1 files changed, 27 insertions, 4 deletions

diff --git a/drivers/media/video/bt8xx/bttv-driver.c b/drivers/media/video/bt8xx/bttv-driver.c
index 939d1e512974..3ff84c8185a9 100644
--- a/drivers/media/video/bt8xx/bttv-driver.c
+++ b/drivers/media/video/bt8xx/bttv-driver.c
@@ -3800,11 +3800,34 @@ bttv_irq_next_video(struct bttv *btv, struct bttv_buffer_set *set)
                 if (!V4L2_FIELD_HAS_BOTH(item->vb.field) &&
                     (item->vb.queue.next != &btv->capture)) {
                         item = list_entry(item->vb.queue.next, struct bttv_buffer, vb.queue);
+                        /* Mike Isely <isely@pobox.com> - Only check
+                         * and set up the bottom field in the logic
+                         * below.  Don't ever do the top field.  This
+                         * of course means that if we set up the
+                         * bottom field in the above code that we'll
+                         * actually skip a field.  But that's OK.
+                         * Having processed only a single buffer this
+                         * time, then the next time around the first
+                         * available buffer should be for a top field.
+                         * That will then cause us here to set up a
+                         * top then a bottom field in the normal way.
+                         * The alternative to this understanding is
+                         * that we set up the second available buffer
+                         * as a top field, but that's out of order
+                         * since this driver always processes the top
+                         * field first - the effect will be the two
+                         * buffers being returned in the wrong order,
+                         * with the second buffer also being delayed
+                         * by one field time (owing to the fifo nature
+                         * of videobuf).  Worse still, we'll be stuck
+                         * doing fields out of order now every time
+                         * until something else causes a field to be
+                         * dropped.  By effectively forcing a field to
+                         * drop this way then we always get back into
+                         * sync within a single frame time.  (Out of
+                         * order fields can screw up deinterlacing
+                         * algorithms.) */
                         if (!V4L2_FIELD_HAS_BOTH(item->vb.field)) {
-                                if (NULL == set->top &&
-                                    V4L2_FIELD_TOP == item->vb.field) {
-                                        set->top = item;
-                                }
                                 if (NULL == set->bottom &&
                                     V4L2_FIELD_BOTTOM == item->vb.field) {
                                         set->bottom = item;