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 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 59 struct dentry *debugfs_create_u8(const char *name, umode_t mode, 60 struct dentry *parent, u8 *value); 61 struct dentry *debugfs_create_u16(const char *name, umode_t mode, 62 struct dentry *parent, u16 *value); 63 struct dentry *debugfs_create_u32(const char *name, umode_t mode, 64 struct dentry *parent, u32 *value); 65 struct dentry *debugfs_create_u64(const char *name, umode_t mode, 66 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 73 struct dentry *debugfs_create_x8(const char *name, umode_t mode, 74 struct dentry *parent, u8 *value); 75 struct dentry *debugfs_create_x16(const char *name, umode_t mode, 76 struct dentry *parent, u16 *value); 77 struct dentry *debugfs_create_x32(const char *name, umode_t mode, 78 struct dentry *parent, u32 *value); 79 struct dentry *debugfs_create_x64(const char *name, umode_t mode, 80 struct dentry *parent, u64 *value); 81 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 87 struct dentry *debugfs_create_size_t(const char *name, umode_t mode, 88 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 96 struct dentry *debugfs_create_bool(const char *name, umode_t mode, 97 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 103Another option is exporting a block of arbitrary binary data, with 104this structure and function: 105 106 struct debugfs_blob_wrapper { 107 void *data; 108 unsigned long size; 109 }; 110 111 struct dentry *debugfs_create_blob(const char *name, umode_t mode, 112 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 122If 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 }; 138 139 struct dentry *debugfs_create_regset32(const char *name, umode_t mode, 140 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 151There 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/ 192