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 ==================================
 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,
                                                 umode_t 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,
                             size_t 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.

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