What are drm structure drm_mode_modeinfo members "hskew" and "vscan" stand for?

Question

In drm KMS there is a structure drm_mode_modeinfo, what's the meaning of "hskew" and "vscan"? How should I use and fill them?

 /**
  * struct drm_mode_modeinfo - Display mode information.
  * @clock: pixel clock in kHz
  * @hdisplay: horizontal display size
  * @hsync_start: horizontal sync start
  * @hsync_end: horizontal sync end
  * @htotal: horizontal total size
  * @hskew: horizontal skew
  * @vdisplay: vertical display size
  * @vsync_start: vertical sync start
  * @vsync_end: vertical sync end
  * @vtotal: vertical total size
  * @vscan: vertical scan
  * @vrefresh: approximate vertical refresh rate in Hz
  * @flags: bitmask of misc. flags, see DRM_MODE_FLAG_* defines
  * @type: bitmask of type flags, see DRM_MODE_TYPE_* defines
  * @name: string describing the mode resolution
  *
  * This is the user-space API display mode information structure. For the
  * kernel version see struct drm_display_mode.
  */
 struct drm_mode_modeinfo {
     __u32 clock;
     __u16 hdisplay;
     __u16 hsync_start;
     __u16 hsync_end;
     __u16 htotal;
     __u16 hskew;
     __u16 vdisplay;
     __u16 vsync_start;
     __u16 vsync_end;
     __u16 vtotal;
     __u16 vscan;

     __u32 vrefresh;

     __u32 flags;
     __u32 type;
     char name[DRM_DISPLAY_MODE_LEN];
 };

I'm a beginner of linux DRM and try to write a driver based on DRM framework.