relay interface (formerly relayfs)
  ==================================
  
  The relay interface provides a means for kernel applications to
  efficiently log and transfer large quantities of data from the kernel
  to userspace via user-defined 'relay channels'.
  
  A 'relay channel' is a kernel->user data relay mechanism implemented
  as a set of per-cpu kernel buffers ('channel buffers'), each
  represented as a regular file ('relay file') in user space.  Kernel
  clients write into the channel buffers using efficient write
  functions; these automatically log into the current cpu's channel
  buffer.  User space applications mmap() or read() from the relay files
  and retrieve the data as it becomes available.  The relay files
  themselves are files created in a host filesystem, e.g. debugfs, and
  are associated with the channel buffers using the API described below.
  
  The format of the data logged into the channel buffers is completely
  up to the kernel client; the relay interface does however provide
  hooks which allow kernel clients to impose some structure on the
  buffer data.  The relay interface doesn't implement any form of data
  filtering - this also is left to the kernel client.  The purpose is to
  keep things as simple as possible.
  
  This document provides an overview of the relay interface API.  The
  details of the function parameters are documented along with the
  functions in the relay interface code - please see that for details.
  
  Semantics
  =========
  
  Each relay 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 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 space occurring because a complete
  message couldn't fit into a sub-buffer.  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.
  
  A relay channel can operate in a mode where it will overwrite data not
  yet collected by userspace, and not wait for it to be consumed.
  
  The relay channel 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.
  
  The read() interface both removes padding and internally consumes the
  read sub-buffers; thus in cases where read(2) is being used to drain
  the channel buffers, special-purpose communication between kernel and
  user isn't necessary for basic operation.
  
  One of the major goals of the relay interface is to provide a low
  overhead mechanism for conveying kernel data to userspace.  While the
  read() interface is easy to use, it's not as efficient as the mmap()
  approach; the example code attempts to make the tradeoff between the
  two approaches as small as possible.
  
  klog and relay-apps example code
  ================================
  
  The relay interface 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 relay 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 relay application.  When 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 even
  whether the relay interface is compiled into the kernel or not.  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 the relay interface 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).  The read() interface both removes padding and internally
  consumes the read sub-buffers; thus in cases where read(2) is being
  used to drain the channel buffers, special-purpose communication
  between kernel and user isn't necessary for basic operation.  Things
  such as buffer-full conditions would still need to be communicated via
  some channel though.
  
  klog and the relay-apps examples can be found in the relay-apps
  tarball on http://relayfs.sourceforge.net
  
  The relay interface user space API
  ==================================
  
  The relay interface implements basic file operations for user space
  access to relay channel buffer data.  Here are the file operations
  that are available and some comments regarding their behavior:
  
  open()	    enables user to open an _existing_ channel 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.  Sub-buffer
  	    padding is automatically removed and will not be seen by
  	    the reader.
  
  sendfile()  transfer data from a channel buffer to an output file
  	    descriptor. Sub-buffer padding is automatically removed
  	    and will not be seen by the reader.
  
  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 relay files, the
  host filesystem must be mounted.  For example,
  
  	mount -t debugfs debugfs /sys/kernel/debug
  
  NOTE:   the host filesystem 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 relay interface kernel API
  ==============================
  
  Here's a summary of the API the relay interface provides to in-kernel clients:
  
  TBD(curr. line MT:/API/)
    channel management functions:
  
      relay_open(base_filename, parent, subbuf_size, n_subbufs,
                 callbacks, private_data)
      relay_close(chan)
      relay_flush(chan)
      relay_reset(chan)
  
    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 host filesystem, which can be and mmapped or
  read from in user space.  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 the parent param is NULL).  If you
  want a directory structure to contain your relay files, you should
  create it using the host filesystem's directory creation function,
  e.g. debugfs_create_dir(), and pass the parent directory to
  relay_open().  Users are responsible for cleaning up any directory
  structure they create, when the channel is closed - again the host
  filesystem's directory removal functions should be used for that,
  e.g. debugfs_remove().
  
  In order for a channel to be created and the host filesystem's files
  associated with its channel buffers, the user must provide definitions
  for two callback functions, create_buf_file() and remove_buf_file().
  create_buf_file() is called once for each per-cpu buffer from
  relay_open() and allows the user to create the file which will be used
  to represent the corresponding channel buffer.  The callback should
  return the dentry of the file created to represent the channel buffer.
  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().
  
  Here are some typical definitions for these callbacks, in this case
  using debugfs:
  
  /*
   * create_buf_file() callback.  Creates relay file in debugfs.
   */
  static struct dentry *create_buf_file_handler(const char *filename,
                                                struct dentry *parent,
                                                int mode,
                                                struct rchan_buf *buf,
                                                int *is_global)
  {
          return debugfs_create_file(filename, mode, parent, buf,
  	                           &relay_file_operations);
  }
  
  /*
   * remove_buf_file() callback.  Removes relay file from debugfs.
   */
  static int remove_buf_file_handler(struct dentry *dentry)
  {
          debugfs_remove(dentry);
  
          return 0;
  }
  
  /*
   * relay interface callbacks
   */
  static struct rchan_callbacks relay_callbacks =
  {
          .create_buf_file = create_buf_file_handler,
          .remove_buf_file = remove_buf_file_handler,
  };
  
  And an example relay_open() invocation using them:
  
    chan = relay_open("cpu", NULL, SUBBUF_SIZE, N_SUBBUFS, &relay_callbacks, NULL);
  
  If the create_buf_file() callback fails, or isn't defined, channel
  creation and thus relay_open() will fail.
  
  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.
  
  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 relay.h and
  creating a local version that internally does the proper locking.
  
  The private_data passed into relay_open() allows clients to associate
  user-defined data with a channel, and is immediately available
  (including in create_buf_file()) via chan->private_data or
  buf->chan->private_data.
  
  Buffer-only channels
  --------------------
  
  These channels have no files associated and can be created with
  relay_open(NULL, NULL, ...). Such channels are useful in scenarios such
  as when doing early tracing in the kernel, before the VFS is up. In these
  cases, one may open a buffer-only channel and then call
  relay_late_setup_files() when the kernel is ready to handle files,
  to expose the buffered data to the userspace.
  
  Channel 'modes'
  ---------------
  
  relay 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.  The default if no
  subbuf_start() callback is defined is 'no-overwrite' mode.  If the
  default mode suits your needs, and you plan to use the read()
  interface to retrieve channel data, you can ignore the details of this
  section, as it pertains mainly to mmap() implementations.
  
  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
  enough, 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 relay 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 move on to the next sub-buffer.
  
  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 responsible for notifying the relay
  interface 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.
  
  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 relay interface homepage:
  
      http://relayfs.sourceforge.net
  
  
  Credits
  =======
  
  The ideas and specs for the relay interface 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.