159 lines
7.0 KiB
Plaintext
159 lines
7.0 KiB
Plaintext
Copyright 2009 Jonathan Corbet <corbet@lwn.net>
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Debugfs exists as a simple way for kernel developers to make information
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available to user space. Unlike /proc, which is only meant for information
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about a process, or sysfs, which has strict one-value-per-file rules,
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debugfs has no rules at all. Developers can put any information they want
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there. The debugfs filesystem is also intended to not serve as a stable
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ABI to user space; in theory, there are no stability constraints placed on
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files exported there. The real world is not always so simple, though [1];
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even debugfs interfaces are best designed with the idea that they will need
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to be maintained forever.
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Debugfs is typically mounted with a command like:
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mount -t debugfs none /sys/kernel/debug
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(Or an equivalent /etc/fstab line).
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Note that the debugfs API is exported GPL-only to modules.
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Code using debugfs should include <linux/debugfs.h>. Then, the first order
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of business will be to create at least one directory to hold a set of
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debugfs files:
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struct dentry *debugfs_create_dir(const char *name, struct dentry *parent);
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This call, if successful, will make a directory called name underneath the
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indicated parent directory. If parent is NULL, the directory will be
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created in the debugfs root. On success, the return value is a struct
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dentry pointer which can be used to create files in the directory (and to
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clean it up at the end). A NULL return value indicates that something went
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wrong. If ERR_PTR(-ENODEV) is returned, that is an indication that the
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kernel has been built without debugfs support and none of the functions
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described below will work.
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The most general way to create a file within a debugfs directory is with:
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struct dentry *debugfs_create_file(const char *name, mode_t mode,
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struct dentry *parent, void *data,
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const struct file_operations *fops);
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Here, name is the name of the file to create, mode describes the access
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permissions the file should have, parent indicates the directory which
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should hold the file, data will be stored in the i_private field of the
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resulting inode structure, and fops is a set of file operations which
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implement the file's behavior. At a minimum, the read() and/or write()
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operations should be provided; others can be included as needed. Again,
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the return value will be a dentry pointer to the created file, NULL for
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error, or ERR_PTR(-ENODEV) if debugfs support is missing.
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In a number of cases, the creation of a set of file operations is not
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actually necessary; the debugfs code provides a number of helper functions
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for simple situations. Files containing a single integer value can be
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created with any of:
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struct dentry *debugfs_create_u8(const char *name, mode_t mode,
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struct dentry *parent, u8 *value);
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struct dentry *debugfs_create_u16(const char *name, mode_t mode,
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struct dentry *parent, u16 *value);
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struct dentry *debugfs_create_u32(const char *name, mode_t mode,
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struct dentry *parent, u32 *value);
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struct dentry *debugfs_create_u64(const char *name, mode_t mode,
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struct dentry *parent, u64 *value);
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These files support both reading and writing the given value; if a specific
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file should not be written to, simply set the mode bits accordingly. The
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values in these files are in decimal; if hexadecimal is more appropriate,
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the following functions can be used instead:
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struct dentry *debugfs_create_x8(const char *name, mode_t mode,
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struct dentry *parent, u8 *value);
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struct dentry *debugfs_create_x16(const char *name, mode_t mode,
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struct dentry *parent, u16 *value);
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struct dentry *debugfs_create_x32(const char *name, mode_t mode,
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struct dentry *parent, u32 *value);
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struct dentry *debugfs_create_x64(const char *name, mode_t mode,
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struct dentry *parent, u64 *value);
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These functions are useful as long as the developer knows the size of the
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value to be exported. Some types can have different widths on different
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architectures, though, complicating the situation somewhat. There is a
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function meant to help out in one special case:
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struct dentry *debugfs_create_size_t(const char *name, mode_t mode,
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struct dentry *parent,
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size_t *value);
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As might be expected, this function will create a debugfs file to represent
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a variable of type size_t.
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Boolean values can be placed in debugfs with:
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struct dentry *debugfs_create_bool(const char *name, mode_t mode,
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struct dentry *parent, u32 *value);
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A read on the resulting file will yield either Y (for non-zero values) or
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N, followed by a newline. If written to, it will accept either upper- or
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lower-case values, or 1 or 0. Any other input will be silently ignored.
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Finally, a block of arbitrary binary data can be exported with:
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struct debugfs_blob_wrapper {
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void *data;
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unsigned long size;
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};
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struct dentry *debugfs_create_blob(const char *name, mode_t mode,
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struct dentry *parent,
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struct debugfs_blob_wrapper *blob);
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A read of this file will return the data pointed to by the
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debugfs_blob_wrapper structure. Some drivers use "blobs" as a simple way
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to return several lines of (static) formatted text output. This function
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can be used to export binary information, but there does not appear to be
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any code which does so in the mainline. Note that all files created with
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debugfs_create_blob() are read-only.
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There are a couple of other directory-oriented helper functions:
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struct dentry *debugfs_rename(struct dentry *old_dir,
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struct dentry *old_dentry,
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struct dentry *new_dir,
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const char *new_name);
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struct dentry *debugfs_create_symlink(const char *name,
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struct dentry *parent,
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const char *target);
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A call to debugfs_rename() will give a new name to an existing debugfs
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file, possibly in a different directory. The new_name must not exist prior
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to the call; the return value is old_dentry with updated information.
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Symbolic links can be created with debugfs_create_symlink().
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There is one important thing that all debugfs users must take into account:
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there is no automatic cleanup of any directories created in debugfs. If a
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module is unloaded without explicitly removing debugfs entries, the result
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will be a lot of stale pointers and no end of highly antisocial behavior.
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So all debugfs users - at least those which can be built as modules - must
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be prepared to remove all files and directories they create there. A file
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can be removed with:
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void debugfs_remove(struct dentry *dentry);
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The dentry value can be NULL, in which case nothing will be removed.
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Once upon a time, debugfs users were required to remember the dentry
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pointer for every debugfs file they created so that all files could be
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cleaned up. We live in more civilized times now, though, and debugfs users
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can call:
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void debugfs_remove_recursive(struct dentry *dentry);
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If this function is passed a pointer for the dentry corresponding to the
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top-level directory, the entire hierarchy below that directory will be
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removed.
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Notes:
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[1] http://lwn.net/Articles/309298/
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