CVT_MIN_V_PORCH
tmp2 = (vdisplay_rnd + 2 * vmargin + CVT_MIN_V_PORCH) * 2 +
if (tmp1 < (vsync + CVT_MIN_V_PORCH))
vsyncandback_porch = vsync + CVT_MIN_V_PORCH;
vsyncandback_porch + CVT_MIN_V_PORCH;
drm_mode->vsync_start = drm_mode->vdisplay + CVT_MIN_V_PORCH;