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-relayfs - a high-speed data relay filesystem
-============================================
-relayfs is a filesystem designed to provide an efficient mechanism for
-tools and facilities to relay large and potentially sustained streams
-of data from kernel space to user space.
-The main abstraction of relayfs is the 'channel'.  A channel consists
-of a set of per-cpu kernel buffers each represented by a file in the
-relayfs filesystem.  Kernel clients write into a channel using
-efficient write functions which automatically log to the current cpu's
-channel buffer.  User space applications mmap() the per-cpu files and
-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
-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.
-This document provides an overview of the relayfs API.  The details of
-the function parameters are documented along with the functions in the
-filesystem code - please see that for details.
-Semantics
-=========
-Each relayfs channel has one buffer per CPU, each buffer has one or
-more sub-buffers. Messages are written to the first sub-buffer until
-it is too full to contain a new message, in which case it it is
-written to the next (if available).  Messages are never split across
-sub-buffers.  At this point, userspace can be notified so it empties
-the first sub-buffer, while the kernel continues writing to the next.
-When notified that a sub-buffer is full, the kernel knows how many
-bytes of it are padding i.e. unused.  Userspace can use this knowledge
-to copy only valid data.
-After copying it, userspace can notify the kernel that a sub-buffer
-has been consumed.
-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 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
-The relayfs user space API
-==========================
-relayfs implements basic file operations for user space access to
-relayfs channel buffer data.  Here are the file operations that are
-available and some comments regarding their behavior:
-open()   enables user to open an _existing_ buffer.
-mmap()   results in channel buffer being mapped into the caller's
-         memory space. Note that you can't do a partial mmap - you must
-         map the entire file, which is NRBUF * SUBBUFSIZE.
-read()   read the contents of a channel buffer.  The bytes read are
-         'consumed' by the reader i.e. they won't be available again
-         to subsequent reads.  If the channel is being used in
-         no-overwrite mode (the default), it can be read at any time
-         even if there's an active kernel writer.  If the channel is
-         being used in overwrite mode and there are active channel
-         writers, results may be unpredictable - users should make
-         sure that all logging to the channel has ended before using
-         read() with overwrite mode.
-poll()   POLLIN/POLLRDNORM/POLLERR supported.  User applications are
-         notified when sub-buffer boundaries are crossed.
-close() decrements the channel buffer's refcount.  When the refcount
-        reaches 0 i.e. when no process or kernel client has the buffer
-        open, the channel buffer is freed.
-In order for a user application to make use of relayfs files, the
-relayfs filesystem must be mounted.  For example,
-        mount -t relayfs relayfs /mnt/relay
-NOTE:   relayfs doesn't need to be mounted for kernel clients to create
-        or use channels - it only needs to be mounted when user space
-        applications need access to the buffer data.
-The relayfs kernel API
-======================
-Here's a summary of the API relayfs provides to in-kernel clients:
-  channel management functions:
-    relay_open(base_filename, parent, subbuf_size, n_subbufs,
-               callbacks)
-    relay_close(chan)
-    relay_flush(chan)
-    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:
-    relay_subbufs_consumed(chan, cpu, subbufs_consumed)
-  write functions:
-    relay_write(chan, data, length)
-    __relay_write(chan, data, length)
-    relay_reserve(chan, length)
-  callbacks:
-    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:
-    relay_buf_full(buf)
-    subbuf_start_reserve(buf, length)
-Creating a channel
------------------
-relay_open() is used to create a channel, along with its per-cpu
-channel buffers.  Each channel buffer will have an associated file
-created for it in the relayfs filesystem, which can be opened and
-mmapped from user space if desired.  The files are named
-basename0...basenameN-1 where N is the number of online cpus, and by
-default will be created in the root of the filesystem.  If you want a
-directory structure to contain your relayfs files, you can create it
-with relayfs_create_dir() and pass the parent directory to
-relay_open().  Clients are responsible for cleaning up any directory
-structure they create when the channel is closed - use
-relayfs_remove_dir() for that.
-The total size of each per-cpu buffer is calculated by multiplying the
-number of sub-buffers by the sub-buffer size passed into relay_open().
-The idea behind sub-buffers is that they're basically an extension of
-double-buffering to N buffers, and they also allow applications to
-easily implement random-access-on-buffer-boundary schemes, which can
-be important for some high-volume applications.  The number and size
-of sub-buffers is completely dependent on the application and even for
-the same application, different conditions will warrant different
-values for these parameters at different times.  Typically, the right
-values to use are best decided after some experimentation; in general,
-though, it's safe to assume that having only 1 sub-buffer is a bad
-idea - you're guaranteed to either overwrite data or lose events
-depending on the channel mode being used.
-Channel 'modes'
---------------
-relayfs channels can be used in either of two modes - 'overwrite' or
-'no-overwrite'.  The mode is entirely determined by the implementation
-of the subbuf_start() callback, as described below.  In 'overwrite'
-mode, also known as 'flight recorder' mode, writes continuously cycle
-around the buffer and will never fail, but will unconditionally
-overwrite old data regardless of whether it's actually been consumed.
-In no-overwrite mode, writes will fail i.e. data will be lost, if the
-number of unconsumed sub-buffers equals the total number of
-sub-buffers in the channel.  It should be clear that if there is no
-consumer or if the consumer can't consume sub-buffers fast enought,
-data will be lost in either case; the only difference is whether data
-is lost from the beginning or the end of a buffer.
-As explained above, a relayfs channel is made of up one or more
-per-cpu channel buffers, each implemented as a circular buffer
-subdivided into one or more sub-buffers.  Messages are written into
-the current sub-buffer of the channel's current per-cpu buffer via the
-write functions described below.  Whenever a message can't fit into
-the current sub-buffer, because there's no room left for it, the
-client is notified via the subbuf_start() callback that a switch to a
-new sub-buffer is about to occur.  The client uses this callback to 1)
-initialize the next sub-buffer if appropriate 2) finalize the previous
-sub-buffer if appropriate and 3) return a boolean value indicating
-whether or not to actually go ahead with the sub-buffer switch.
-To implement 'no-overwrite' mode, the userspace client would provide
-an implementation of the subbuf_start() callback something like the
-following:
-static int subbuf_start(struct rchan_buf *buf,
-                        void *subbuf,
-                        void *prev_subbuf,
-                        unsigned int prev_padding)
-{
-        if (prev_subbuf)
-                *((unsigned *)prev_subbuf) = prev_padding;
-        if (relay_buf_full(buf))
-                return 0;
-        subbuf_start_reserve(buf, sizeof(unsigned int));
-        return 1;
-}
-If the current buffer is full i.e. all sub-buffers remain unconsumed,
-the callback returns 0 to indicate that the buffer switch should not
-occur yet i.e. until the consumer has had a chance to read the current
-set of ready sub-buffers.  For the relay_buf_full() function to make
-sense, the consumer is reponsible for notifying relayfs when
-sub-buffers have been consumed via relay_subbufs_consumed().  Any
-subsequent attempts to write into the buffer will again invoke the
-subbuf_start() callback with the same parameters; only when the
-consumer has consumed one or more of the ready sub-buffers will
-relay_buf_full() return 0, in which case the buffer switch can
-continue.
-The implementation of the subbuf_start() callback for 'overwrite' mode
-would be very similar:
-static int subbuf_start(struct rchan_buf *buf,
-                        void *subbuf,
-                        void *prev_subbuf,
-                        unsigned int prev_padding)
-{
-        if (prev_subbuf)
-                *((unsigned *)prev_subbuf) = prev_padding;
-        subbuf_start_reserve(buf, sizeof(unsigned int));
-        return 1;
-}
-In this case, the relay_buf_full() check is meaningless and the
-callback always returns 1, causing the buffer switch to occur
-unconditionally.  It's also meaningless for the client to use the
-relay_subbufs_consumed() function in this mode, as it's never
-consulted.
-The default subbuf_start() implementation, used if the client doesn't
-define any callbacks, or doesn't define the subbuf_start() callback,
-implements the simplest possible 'no-overwrite' mode i.e. it does
-nothing but return 0.
-Header information can be reserved at the beginning of each sub-buffer
-by calling the subbuf_start_reserve() helper function from within the
-subbuf_start() callback.  This reserved area can be used to store
-whatever information the client wants.  In the example above, room is
-reserved in each sub-buffer to store the padding count for that
-sub-buffer.  This is filled in for the previous sub-buffer in the
-subbuf_start() implementation; the padding value for the previous
-sub-buffer is passed into the subbuf_start() callback along with a
-pointer to the previous sub-buffer, since the padding value isn't
-known until a sub-buffer is filled.  The subbuf_start() callback is
-also called for the first sub-buffer when the channel is opened, to
-give the client a chance to reserve space in it.  In this case the
-previous sub-buffer pointer passed into the callback will be NULL, so
-the client should check the value of the prev_subbuf pointer before
-writing into the previous sub-buffer.
-Writing to a channel
--------------------
-kernel clients write data into the current cpu's channel buffer using
-relay_write() or __relay_write().  relay_write() is the main logging
-function - it uses local_irqsave() to protect the buffer and should be
-used if you might be logging from interrupt context.  If you know
-you'll never be logging from interrupt context, you can use
-__relay_write(), which only disables preemption.  These functions
-don't return a value, so you can't determine whether or not they
-failed - the assumption is that you wouldn't want to check a return
-value in the fast logging path anyway, and that they'll always succeed
-unless the buffer is full and no-overwrite mode is being used, in
-which case you can detect a failed write in the subbuf_start()
-callback by calling the relay_buf_full() helper function.
-relay_reserve() is used to reserve a slot in a channel buffer which
-can be written to later.  This would typically be used in applications
-that need to write directly into a channel buffer without having to
-stage data in a temporary buffer beforehand.  Because the actual write
-may not happen immediately after the slot is reserved, applications
-using relay_reserve() can keep a count of the number of bytes actually
-written, either in space reserved in the sub-buffers themselves or as
-a separate array.  See the 'reserve' example in the relay-apps tarball
-at http://relayfs.sourceforge.net for an example of how this can be
-done.  Because the write is under control of the client and is
-separated from the reserve, relay_reserve() doesn't protect the buffer
-at all - it's up to the client to provide the appropriate
-synchronization when using relay_reserve().
-Closing a channel
-----------------
-The client calls relay_close() when it's finished using the channel.
-The channel and its associated buffers are destroyed when there are no
-longer any references to any of the channel buffers.  relay_flush()
-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
----
-Some applications may want to keep a channel around and re-use it
-rather than open and close a new channel for each use.  relay_reset()
-can be used for this purpose - it resets a channel to its initial
-state without reallocating channel buffer memory or destroying
-existing mappings.  It should however only be called when it's safe to
-do so i.e. when the channel isn't currently being written to.
-Finally, there are a couple of utility callbacks that can be used for
-different purposes.  buf_mapped() is called whenever a channel buffer
-is mmapped from user space and buf_unmapped() is called when it's
-unmapped.  The client can use this notification to trigger actions
-within the kernel application, such as enabling/disabling logging to
-the channel.
-Resources
-=========
-For news, example code, mailing list, etc. see the relayfs homepage:
-    http://relayfs.sourceforge.net
-Credits
-=======
-The ideas and specs for relayfs came about as a result of discussions
-on tracing involving the following:
-Michel Dagenais         <michel.dagenais@polymtl.ca>
-Richard Moore           <richardj_moore@uk.ibm.com>
-Bob Wisniewski          <bob@watson.ibm.com>
-Karim Yaghmour          <karim@opersys.com>
-Tom Zanussi             <zanussi@us.ibm.com>
-Also thanks to Hubertus Franke for a lot of useful suggestions and bug
-reports.