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Jonathan Corbetf89d7ea2009-06-04 16:35:25 -06001Copyright 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
Ludwig Nusseld6e48682012-01-25 11:52:28 +010017(Or an equivalent /etc/fstab line).
18The debugfs root directory is accessible by anyone by default. To
19restrict access to the tree the "uid", "gid" and "mode" mount
20options can be used.
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -060021
22Note that the debugfs API is exported GPL-only to modules.
23
24Code using debugfs should include <linux/debugfs.h>. Then, the first order
25of business will be to create at least one directory to hold a set of
26debugfs files:
27
28 struct dentry *debugfs_create_dir(const char *name, struct dentry *parent);
29
30This call, if successful, will make a directory called name underneath the
31indicated parent directory. If parent is NULL, the directory will be
32created in the debugfs root. On success, the return value is a struct
33dentry pointer which can be used to create files in the directory (and to
34clean it up at the end). A NULL return value indicates that something went
35wrong. If ERR_PTR(-ENODEV) is returned, that is an indication that the
36kernel has been built without debugfs support and none of the functions
37described below will work.
38
39The most general way to create a file within a debugfs directory is with:
40
Al Virof4ae40a2011-07-24 04:33:43 -040041 struct dentry *debugfs_create_file(const char *name, umode_t mode,
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -060042 struct dentry *parent, void *data,
43 const struct file_operations *fops);
44
45Here, name is the name of the file to create, mode describes the access
46permissions the file should have, parent indicates the directory which
47should hold the file, data will be stored in the i_private field of the
48resulting inode structure, and fops is a set of file operations which
49implement the file's behavior. At a minimum, the read() and/or write()
50operations should be provided; others can be included as needed. Again,
51the return value will be a dentry pointer to the created file, NULL for
52error, or ERR_PTR(-ENODEV) if debugfs support is missing.
53
54In a number of cases, the creation of a set of file operations is not
55actually necessary; the debugfs code provides a number of helper functions
56for simple situations. Files containing a single integer value can be
57created with any of:
58
Al Virof4ae40a2011-07-24 04:33:43 -040059 struct dentry *debugfs_create_u8(const char *name, umode_t mode,
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -060060 struct dentry *parent, u8 *value);
Al Virof4ae40a2011-07-24 04:33:43 -040061 struct dentry *debugfs_create_u16(const char *name, umode_t mode,
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -060062 struct dentry *parent, u16 *value);
Al Virof4ae40a2011-07-24 04:33:43 -040063 struct dentry *debugfs_create_u32(const char *name, umode_t mode,
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -060064 struct dentry *parent, u32 *value);
Al Virof4ae40a2011-07-24 04:33:43 -040065 struct dentry *debugfs_create_u64(const char *name, umode_t mode,
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -060066 struct dentry *parent, u64 *value);
67
68These files support both reading and writing the given value; if a specific
69file should not be written to, simply set the mode bits accordingly. The
70values in these files are in decimal; if hexadecimal is more appropriate,
71the following functions can be used instead:
72
Al Virof4ae40a2011-07-24 04:33:43 -040073 struct dentry *debugfs_create_x8(const char *name, umode_t mode,
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -060074 struct dentry *parent, u8 *value);
Al Virof4ae40a2011-07-24 04:33:43 -040075 struct dentry *debugfs_create_x16(const char *name, umode_t mode,
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -060076 struct dentry *parent, u16 *value);
Al Virof4ae40a2011-07-24 04:33:43 -040077 struct dentry *debugfs_create_x32(const char *name, umode_t mode,
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -060078 struct dentry *parent, u32 *value);
Al Virof4ae40a2011-07-24 04:33:43 -040079 struct dentry *debugfs_create_x64(const char *name, umode_t mode,
Akinobu Mitad0a54262011-07-09 14:01:17 +090080 struct dentry *parent, u64 *value);
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -060081
82These functions are useful as long as the developer knows the size of the
83value to be exported. Some types can have different widths on different
84architectures, though, complicating the situation somewhat. There is a
85function meant to help out in one special case:
86
Al Virof4ae40a2011-07-24 04:33:43 -040087 struct dentry *debugfs_create_size_t(const char *name, umode_t mode,
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -060088 struct dentry *parent,
89 size_t *value);
90
91As might be expected, this function will create a debugfs file to represent
92a variable of type size_t.
93
94Boolean values can be placed in debugfs with:
95
Al Virof4ae40a2011-07-24 04:33:43 -040096 struct dentry *debugfs_create_bool(const char *name, umode_t mode,
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -060097 struct dentry *parent, u32 *value);
98
99A read on the resulting file will yield either Y (for non-zero values) or
100N, followed by a newline. If written to, it will accept either upper- or
101lower-case values, or 1 or 0. Any other input will be silently ignored.
102
Alessandro Rubini1a087c62011-11-18 14:50:21 +0100103Another option is exporting a block of arbitrary binary data, with
104this structure and function:
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -0600105
106 struct debugfs_blob_wrapper {
107 void *data;
108 unsigned long size;
109 };
110
Al Virof4ae40a2011-07-24 04:33:43 -0400111 struct dentry *debugfs_create_blob(const char *name, umode_t mode,
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -0600112 struct dentry *parent,
113 struct debugfs_blob_wrapper *blob);
114
115A read of this file will return the data pointed to by the
116debugfs_blob_wrapper structure. Some drivers use "blobs" as a simple way
117to return several lines of (static) formatted text output. This function
118can be used to export binary information, but there does not appear to be
119any code which does so in the mainline. Note that all files created with
120debugfs_create_blob() are read-only.
121
Alessandro Rubini1a087c62011-11-18 14:50:21 +0100122If you want to dump a block of registers (something that happens quite
123often during development, even if little such code reaches mainline.
124Debugfs offers two functions: one to make a registers-only file, and
125another to insert a register block in the middle of another sequential
126file.
127
128 struct debugfs_reg32 {
129 char *name;
130 unsigned long offset;
131 };
132
133 struct debugfs_regset32 {
134 struct debugfs_reg32 *regs;
135 int nregs;
136 void __iomem *base;
137 };
138
Al Viro88187392012-03-20 06:00:24 -0400139 struct dentry *debugfs_create_regset32(const char *name, umode_t mode,
Alessandro Rubini1a087c62011-11-18 14:50:21 +0100140 struct dentry *parent,
141 struct debugfs_regset32 *regset);
142
143 int debugfs_print_regs32(struct seq_file *s, struct debugfs_reg32 *regs,
144 int nregs, void __iomem *base, char *prefix);
145
146The "base" argument may be 0, but you may want to build the reg32 array
147using __stringify, and a number of register names (macros) are actually
148byte offsets over a base for the register block.
149
150
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -0600151There are a couple of other directory-oriented helper functions:
152
153 struct dentry *debugfs_rename(struct dentry *old_dir,
154 struct dentry *old_dentry,
155 struct dentry *new_dir,
156 const char *new_name);
157
158 struct dentry *debugfs_create_symlink(const char *name,
159 struct dentry *parent,
160 const char *target);
161
162A call to debugfs_rename() will give a new name to an existing debugfs
163file, possibly in a different directory. The new_name must not exist prior
164to the call; the return value is old_dentry with updated information.
165Symbolic links can be created with debugfs_create_symlink().
166
167There is one important thing that all debugfs users must take into account:
168there is no automatic cleanup of any directories created in debugfs. If a
169module is unloaded without explicitly removing debugfs entries, the result
170will be a lot of stale pointers and no end of highly antisocial behavior.
171So all debugfs users - at least those which can be built as modules - must
172be prepared to remove all files and directories they create there. A file
173can be removed with:
174
175 void debugfs_remove(struct dentry *dentry);
176
177The dentry value can be NULL, in which case nothing will be removed.
178
179Once upon a time, debugfs users were required to remember the dentry
180pointer for every debugfs file they created so that all files could be
181cleaned up. We live in more civilized times now, though, and debugfs users
182can call:
183
184 void debugfs_remove_recursive(struct dentry *dentry);
185
186If this function is passed a pointer for the dentry corresponding to the
187top-level directory, the entire hierarchy below that directory will be
188removed.
189
190Notes:
191 [1] http://lwn.net/Articles/309298/