docs: filesystems: convert debugfs.txt to ReST

- Add a SPDX header;
- Use copyright symbol;
- Add a document title;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Use footnoote markups;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/42db8f9db17a5d8b619130815ae63d1615951d50.1581955849.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 247 insertions, 0 deletions

diff --git a/Documentation/filesystems/debugfs.rst b/Documentation/filesystems/debugfs.rst
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+.. SPDX-License-Identifier: GPL-2.0
+.. include:: <isonum.txt>
+
+=======
+DebugFS
+=======
+
+Copyright |copy| 2009 Jonathan Corbet <corbet@lwn.net>
+
+Debugfs exists as a simple way for kernel developers to make information
+available to user space.  Unlike /proc, which is only meant for information
+about a process, or sysfs, which has strict one-value-per-file rules,
+debugfs has no rules at all.  Developers can put any information they want
+there.  The debugfs filesystem is also intended to not serve as a stable
+ABI to user space; in theory, there are no stability constraints placed on
+files exported there.  The real world is not always so simple, though [1]_;
+even debugfs interfaces are best designed with the idea that they will need
+to be maintained forever.
+
+Debugfs is typically mounted with a command like::
+
+    mount -t debugfs none /sys/kernel/debug
+
+(Or an equivalent /etc/fstab line).
+The debugfs root directory is accessible only to the root user by
+default. To change access to the tree the "uid", "gid" and "mode" mount
+options can be used.
+
+Note that the debugfs API is exported GPL-only to modules.
+
+Code using debugfs should include <linux/debugfs.h>.  Then, the first order
+of business will be to create at least one directory to hold a set of
+debugfs files::
+
+    struct dentry *debugfs_create_dir(const char *name, struct dentry *parent);
+
+This call, if successful, will make a directory called name underneath the
+indicated parent directory.  If parent is NULL, the directory will be
+created in the debugfs root.  On success, the return value is a struct
+dentry pointer which can be used to create files in the directory (and to
+clean it up at the end).  An ERR_PTR(-ERROR) return value indicates that
+something went wrong.  If ERR_PTR(-ENODEV) is returned, that is an
+indication that the kernel has been built without debugfs support and none
+of the functions described below will work.
+
+The most general way to create a file within a debugfs directory is with::
+
+    struct dentry *debugfs_create_file(const char *name, umode_t mode,
+				       struct dentry *parent, void *data,
+				       const struct file_operations *fops);
+
+Here, name is the name of the file to create, mode describes the access
+permissions the file should have, parent indicates the directory which
+should hold the file, data will be stored in the i_private field of the
+resulting inode structure, and fops is a set of file operations which
+implement the file's behavior.  At a minimum, the read() and/or write()
+operations should be provided; others can be included as needed.  Again,
+the return value will be a dentry pointer to the created file,
+ERR_PTR(-ERROR) on error, or ERR_PTR(-ENODEV) if debugfs support is
+missing.
+
+Create a file with an initial size, the following function can be used
+instead::
+
+    struct dentry *debugfs_create_file_size(const char *name, umode_t mode,
+				struct dentry *parent, void *data,
+				const struct file_operations *fops,
+				loff_t file_size);
+
+file_size is the initial file size. The other parameters are the same
+as the function debugfs_create_file.
+
+In a number of cases, the creation of a set of file operations is not
+actually necessary; the debugfs code provides a number of helper functions
+for simple situations.  Files containing a single integer value can be
+created with any of::
+
+    void debugfs_create_u8(const char *name, umode_t mode,
+			   struct dentry *parent, u8 *value);
+    void debugfs_create_u16(const char *name, umode_t mode,
+			    struct dentry *parent, u16 *value);
+    struct dentry *debugfs_create_u32(const char *name, umode_t mode,
+				      struct dentry *parent, u32 *value);
+    void debugfs_create_u64(const char *name, umode_t mode,
+			    struct dentry *parent, u64 *value);
+
+These files support both reading and writing the given value; if a specific
+file should not be written to, simply set the mode bits accordingly.  The
+values in these files are in decimal; if hexadecimal is more appropriate,
+the following functions can be used instead::
+
+    void debugfs_create_x8(const char *name, umode_t mode,
+			   struct dentry *parent, u8 *value);
+    void debugfs_create_x16(const char *name, umode_t mode,
+			    struct dentry *parent, u16 *value);
+    void debugfs_create_x32(const char *name, umode_t mode,
+			    struct dentry *parent, u32 *value);
+    void debugfs_create_x64(const char *name, umode_t mode,
+			    struct dentry *parent, u64 *value);
+
+These functions are useful as long as the developer knows the size of the
+value to be exported.  Some types can have different widths on different
+architectures, though, complicating the situation somewhat.  There are
+functions meant to help out in such special cases::
+
+    void debugfs_create_size_t(const char *name, umode_t mode,
+			       struct dentry *parent, size_t *value);
+
+As might be expected, this function will create a debugfs file to represent
+a variable of type size_t.
+
+Similarly, there are helpers for variables of type unsigned long, in decimal
+and hexadecimal::
+
+    struct dentry *debugfs_create_ulong(const char *name, umode_t mode,
+					struct dentry *parent,
+					unsigned long *value);
+    void debugfs_create_xul(const char *name, umode_t mode,
+			    struct dentry *parent, unsigned long *value);
+
+Boolean values can be placed in debugfs with::
+
+    struct dentry *debugfs_create_bool(const char *name, umode_t mode,
+				       struct dentry *parent, bool *value);
+
+A read on the resulting file will yield either Y (for non-zero values) or
+N, followed by a newline.  If written to, it will accept either upper- or
+lower-case values, or 1 or 0.  Any other input will be silently ignored.
+
+Also, atomic_t values can be placed in debugfs with::
+
+    void debugfs_create_atomic_t(const char *name, umode_t mode,
+				 struct dentry *parent, atomic_t *value)
+
+A read of this file will get atomic_t values, and a write of this file
+will set atomic_t values.
+
+Another option is exporting a block of arbitrary binary data, with
+this structure and function::
+
+    struct debugfs_blob_wrapper {
+	void *data;
+	unsigned long size;
+    };
+
+    struct dentry *debugfs_create_blob(const char *name, umode_t mode,
+				       struct dentry *parent,
+				       struct debugfs_blob_wrapper *blob);
+
+A read of this file will return the data pointed to by the
+debugfs_blob_wrapper structure.  Some drivers use "blobs" as a simple way
+to return several lines of (static) formatted text output.  This function
+can be used to export binary information, but there does not appear to be
+any code which does so in the mainline.  Note that all files created with
+debugfs_create_blob() are read-only.
+
+If you want to dump a block of registers (something that happens quite
+often during development, even if little such code reaches mainline.
+Debugfs offers two functions: one to make a registers-only file, and
+another to insert a register block in the middle of another sequential
+file::
+
+    struct debugfs_reg32 {
+	char *name;
+	unsigned long offset;
+    };
+
+    struct debugfs_regset32 {
+	struct debugfs_reg32 *regs;
+	int nregs;
+	void __iomem *base;
+    };
+
+    struct dentry *debugfs_create_regset32(const char *name, umode_t mode,
+				     struct dentry *parent,
+				     struct debugfs_regset32 *regset);
+
+    void debugfs_print_regs32(struct seq_file *s, struct debugfs_reg32 *regs,
+			 int nregs, void __iomem *base, char *prefix);
+
+The "base" argument may be 0, but you may want to build the reg32 array
+using __stringify, and a number of register names (macros) are actually
+byte offsets over a base for the register block.
+
+If you want to dump an u32 array in debugfs, you can create file with::
+
+    void debugfs_create_u32_array(const char *name, umode_t mode,
+			struct dentry *parent,
+			u32 *array, u32 elements);
+
+The "array" argument provides data, and the "elements" argument is
+the number of elements in the array. Note: Once array is created its
+size can not be changed.
+
+There is a helper function to create device related seq_file::
+
+   struct dentry *debugfs_create_devm_seqfile(struct device *dev,
+				const char *name,
+				struct dentry *parent,
+				int (*read_fn)(struct seq_file *s,
+					void *data));
+
+The "dev" argument is the device related to this debugfs file, and
+the "read_fn" is a function pointer which to be called to print the
+seq_file content.
+
+There are a couple of other directory-oriented helper functions::
+
+    struct dentry *debugfs_rename(struct dentry *old_dir,
+    				  struct dentry *old_dentry,
+		                  struct dentry *new_dir,
+				  const char *new_name);
+
+    struct dentry *debugfs_create_symlink(const char *name,
+                                          struct dentry *parent,
+				      	  const char *target);
+
+A call to debugfs_rename() will give a new name to an existing debugfs
+file, possibly in a different directory.  The new_name must not exist prior
+to the call; the return value is old_dentry with updated information.
+Symbolic links can be created with debugfs_create_symlink().
+
+There is one important thing that all debugfs users must take into account:
+there is no automatic cleanup of any directories created in debugfs.  If a
+module is unloaded without explicitly removing debugfs entries, the result
+will be a lot of stale pointers and no end of highly antisocial behavior.
+So all debugfs users - at least those which can be built as modules - must
+be prepared to remove all files and directories they create there.  A file
+can be removed with::
+
+    void debugfs_remove(struct dentry *dentry);
+
+The dentry value can be NULL or an error value, in which case nothing will
+be removed.
+
+Once upon a time, debugfs users were required to remember the dentry
+pointer for every debugfs file they created so that all files could be
+cleaned up.  We live in more civilized times now, though, and debugfs users
+can call::
+
+    void debugfs_remove_recursive(struct dentry *dentry);
+
+If this function is passed a pointer for the dentry corresponding to the
+top-level directory, the entire hierarchy below that directory will be
+removed.
+
+.. [1] http://lwn.net/Articles/309298/