splice: completely document external interface with kerneldoc

Also add fs/splice.c as a kerneldoc target with a smaller blurb that
should be expanded to better explain the overview of splice.
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>

Documentation/DocBook/kernel-api.tmpl

@@ -643,4 +643,15 @@ X!Idrivers/video/console/fonts.c
 !Edrivers/spi/spi.c
   </chapter>
 
+  <chapter id="splice">
+      <title>splice API</title>
+  <para>)
+	splice is a method for moving blocks of data around inside the
+	kernel, without continually transferring it between the kernel
+	and user space.
+  </para>
+!Iinclude/linux/splice.h
+!Ffs/splice.c
+  </chapter>
+
 </book>

fs/splice.c

@@ -153,9 +153,16 @@ static const struct pipe_buf_operations user_page_pipe_buf_ops = {
 	.get = generic_pipe_buf_get,
 };
 
-/*
- * Pipe output worker. This fills a pipe with the information contained
- * from splice_pipe_desc().
+/**
+ * splice_to_pipe - fill passed data into a pipe
+ * @pipe:	pipe to fill
+ * @spd:	data to fill
+ *
+ * Description:
+ *    @spd contains a map of pages and len/offset tupples, a long with
+ *    the struct pipe_buf_operations associated with these pages. This
+ *    function will link that data to the pipe.
+ *
  */
 ssize_t splice_to_pipe(struct pipe_inode_info *pipe,
 		       struct splice_pipe_desc *spd)
@@ -280,11 +287,6 @@ __generic_file_splice_read(struct file *in, loff_t *ppos,
 	 */
 	page_cache_readahead(mapping, &in->f_ra, in, index, nr_pages);
 
-	/*
-	 * Now fill in the holes:
-	 */
-	error = 0;
-
 	/*
 	 * Lookup the (hopefully) full range of pages we need.
 	 */
@@ -292,8 +294,9 @@ __generic_file_splice_read(struct file *in, loff_t *ppos,
 
 	/*
 	 * If find_get_pages_contig() returned fewer pages than we needed,
-	 * allocate the rest.
+	 * allocate the rest and fill in the holes.
 	 */
+	error = 0;
 	index += spd.nr_pages;
 	while (spd.nr_pages < nr_pages) {
 		/*
@@ -455,11 +458,16 @@ __generic_file_splice_read(struct file *in, loff_t *ppos,
 /**
  * generic_file_splice_read - splice data from file to a pipe
  * @in:		file to splice from
+ * @ppos:	position in @in
  * @pipe:	pipe to splice to
  * @len:	number of bytes to splice
  * @flags:	splice modifier flags
  *
- * Will read pages from given file and fill them into a pipe.
+ * Description:
+ *    Will read pages from given file and fill them into a pipe. Can be
+ *    used as long as the address_space operations for the source implements
+ *    a readpage() hook.
+ *
  */
 ssize_t generic_file_splice_read(struct file *in, loff_t *ppos,
 				 struct pipe_inode_info *pipe, size_t len,
@@ -648,10 +656,18 @@ static int pipe_to_file(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
 	return ret;
 }
 
-/*
- * Pipe input worker. Most of this logic works like a regular pipe, the
- * key here is the 'actor' worker passed in that actually moves the data
- * to the wanted destination. See pipe_to_file/pipe_to_sendpage above.
+/**
+ * __splice_from_pipe - splice data from a pipe to given actor
+ * @pipe:	pipe to splice from
+ * @sd:		information to @actor
+ * @actor:	handler that splices the data
+ *
+ * Description:
+ *    This function does little more than loop over the pipe and call
+ *    @actor to do the actual moving of a single struct pipe_buffer to
+ *    the desired destination. See pipe_to_file, pipe_to_sendpage, or
+ *    pipe_to_user.
+ *
  */
 ssize_t __splice_from_pipe(struct pipe_inode_info *pipe, struct splice_desc *sd,
 			   splice_actor *actor)
@@ -744,6 +760,20 @@ ssize_t __splice_from_pipe(struct pipe_inode_info *pipe, struct splice_desc *sd,
 }
 EXPORT_SYMBOL(__splice_from_pipe);
 
+/**
+ * splice_from_pipe - splice data from a pipe to a file
+ * @pipe:	pipe to splice from
+ * @out:	file to splice to
+ * @ppos:	position in @out
+ * @len:	how many bytes to splice
+ * @flags:	splice modifier flags
+ * @actor:	handler that splices the data
+ *
+ * Description:
+ *    See __splice_from_pipe. This function locks the input and output inodes,
+ *    otherwise it's identical to __splice_from_pipe().
+ *
+ */
 ssize_t splice_from_pipe(struct pipe_inode_info *pipe, struct file *out,
 			 loff_t *ppos, size_t len, unsigned int flags,
 			 splice_actor *actor)
@@ -774,9 +804,11 @@ ssize_t splice_from_pipe(struct pipe_inode_info *pipe, struct file *out,
  * generic_file_splice_write_nolock - generic_file_splice_write without mutexes
  * @pipe:	pipe info
  * @out:	file to write to
+ * @ppos:	position in @out
  * @len:	number of bytes to splice
  * @flags:	splice modifier flags
  *
+ * Description:
  *    Will either move or copy pages (determined by @flags options) from
  *    the given pipe inode to the given file. The caller is responsible
  *    for acquiring i_mutex on both inodes.
@@ -831,9 +863,11 @@ EXPORT_SYMBOL(generic_file_splice_write_nolock);
  * generic_file_splice_write - splice data from a pipe to a file
  * @pipe:	pipe info
  * @out:	file to write to
+ * @ppos:	position in @out
  * @len:	number of bytes to splice
  * @flags:	splice modifier flags
  *
+ * Description:
  *    Will either move or copy pages (determined by @flags options) from
  *    the given pipe inode to the given file.
  *
@@ -886,11 +920,13 @@ EXPORT_SYMBOL(generic_file_splice_write);
 
 /**
  * generic_splice_sendpage - splice data from a pipe to a socket
- * @inode:	pipe inode
+ * @pipe:	pipe to splice from
  * @out:	socket to write to
+ * @ppos:	position in @out
  * @len:	number of bytes to splice
  * @flags:	splice modifier flags
  *
+ * Description:
  *    Will send @len bytes from the pipe to a network socket. No data copying
  *    is involved.
  *
@@ -946,8 +982,18 @@ static long do_splice_to(struct file *in, loff_t *ppos,
 	return in->f_op->splice_read(in, ppos, pipe, len, flags);
 }
 
-/*
- * Splices from an input file to an actor, using a 'direct' pipe.
+/**
+ * splice_direct_to_actor - splices data directly between two non-pipes
+ * @in:		file to splice from
+ * @sd:		actor information on where to splice to
+ * @actor:	handles the data splicing
+ *
+ * Description:
+ *    This is a special case helper to splice directly between two
+ *    points, without requiring an explicit pipe. Internally an allocated
+ *    pipe is cached in the process, and reused during the life time of
+ *    that process.
+ *
  */
 ssize_t splice_direct_to_actor(struct file *in, struct splice_desc *sd,
 			       splice_direct_actor *actor)
@@ -1077,6 +1123,21 @@ static int direct_splice_actor(struct pipe_inode_info *pipe,
 	return do_splice_from(pipe, file, &sd->pos, sd->total_len, sd->flags);
 }
 
+/**
+ * do_splice_direct - splices data directly between two files
+ * @in:		file to splice from
+ * @ppos:	input file offset
+ * @out:	file to splice to
+ * @len:	number of bytes to splice
+ * @flags:	splice modifier flags
+ *
+ * Description:
+ *    For use by do_sendfile(). splice can easily emulate sendfile, but
+ *    doing it in the application would incur an extra system call
+ *    (splice in + splice out, as compared to just sendfile()). So this helper
+ *    can splice directly through a process-private pipe.
+ *
+ */
 long do_splice_direct(struct file *in, loff_t *ppos, struct file *out,
 		      size_t len, unsigned int flags)
 {