tcmu: clean up the scatter helper

Add some comments to make the scatter code to be more readable,
and drop unused arg to new_iov.

Signed-off-by: Xiubo Li <lixiubo@cmss.chinamobile.com>
Signed-off-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

1 files changed, 25 insertions, 5 deletions

diff --git a/drivers/target/target_core_user.c b/drivers/target/target_core_user.c
index 5d1daea51079..8d0dc471fce8 100644
--- a/drivers/target/target_core_user.c
+++ b/drivers/target/target_core_user.c
@@ -492,8 +492,7 @@ static inline size_t head_to_end(size_t head, size_t size)
         return size - head;
 }
 
-static inline void new_iov(struct iovec **iov, int *iov_cnt,
-                           struct tcmu_dev *udev)
+static inline void new_iov(struct iovec **iov, int *iov_cnt)
 {
         struct iovec *iovec;
 
@@ -546,19 +545,38 @@ static void scatter_data_area(struct tcmu_dev *udev,
                                 to = kmap_atomic(page);
                         }
 
-                        copy_bytes = min_t(size_t, sg_remaining,
-                                        block_remaining);
+                        /*
+                         * Covert to virtual offset of the ring data area.
+                         */
                         to_offset = get_block_offset_user(udev, dbi,
                                         block_remaining);
 
+                        /*
+                         * The following code will gather and map the blocks
+                         * to the same iovec when the blocks are all next to
+                         * each other.
+                         */
+                        copy_bytes = min_t(size_t, sg_remaining,
+                                        block_remaining);
                         if (*iov_cnt != 0 &&
                             to_offset == iov_tail(*iov)) {
+                                /*
+                                 * Will append to the current iovec, because
+                                 * the current block page is next to the
+                                 * previous one.
+                                 */
                                 (*iov)->iov_len += copy_bytes;
                         } else {
-                                new_iov(iov, iov_cnt, udev);
+                                /*
+                                 * Will allocate a new iovec because we are
+                                 * first time here or the current block page
+                                 * is not next to the previous one.
+                                 */
+                                new_iov(iov, iov_cnt);
                                 (*iov)->iov_base = (void __user *)to_offset;
                                 (*iov)->iov_len = copy_bytes;
                         }
+
                         if (copy_data) {
                                 offset = DATA_BLOCK_SIZE - block_remaining;
                                 memcpy(to + offset,
@@ -566,11 +584,13 @@ static void scatter_data_area(struct tcmu_dev *udev,
                                        copy_bytes);
                                 tcmu_flush_dcache_range(to, copy_bytes);
                         }
+
                         sg_remaining -= copy_bytes;
                         block_remaining -= copy_bytes;
                 }
                 kunmap_atomic(from - sg->offset);
         }
+
         if (to)
                 kunmap_atomic(to);
 }