dm: document iterate_devices

Document iterate_devices in device-mapper.h.
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

include/linux/device-mapper.h

@@ -79,11 +79,26 @@ typedef int (*dm_ioctl_fn) (struct dm_target *ti, unsigned int cmd,
 typedef int (*dm_merge_fn) (struct dm_target *ti, struct bvec_merge_data *bvm,
 			    struct bio_vec *biovec, int max_size);
 
+/*
+ * These iteration functions are typically used to check (and combine)
+ * properties of underlying devices.
+ * E.g. Does at least one underlying device support flush?
+ *      Does any underlying device not support WRITE_SAME?
+ *
+ * The callout function is called once for each contiguous section of
+ * an underlying device.  State can be maintained in *data.
+ * Return non-zero to stop iterating through any further devices.
+ */
 typedef int (*iterate_devices_callout_fn) (struct dm_target *ti,
 					   struct dm_dev *dev,
 					   sector_t start, sector_t len,
 					   void *data);
 
+/*
+ * This function must iterate through each section of device used by the
+ * target until it encounters a non-zero return code, which it then returns.
+ * Returns zero if no callout returned non-zero.
+ */
 typedef int (*dm_iterate_devices_fn) (struct dm_target *ti,
 				      iterate_devices_callout_fn fn,
 				      void *data);