[media] vb2: reject output buffers with V4L2_FIELD_ALTERNATE

This is not allowed by the spec and does in fact not make any sense.
Return -EINVAL if this is the case.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Pawel Osciak <pawel@osciak.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

drivers/media/v4l2-core/videobuf2-core.c

@@ -1564,6 +1564,19 @@ static int __buf_prepare(struct vb2_buffer *vb, const struct v4l2_buffer *b)
 		dprintk(1, "plane parameters verification failed: %d\n", ret);
 		return ret;
 	}
+	if (b->field == V4L2_FIELD_ALTERNATE && V4L2_TYPE_IS_OUTPUT(q->type)) {
+		/*
+		 * If the format's field is ALTERNATE, then the buffer's field
+		 * should be either TOP or BOTTOM, not ALTERNATE since that
+		 * makes no sense. The driver has to know whether the
+		 * buffer represents a top or a bottom field in order to
+		 * program any DMA correctly. Using ALTERNATE is wrong, since
+		 * that just says that it is either a top or a bottom field,
+		 * but not which of the two it is.
+		 */
+		dprintk(1, "the field is incorrectly set to ALTERNATE for an output buffer\n");
+		return -EINVAL;
+	}
 
 	vb->state = VB2_BUF_STATE_PREPARING;
 	vb->v4l2_buf.timestamp.tv_sec = 0;