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authorJens Axboe <jens.axboe@oracle.com>2007-06-12 20:51:32 +0200
committerJens Axboe <jens.axboe@oracle.com>2007-07-10 08:04:16 +0200
commit0845718dafea3e16041d270c256e8516acf4e13d (patch)
tree5b572e9bf4a13e05d0c3a9b8c36745ef06a92d58 /include/linux/pipe_fs_i.h
parentcac36bb06efe4880234524e117e0e712b10b1f16 (diff)
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pipe: add documentation and comments
As per Andrew Mortons request, here's a set of documentation for the generic pipe_buf_operations hooks, the pipe, and pipe_buffer structures. Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
Diffstat (limited to 'include/linux/pipe_fs_i.h')
-rw-r--r--include/linux/pipe_fs_i.h77
1 files changed, 76 insertions, 1 deletions
diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h
index cc09fe89bf07..8e4120285f72 100644
--- a/include/linux/pipe_fs_i.h
+++ b/include/linux/pipe_fs_i.h
@@ -9,6 +9,15 @@
#define PIPE_BUF_FLAG_ATOMIC 0x02 /* was atomically mapped */
#define PIPE_BUF_FLAG_GIFT 0x04 /* page is a gift */
+/**
+ * struct pipe_buffer - a linux kernel pipe buffer
+ * @page: the page containing the data for the pipe buffer
+ * @offset: offset of data inside the @page
+ * @len: length of data inside the @page
+ * @ops: operations associated with this buffer. See @pipe_buf_operations.
+ * @flags: pipe buffer flags. See above.
+ * @private: private data owned by the ops.
+ **/
struct pipe_buffer {
struct page *page;
unsigned int offset, len;
@@ -17,6 +26,22 @@ struct pipe_buffer {
unsigned long private;
};
+/**
+ * struct pipe_inode_info - a linux kernel pipe
+ * @wait: reader/writer wait point in case of empty/full pipe
+ * @nrbufs: the number of non-empty pipe buffers in this pipe
+ * @curbuf: the current pipe buffer entry
+ * @tmp_page: cached released page
+ * @readers: number of current readers of this pipe
+ * @writers: number of current writers of this pipe
+ * @waiting_writers: number of writers blocked waiting for room
+ * @r_counter: reader counter
+ * @w_counter: writer counter
+ * @fasync_readers: reader side fasync
+ * @fasync_writers: writer side fasync
+ * @inode: inode this pipe is attached to
+ * @bufs: the circular array of pipe buffers
+ **/
struct pipe_inode_info {
wait_queue_head_t wait;
unsigned int nrbufs, curbuf;
@@ -43,15 +68,65 @@ struct pipe_inode_info {
* ->unmap()
*
* That is, ->map() must be called on a confirmed buffer,
- * same goes for ->steal().
+ * same goes for ->steal(). See below for the meaning of each
+ * operation. Also see kerneldoc in fs/pipe.c for the pipe
+ * and generic variants of these hooks.
*/
struct pipe_buf_operations {
+ /*
+ * This is set to 1, if the generic pipe read/write may coalesce
+ * data into an existing buffer. If this is set to 0, a new pipe
+ * page segment is always used for new data.
+ */
int can_merge;
+
+ /*
+ * ->map() returns a virtual address mapping of the pipe buffer.
+ * The last integer flag reflects whether this should be an atomic
+ * mapping or not. The atomic map is faster, however you can't take
+ * page faults before calling ->unmap() again. So if you need to eg
+ * access user data through copy_to/from_user(), then you must get
+ * a non-atomic map. ->map() uses the KM_USER0 atomic slot for
+ * atomic maps, so you can't map more than one pipe_buffer at once
+ * and you have to be careful if mapping another page as source
+ * or destination for a copy (IOW, it has to use something else
+ * than KM_USER0).
+ */
void * (*map)(struct pipe_inode_info *, struct pipe_buffer *, int);
+
+ /*
+ * Undoes ->map(), finishes the virtual mapping of the pipe buffer.
+ */
void (*unmap)(struct pipe_inode_info *, struct pipe_buffer *, void *);
+
+ /*
+ * ->confirm() verifies that the data in the pipe buffer is there
+ * and that the contents are good. If the pages in the pipe belong
+ * to a file system, we may need to wait for IO completion in this
+ * hook. Returns 0 for good, or a negative error value in case of
+ * error.
+ */
int (*confirm)(struct pipe_inode_info *, struct pipe_buffer *);
+
+ /*
+ * When the contents of this pipe buffer has been completely
+ * consumed by a reader, ->release() is called.
+ */
void (*release)(struct pipe_inode_info *, struct pipe_buffer *);
+
+ /*
+ * Attempt to take ownership of the pipe buffer and its contents.
+ * ->steal() returns 0 for success, in which case the contents
+ * of the pipe (the buf->page) is locked and now completely owned
+ * by the caller. The page may then be transferred to a different
+ * mapping, the most often used case is insertion into different
+ * file address space cache.
+ */
int (*steal)(struct pipe_inode_info *, struct pipe_buffer *);
+
+ /*
+ * Get a reference to the pipe buffer.
+ */
void (*get)(struct pipe_inode_info *, struct pipe_buffer *);
};