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>

diff --git a/drivers/target/target_core_user.c b/drivers/target/target_core_user.c
index 5d1daea..8d0dc47 100644 (file)
--- 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);
 }