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). 18The debugfs root directory is accessible only to the root user by 19default. To change access to the tree the "uid", "gid" and "mode" mount 20options can be used. 21 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 41 struct dentry *debugfs_create_file(const char *name, umode_t mode, 42 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 54Create a file with an initial size, the following function can be used 55instead: 56 57 struct dentry *debugfs_create_file_size(const char *name, umode_t mode, 58 struct dentry *parent, void *data, 59 const struct file_operations *fops, 60 loff_t file_size); 61 62file_size is the initial file size. The other parameters are the same 63as the function debugfs_create_file. 64 65In a number of cases, the creation of a set of file operations is not 66actually necessary; the debugfs code provides a number of helper functions 67for simple situations. Files containing a single integer value can be 68created with any of: 69 70 struct dentry *debugfs_create_u8(const char *name, umode_t mode, 71 struct dentry *parent, u8 *value); 72 struct dentry *debugfs_create_u16(const char *name, umode_t mode, 73 struct dentry *parent, u16 *value); 74 struct dentry *debugfs_create_u32(const char *name, umode_t mode, 75 struct dentry *parent, u32 *value); 76 struct dentry *debugfs_create_u64(const char *name, umode_t mode, 77 struct dentry *parent, u64 *value); 78 79These files support both reading and writing the given value; if a specific 80file should not be written to, simply set the mode bits accordingly. The 81values in these files are in decimal; if hexadecimal is more appropriate, 82the following functions can be used instead: 83 84 struct dentry *debugfs_create_x8(const char *name, umode_t mode, 85 struct dentry *parent, u8 *value); 86 struct dentry *debugfs_create_x16(const char *name, umode_t mode, 87 struct dentry *parent, u16 *value); 88 struct dentry *debugfs_create_x32(const char *name, umode_t mode, 89 struct dentry *parent, u32 *value); 90 struct dentry *debugfs_create_x64(const char *name, umode_t mode, 91 struct dentry *parent, u64 *value); 92 93These functions are useful as long as the developer knows the size of the 94value to be exported. Some types can have different widths on different 95architectures, though, complicating the situation somewhat. There is a 96function meant to help out in one special case: 97 98 struct dentry *debugfs_create_size_t(const char *name, umode_t mode, 99 struct dentry *parent, 100 size_t *value); 101 102As might be expected, this function will create a debugfs file to represent 103a variable of type size_t. 104 105Boolean values can be placed in debugfs with: 106 107 struct dentry *debugfs_create_bool(const char *name, umode_t mode, 108 struct dentry *parent, bool *value); 109 110A read on the resulting file will yield either Y (for non-zero values) or 111N, followed by a newline. If written to, it will accept either upper- or 112lower-case values, or 1 or 0. Any other input will be silently ignored. 113 114Also, atomic_t values can be placed in debugfs with: 115 116 struct dentry *debugfs_create_atomic_t(const char *name, umode_t mode, 117 struct dentry *parent, atomic_t *value) 118 119A read of this file will get atomic_t values, and a write of this file 120will set atomic_t values. 121 122Another option is exporting a block of arbitrary binary data, with 123this structure and function: 124 125 struct debugfs_blob_wrapper { 126 void *data; 127 unsigned long size; 128 }; 129 130 struct dentry *debugfs_create_blob(const char *name, umode_t mode, 131 struct dentry *parent, 132 struct debugfs_blob_wrapper *blob); 133 134A read of this file will return the data pointed to by the 135debugfs_blob_wrapper structure. Some drivers use "blobs" as a simple way 136to return several lines of (static) formatted text output. This function 137can be used to export binary information, but there does not appear to be 138any code which does so in the mainline. Note that all files created with 139debugfs_create_blob() are read-only. 140 141If you want to dump a block of registers (something that happens quite 142often during development, even if little such code reaches mainline. 143Debugfs offers two functions: one to make a registers-only file, and 144another to insert a register block in the middle of another sequential 145file. 146 147 struct debugfs_reg32 { 148 char *name; 149 unsigned long offset; 150 }; 151 152 struct debugfs_regset32 { 153 struct debugfs_reg32 *regs; 154 int nregs; 155 void __iomem *base; 156 }; 157 158 struct dentry *debugfs_create_regset32(const char *name, umode_t mode, 159 struct dentry *parent, 160 struct debugfs_regset32 *regset); 161 162 void debugfs_print_regs32(struct seq_file *s, struct debugfs_reg32 *regs, 163 int nregs, void __iomem *base, char *prefix); 164 165The "base" argument may be 0, but you may want to build the reg32 array 166using __stringify, and a number of register names (macros) are actually 167byte offsets over a base for the register block. 168 169If you want to dump an u32 array in debugfs, you can create file with: 170 171 struct dentry *debugfs_create_u32_array(const char *name, umode_t mode, 172 struct dentry *parent, 173 u32 *array, u32 elements); 174 175The "array" argument provides data, and the "elements" argument is 176the number of elements in the array. Note: Once array is created its 177size can not be changed. 178 179There is a helper function to create device related seq_file: 180 181 struct dentry *debugfs_create_devm_seqfile(struct device *dev, 182 const char *name, 183 struct dentry *parent, 184 int (*read_fn)(struct seq_file *s, 185 void *data)); 186 187The "dev" argument is the device related to this debugfs file, and 188the "read_fn" is a function pointer which to be called to print the 189seq_file content. 190 191There are a couple of other directory-oriented helper functions: 192 193 struct dentry *debugfs_rename(struct dentry *old_dir, 194 struct dentry *old_dentry, 195 struct dentry *new_dir, 196 const char *new_name); 197 198 struct dentry *debugfs_create_symlink(const char *name, 199 struct dentry *parent, 200 const char *target); 201 202A call to debugfs_rename() will give a new name to an existing debugfs 203file, possibly in a different directory. The new_name must not exist prior 204to the call; the return value is old_dentry with updated information. 205Symbolic links can be created with debugfs_create_symlink(). 206 207There is one important thing that all debugfs users must take into account: 208there is no automatic cleanup of any directories created in debugfs. If a 209module is unloaded without explicitly removing debugfs entries, the result 210will be a lot of stale pointers and no end of highly antisocial behavior. 211So all debugfs users - at least those which can be built as modules - must 212be prepared to remove all files and directories they create there. A file 213can be removed with: 214 215 void debugfs_remove(struct dentry *dentry); 216 217The dentry value can be NULL, in which case nothing will be removed. 218 219Once upon a time, debugfs users were required to remember the dentry 220pointer for every debugfs file they created so that all files could be 221cleaned up. We live in more civilized times now, though, and debugfs users 222can call: 223 224 void debugfs_remove_recursive(struct dentry *dentry); 225 226If this function is passed a pointer for the dentry corresponding to the 227top-level directory, the entire hierarchy below that directory will be 228removed. 229 230Notes: 231 [1] http://lwn.net/Articles/309298/ 232