splice: kdoc for filemap_splice_read() and copy_splice_read()

Provide kerneldoc comments for filemap_splice_read() and
copy_splice_read().

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Christian Brauner <brauner@kernel.org>
cc: Christoph Hellwig <hch@lst.de>
cc: Jens Axboe <axboe@kernel.dk>
cc: Steve French <smfrench@gmail.com>
cc: Al Viro <viro@zeniv.linux.org.uk>
cc: linux-mm@kvack.org
cc: linux-block@vger.kernel.org
cc: linux-cifs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/20230522135018.2742245-32-dhowells@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: dhowells

fs/splice.c

@@ -299,8 +299,25 @@ void splice_shrink_spd(struct splice_pipe_desc *spd)
 	kfree(spd->partial);
 }
 
-/*
- * Copy data from a file into pages and then splice those into the output pipe.
+/**
+ * copy_splice_read -  Copy data from a file and splice the copy into a pipe
+ * @in: The file to read from
+ * @ppos: Pointer to the file position to read from
+ * @pipe: The pipe to splice into
+ * @len: The amount to splice
+ * @flags: The SPLICE_F_* flags
+ *
+ * This function allocates a bunch of pages sufficient to hold the requested
+ * amount of data (but limited by the remaining pipe capacity), passes it to
+ * the file's ->read_iter() to read into and then splices the used pages into
+ * the pipe.
+ *
+ * Return: On success, the number of bytes read will be returned and *@ppos
+ * will be updated if appropriate; 0 will be returned if there is no more data
+ * to be read; -EAGAIN will be returned if the pipe had no space, and some
+ * other negative error code will be returned on error.  A short read may occur
+ * if the pipe has insufficient space, we reach the end of the data or we hit a
+ * hole.
  */
 ssize_t copy_splice_read(struct file *in, loff_t *ppos,
 			 struct pipe_inode_info *pipe,

mm/filemap.c

@@ -2871,9 +2871,24 @@ size_t splice_folio_into_pipe(struct pipe_inode_info *pipe,
 	return spliced;
 }
 
-/*
- * Splice folios from the pagecache of a buffered (ie. non-O_DIRECT) file into
- * a pipe.
+/**
+ * filemap_splice_read -  Splice data from a file's pagecache into a pipe
+ * @in: The file to read from
+ * @ppos: Pointer to the file position to read from
+ * @pipe: The pipe to splice into
+ * @len: The amount to splice
+ * @flags: The SPLICE_F_* flags
+ *
+ * This function gets folios from a file's pagecache and splices them into the
+ * pipe.  Readahead will be called as necessary to fill more folios.  This may
+ * be used for blockdevs also.
+ *
+ * Return: On success, the number of bytes read will be returned and *@ppos
+ * will be updated if appropriate; 0 will be returned if there is no more data
+ * to be read; -EAGAIN will be returned if the pipe had no space, and some
+ * other negative error code will be returned on error.  A short read may occur
+ * if the pipe has insufficient space, we reach the end of the data or we hit a
+ * hole.
  */
 ssize_t filemap_splice_read(struct file *in, loff_t *ppos,
 			    struct pipe_inode_info *pipe,