Merge branch 'sundance'

[powerpc.git] / Documentation / filesystems / relayfs.txt

diff --git a/Documentation/filesystems/relayfs.txt b/Documentation/filesystems/relayfs.txt
index d24e1b0..5832377 100644 (file)
--- a/Documentation/filesystems/relayfs.txt
+++ b/Documentation/filesystems/relayfs.txt
@@ -15,7 +15,7 @@ retrieve the data as it becomes available.
 
 The format of the data logged into the channel buffers is completely
 up to the relayfs client; relayfs does however provide hooks which
 
 The format of the data logged into the channel buffers is completely
 up to the relayfs client; relayfs does however provide hooks which

-allow clients to impose some stucture on the buffer data.  Nor does
+allow clients to impose some structure on the buffer data.  Nor does

 relayfs implement any form of data filtering - this also is left to
 the client.  The purpose is to keep relayfs as simple as possible.
 
 relayfs implement any form of data filtering - this also is left to
 the client.  The purpose is to keep relayfs as simple as possible.
 


@@ -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
 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
 ==========================
 
 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)
     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:
 
 
   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)
     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:
 
 
   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.
 
 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
 ----
 
 Misc
 ----