Documentation/filesystems/debugfs.rst

   1 .. SPDX-License-Identifier: GPL-2.0
   2 .. include:: <isonum.txt>
   3
   4 =======
   5 DebugFS
   6 =======
   7
   8 Copyright |copy| 2009 Jonathan Corbet <corbet@lwn.net>
   9
  10 Debugfs exists as a simple way for kernel developers to make information
  11 available to user space.  Unlike /proc, which is only meant for information
  12 about a process, or sysfs, which has strict one-value-per-file rules,
  13 debugfs has no rules at all.  Developers can put any information they want
  14 there.  The debugfs filesystem is also intended to not serve as a stable
  15 ABI to user space; in theory, there are no stability constraints placed on
  16 files exported there.  The real world is not always so simple, though [1]_;
  17 even debugfs interfaces are best designed with the idea that they will need
  18 to be maintained forever.
  19
  20 Debugfs is typically mounted with a command like::
  21
  22     mount -t debugfs none /sys/kernel/debug
  23
  24 (Or an equivalent /etc/fstab line).
  25 The debugfs root directory is accessible only to the root user by
  26 default. To change access to the tree the "uid", "gid" and "mode" mount
  27 options can be used.
  28
  29 Note that the debugfs API is exported GPL-only to modules.
  30
  31 Code using debugfs should include <linux/debugfs.h>.  Then, the first order
  32 of business will be to create at least one directory to hold a set of
  33 debugfs files::
  34
  35     struct dentry *debugfs_create_dir(const char *name, struct dentry *parent);
  36
  37 This call, if successful, will make a directory called name underneath the
  38 indicated parent directory.  If parent is NULL, the directory will be
  39 created in the debugfs root.  On success, the return value is a struct
  40 dentry pointer which can be used to create files in the directory (and to
  41 clean it up at the end).  An ERR_PTR(-ERROR) return value indicates that
  42 something went wrong.  If ERR_PTR(-ENODEV) is returned, that is an
  43 indication that the kernel has been built without debugfs support and none
  44 of the functions described below will work.
  45
  46 The most general way to create a file within a debugfs directory is with::
  47
  48     struct dentry *debugfs_create_file(const char *name, umode_t mode,
  49                                        struct dentry *parent, void *data,
  50                                        const struct file_operations *fops);
  51
  52 Here, name is the name of the file to create, mode describes the access
  53 permissions the file should have, parent indicates the directory which
  54 should hold the file, data will be stored in the i_private field of the
  55 resulting inode structure, and fops is a set of file operations which
  56 implement the file's behavior.  At a minimum, the read() and/or write()
  57 operations should be provided; others can be included as needed.  Again,
  58 the return value will be a dentry pointer to the created file,
  59 ERR_PTR(-ERROR) on error, or ERR_PTR(-ENODEV) if debugfs support is
  60 missing.
  61
  62 Create a file with an initial size, the following function can be used
  63 instead::
  64
  65     void debugfs_create_file_size(const char *name, umode_t mode,
  66                                   struct dentry *parent, void *data,
  67                                   const struct file_operations *fops,
  68                                   loff_t file_size);
  69
  70 file_size is the initial file size. The other parameters are the same
  71 as the function debugfs_create_file.
  72
  73 In a number of cases, the creation of a set of file operations is not
  74 actually necessary; the debugfs code provides a number of helper functions
  75 for simple situations.  Files containing a single integer value can be
  76 created with any of::
  77
  78     void debugfs_create_u8(const char *name, umode_t mode,
  79                            struct dentry *parent, u8 *value);
  80     void debugfs_create_u16(const char *name, umode_t mode,
  81                             struct dentry *parent, u16 *value);
  82     struct dentry *debugfs_create_u32(const char *name, umode_t mode,
  83                                       struct dentry *parent, u32 *value);
  84     void debugfs_create_u64(const char *name, umode_t mode,
  85                             struct dentry *parent, u64 *value);
  86
  87 These files support both reading and writing the given value; if a specific
  88 file should not be written to, simply set the mode bits accordingly.  The
  89 values in these files are in decimal; if hexadecimal is more appropriate,
  90 the following functions can be used instead::
  91
  92     void debugfs_create_x8(const char *name, umode_t mode,
  93                            struct dentry *parent, u8 *value);
  94     void debugfs_create_x16(const char *name, umode_t mode,
  95                             struct dentry *parent, u16 *value);
  96     void debugfs_create_x32(const char *name, umode_t mode,
  97                             struct dentry *parent, u32 *value);
  98     void debugfs_create_x64(const char *name, umode_t mode,
  99                             struct dentry *parent, u64 *value);
 100
 101 These functions are useful as long as the developer knows the size of the
 102 value to be exported.  Some types can have different widths on different
 103 architectures, though, complicating the situation somewhat.  There are
 104 functions meant to help out in such special cases::
 105
 106     void debugfs_create_size_t(const char *name, umode_t mode,
 107                                struct dentry *parent, size_t *value);
 108
 109 As might be expected, this function will create a debugfs file to represent
 110 a variable of type size_t.
 111
 112 Similarly, there are helpers for variables of type unsigned long, in decimal
 113 and hexadecimal::
 114
 115     struct dentry *debugfs_create_ulong(const char *name, umode_t mode,
 116                                         struct dentry *parent,
 117                                         unsigned long *value);
 118     void debugfs_create_xul(const char *name, umode_t mode,
 119                             struct dentry *parent, unsigned long *value);
 120
 121 Boolean values can be placed in debugfs with::
 122
 123     struct dentry *debugfs_create_bool(const char *name, umode_t mode,
 124                                        struct dentry *parent, bool *value);
 125
 126 A read on the resulting file will yield either Y (for non-zero values) or
 127 N, followed by a newline.  If written to, it will accept either upper- or
 128 lower-case values, or 1 or 0.  Any other input will be silently ignored.
 129
 130 Also, atomic_t values can be placed in debugfs with::
 131
 132     void debugfs_create_atomic_t(const char *name, umode_t mode,
 133                                  struct dentry *parent, atomic_t *value)
 134
 135 A read of this file will get atomic_t values, and a write of this file
 136 will set atomic_t values.
 137
 138 Another option is exporting a block of arbitrary binary data, with
 139 this structure and function::
 140
 141     struct debugfs_blob_wrapper {
 142         void *data;
 143         unsigned long size;
 144     };
 145
 146     struct dentry *debugfs_create_blob(const char *name, umode_t mode,
 147                                        struct dentry *parent,
 148                                        struct debugfs_blob_wrapper *blob);
 149
 150 A read of this file will return the data pointed to by the
 151 debugfs_blob_wrapper structure.  Some drivers use "blobs" as a simple way
 152 to return several lines of (static) formatted text output.  This function
 153 can be used to export binary information, but there does not appear to be
 154 any code which does so in the mainline.  Note that all files created with
 155 debugfs_create_blob() are read-only.
 156
 157 If you want to dump a block of registers (something that happens quite
 158 often during development, even if little such code reaches mainline.
 159 Debugfs offers two functions: one to make a registers-only file, and
 160 another to insert a register block in the middle of another sequential
 161 file::
 162
 163     struct debugfs_reg32 {
 164         char *name;
 165         unsigned long offset;
 166     };
 167
 168     struct debugfs_regset32 {
 169         struct debugfs_reg32 *regs;
 170         int nregs;
 171         void __iomem *base;
 172     };
 173
 174     debugfs_create_regset32(const char *name, umode_t mode,
 175                             struct dentry *parent,
 176                             struct debugfs_regset32 *regset);
 177
 178     void debugfs_print_regs32(struct seq_file *s, struct debugfs_reg32 *regs,
 179                          int nregs, void __iomem *base, char *prefix);
 180
 181 The "base" argument may be 0, but you may want to build the reg32 array
 182 using __stringify, and a number of register names (macros) are actually
 183 byte offsets over a base for the register block.
 184
 185 If you want to dump an u32 array in debugfs, you can create file with::
 186
 187     void debugfs_create_u32_array(const char *name, umode_t mode,
 188                         struct dentry *parent,
 189                         u32 *array, u32 elements);
 190
 191 The "array" argument provides data, and the "elements" argument is
 192 the number of elements in the array. Note: Once array is created its
 193 size can not be changed.
 194
 195 There is a helper function to create device related seq_file::
 196
 197    struct dentry *debugfs_create_devm_seqfile(struct device *dev,
 198                                 const char *name,
 199                                 struct dentry *parent,
 200                                 int (*read_fn)(struct seq_file *s,
 201                                         void *data));
 202
 203 The "dev" argument is the device related to this debugfs file, and
 204 the "read_fn" is a function pointer which to be called to print the
 205 seq_file content.
 206
 207 There are a couple of other directory-oriented helper functions::
 208
 209     struct dentry *debugfs_rename(struct dentry *old_dir,
 210                                   struct dentry *old_dentry,
 211                                   struct dentry *new_dir,
 212                                   const char *new_name);
 213
 214     struct dentry *debugfs_create_symlink(const char *name,
 215                                           struct dentry *parent,
 216                                           const char *target);
 217
 218 A call to debugfs_rename() will give a new name to an existing debugfs
 219 file, possibly in a different directory.  The new_name must not exist prior
 220 to the call; the return value is old_dentry with updated information.
 221 Symbolic links can be created with debugfs_create_symlink().
 222
 223 There is one important thing that all debugfs users must take into account:
 224 there is no automatic cleanup of any directories created in debugfs.  If a
 225 module is unloaded without explicitly removing debugfs entries, the result
 226 will be a lot of stale pointers and no end of highly antisocial behavior.
 227 So all debugfs users - at least those which can be built as modules - must
 228 be prepared to remove all files and directories they create there.  A file
 229 can be removed with::
 230
 231     void debugfs_remove(struct dentry *dentry);
 232
 233 The dentry value can be NULL or an error value, in which case nothing will
 234 be removed.
 235
 236 Once upon a time, debugfs users were required to remember the dentry
 237 pointer for every debugfs file they created so that all files could be
 238 cleaned up.  We live in more civilized times now, though, and debugfs users
 239 can call::
 240
 241     void debugfs_remove_recursive(struct dentry *dentry);
 242
 243 If this function is passed a pointer for the dentry corresponding to the
 244 top-level directory, the entire hierarchy below that directory will be
 245 removed.
 246
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