1 files changed, 103 insertions, 23 deletions

diff --git a/Documentation/filesystems/relayfs.txt b/Documentation/filesystems/relayfs.txt
index d803abed29f0..5832377b7340 100644
--- a/Documentation/filesystems/relayfs.txt
+++ b/Documentation/filesystems/relayfs.txt
@@ -44,30 +44,41 @@ relayfs can operate in a mode where it will overwrite data not yet
 collected by userspace, and not wait for it to consume it.
 
 relayfs itself does not provide for communication of such data between
-userspace and kernel, allowing the kernel side to remain simple and not
-impose a single interface on userspace. It does provide a separate
-helper though, described below.
+userspace and kernel, allowing the kernel side to remain simple and
+not impose a single interface on userspace. It does provide a set of
+examples and a separate helper though, described below.
+
+klog and relay-apps example code
+================================
+
+relayfs itself is ready to use, but to make things easier, a couple
+simple utility functions and a set of examples are provided.
+
+The relay-apps example tarball, available on the relayfs sourceforge
+site, contains a set of self-contained examples, each consisting of a
+pair of .c files containing boilerplate code for each of the user and
+kernel sides of a relayfs application; combined these two sets of
+boilerplate code provide glue to easily stream data to disk, without
+having to bother with mundane housekeeping chores.
+
+The 'klog debugging functions' patch (klog.patch in the relay-apps
+tarball) provides a couple of high-level logging functions to the
+kernel which allow writing formatted text or raw data to a channel,
+regardless of whether a channel to write into exists or not, or
+whether relayfs is compiled into the kernel or is configured as a
+module.  These functions allow you to put unconditional 'trace'
+statements anywhere in the kernel or kernel modules; only when there
+is a 'klog handler' registered will data actually be logged (see the
+klog and kleak examples for details).
+
+It is of course possible to use relayfs from scratch i.e. without
+using any of the relay-apps example code or klog, but you'll have to
+implement communication between userspace and kernel, allowing both to
+convey the state of buffers (full, empty, amount of padding).
+
+klog and the relay-apps examples can be found in the relay-apps
+tarball on http://relayfs.sourceforge.net
 
-klog, relay-app & librelay
-==========================
-
-relayfs itself is ready to use, but to make things easier, two
-additional systems are provided.  klog is a simple wrapper to make
-writing formatted text or raw data to a channel simpler, regardless of
-whether a channel to write into exists or not, or whether relayfs is
-compiled into the kernel or is configured as a module.  relay-app is
-the kernel counterpart of userspace librelay.c, combined these two
-files provide glue to easily stream data to disk, without having to
-bother with housekeeping.  klog and relay-app can be used together,
-with klog providing high-level logging functions to the kernel and
-relay-app taking care of kernel-user control and disk-logging chores.
-
-It is possible to use relayfs without relay-app & librelay, but you'll
-have to implement communication between userspace and kernel, allowing
-both to convey the state of buffers (full, empty, amount of padding).
-
-klog, relay-app and librelay can be found in the relay-apps tarball on
-http://relayfs.sourceforge.net
 
 The relayfs user space API
 ==========================
@@ -125,6 +136,8 @@ Here's a summary of the API relayfs provides to in-kernel clients:
     relay_reset(chan)
     relayfs_create_dir(name, parent)
     relayfs_remove_dir(dentry)
+    relayfs_create_file(name, parent, mode, fops, data)
+    relayfs_remove_file(dentry)
 
   channel management typically called on instigation of userspace:
 
@@ -141,6 +154,8 @@ Here's a summary of the API relayfs provides to in-kernel clients:
     subbuf_start(buf, subbuf, prev_subbuf, prev_padding)
     buf_mapped(buf, filp)
     buf_unmapped(buf, filp)
+    create_buf_file(filename, parent, mode, buf, is_global)
+    remove_buf_file(dentry)
 
   helper functions:
 
@@ -320,6 +335,71 @@ forces a sub-buffer switch on all the channel buffers, and can be used
 to finalize and process the last sub-buffers before the channel is
 closed.
 
+Creating non-relay files
+------------------------
+
+relay_open() automatically creates files in the relayfs filesystem to
+represent the per-cpu kernel buffers; it's often useful for
+applications to be able to create their own files alongside the relay
+files in the relayfs filesystem as well e.g. 'control' files much like
+those created in /proc or debugfs for similar purposes, used to
+communicate control information between the kernel and user sides of a
+relayfs application.  For this purpose the relayfs_create_file() and
+relayfs_remove_file() API functions exist.  For relayfs_create_file(),
+the caller passes in a set of user-defined file operations to be used
+for the file and an optional void * to a user-specified data item,
+which will be accessible via inode->u.generic_ip (see the relay-apps
+tarball for examples).  The file_operations are a required parameter
+to relayfs_create_file() and thus the semantics of these files are
+completely defined by the caller.
+
+See the relay-apps tarball at http://relayfs.sourceforge.net for
+examples of how these non-relay files are meant to be used.
+
+Creating relay files in other filesystems
+-----------------------------------------
+
+By default of course, relay_open() creates relay files in the relayfs
+filesystem.  Because relay_file_operations is exported, however, it's
+also possible to create and use relay files in other pseudo-filesytems
+such as debugfs.
+
+For this purpose, two callback functions are provided,
+create_buf_file() and remove_buf_file().  create_buf_file() is called
+once for each per-cpu buffer from relay_open() to allow the client to
+create a file to be used to represent the corresponding buffer; if
+this callback is not defined, the default implementation will create
+and return a file in the relayfs filesystem to represent the buffer.
+The callback should return the dentry of the file created to represent
+the relay buffer.  Note that the parent directory passed to
+relay_open() (and passed along to the callback), if specified, must
+exist in the same filesystem the new relay file is created in.  If
+create_buf_file() is defined, remove_buf_file() must also be defined;
+it's responsible for deleting the file(s) created in create_buf_file()
+and is called during relay_close().
+
+The create_buf_file() implementation can also be defined in such a way
+as to allow the creation of a single 'global' buffer instead of the
+default per-cpu set.  This can be useful for applications interested
+mainly in seeing the relative ordering of system-wide events without
+the need to bother with saving explicit timestamps for the purpose of
+merging/sorting per-cpu files in a postprocessing step.
+
+To have relay_open() create a global buffer, the create_buf_file()
+implementation should set the value of the is_global outparam to a
+non-zero value in addition to creating the file that will be used to
+represent the single buffer.  In the case of a global buffer,
+create_buf_file() and remove_buf_file() will be called only once.  The
+normal channel-writing functions e.g. relay_write() can still be used
+- writes from any cpu will transparently end up in the global buffer -
+but since it is a global buffer, callers should make sure they use the
+proper locking for such a buffer, either by wrapping writes in a
+spinlock, or by copying a write function from relayfs_fs.h and
+creating a local version that internally does the proper locking.
+
+See the 'exported-relayfile' examples in the relay-apps tarball for
+examples of creating and using relay files in debugfs.
+
 Misc
 ----