean it up at the end).  A NULL return value indicates that something went
32wrong.  If ERR_PTR(-ENODEV) is returned, that is an indication that the
33kernel has been built without debugfs support and none of the functions
34described below will work.
35
36The most general way to create a file within a debugfs directory is with:
37
38    struct dentry *debugfs_create_file(const char *name, umode_t mode,
39				       struct dentry *parent, void *data,
40				       const struct file_operations *fops);
41
42Here, name is the name of the file to create, mode describes the access
43permissions the file should have, parent indicates the directory which
44should hold the file, data will be stored in the i_private field of the
45resulting inode structure, and fops is a set of file operations which
46implement the file's behavior.  At a minimum, the read() and/or write()
47operations should be provided; others can be included as needed.  Again,
48the return value will be a dentry pointer to the created file, NULL for
49error, or ERR_PTR(-ENODEV) if debugfs support is missing.
50
51In a number of cases, the creation of a set of file operations is not
52actually necessary; the debugfs code provides a number of helper functions
53for simple situations.  Files containing a single integer value can be
54created with any of:
55
56    struct dentry *debugfs_create_u8(const char *name, umode_t mode,
57				     struct dentry *parent, u8 *value);
58    struct dentry *debugfs_create_u16(const char *name, umode_t mode,
59				      struct dentry *parent, u16 *value);
60    struct dentry *debugfs_create_u32(const char *name, umode_t mode,
61				      struct dentry *parent, u32 *value);
62    struct dentry *debugfs_create_u64(const char *name, umode_t mode,
63				      struct dentry *parent, u64 *value);
64
65These files support both reading and writing the given value; if a specific
66file should not be written to, simply set the mode bits accordingly.  The
67values in these files are in decimal; if hexadecimal is more appropriate,
68the following functions can be used instead:
69
70    struct dentry *debugfs_create_x8(const char *name, umode_t mode,
71				     struct dentry *parent, u8 *value);
72    struct dentry *debugfs_create_x16(const char *name, umode_t mode,
73				      struct dentry *parent, u16 *value);
74    struct dentry *debugfs_create_x32(const char *name, umode_t mode,
75				      struct dentry *parent, u32 *value);
76    struct dentry *debugfs_create_x64(const char *name, umode_t mode,
77				      struct dentry *parent, u64 *value);
78
79These functions are useful as long as the developer knows the size of the
80value to be exported.  Some types can have different widths on different
81architectures, though, complicating the situation somewhat.  There is a
82function meant to help out in one special case:
83
84    struct dentry *debugfs_create_size_t(const char *name, umode_t mode,
85				         struct dentry *parent,
86					 size_t *value);
87
88As might be expected, this function will create a debugfs file to represent
89a variable of type size_t.
90
91Boolean values can be placed in debugfs with:
92
93    struct dentry *debugfs_create_bool(const char *name, umode_t mode,
94				       struct dentry *parent, u32 *value);
95
96A read on the resulting file will yield either Y (for non-zero values) or
97N, followed by a newline.  If written to, it will accept either upper- or
98lower-case values, or 1 or 0.  Any other input will be silently ignored.
99
100Another option is exporting a block of arbitrary binary data, with
101this structure and function:
102
103    struct debugfs_blob_wrapper {
104	void *data;
105	unsigned long size;
106    };
107
108    struct dentry *debugfs_create_blob(const char *name, umode_t mode,
109				       struct dentry *parent,
110				       struct debugfs_blob_wrapper *blob);
111
112A read of this file will return the data pointed to by the
113debugfs_blob_wrapper structure.  Some drivers use "blobs" as a simple way
114to return several lines of (static) formatted text output.  This function
115can be used to export binary information, but there does not appear to be
116any code which does so in the mainline.  Note that all files created with
117debugfs_create_blob() are read-only.
118
119If you want to dump a block of registers (something that happens quite
120often during development, even if little such code reaches mainline.
121Debugfs offers two functions: one to make a registers-only file, and
122another to insert a register block in the middle of another sequential
123file.
124
125    struct debugfs_reg32 {
126	char *name;
127	unsigned long offset;
128    };
129
130    struct debugfs_regset32 {
131	struct debugfs_reg32 *regs;
132	int nregs;
133	void __iomem *base;
134    };
135
136    struct dentry *debugfs_create_regset32(const char *name, mode_t mode,
137				     struct dentry *parent,
138				     struct debugfs_regset32 *regset);
139
140    int debugfs_print_regs32(struct seq_file *s, struct debugfs_reg32 *regs,
141			 int nregs, void __iomem *base, char *prefix);
142
143The "base" argument may be 0, but you may want to build the reg32 array
144using __stringify, and a number of register names (macros) are actually
145byte offsets over a base for the register block.
146
147
148There are a couple of other directory-oriented helper functions:
149
150    struct dentry *debugfs_rename(struct dentry *old_dir,
151    				  struct dentry *old_dentry,
152		                  struct dentry *new_dir,
153				  const char *new_name);
154
155    struct dentry *debugfs_create_symlink(const char *name,
156                                          struct dentry *parent,
157				      	  const char *target);
158
159A call to debugfs_rename() will give a new name to an existing debugfs
160file, possibly in a different directory.  The new_name must not exist prior
161to the call; the return value is old_dentry with updated information.
162Symbolic links can be created with debugfs_create_symlink().
163
164There is one important thing that all debugfs users must take into account:
165there is no automatic cleanup of any directories created in debugfs.  If a
166module is unloaded without explicitly removing debugfs entries, the result
167will be a lot of stale pointers and no end of highly antisocial behavior.
168So all debugfs users - at least those which can be built as modules - must
169be prepared to remove all files and directories they create there.  A file
170can be removed with:
171
172    void debugfs_remove(struct dentry *dentry);
173
174The dentry value can be NULL, in which case nothing will be removed.
175
176Once upon a time, debugfs users were required to remember the dentry
177pointer for every debugfs file they created so that all files could be
178cleaned up.  We live in more civilized times now, though, and debugfs users
179can call:
180
181    void debugfs_remove_recursive(struct dentry *dentry);
182
183If this function is passed a pointer for the dentry corresponding to the
184top-level directory, the entire hierarchy below that directory will be
185removed.
186
187Notes:
188	[1] http://lwn.net/Articles/309298/
189