drm/radeon/kms: try to detect tv vs monitor for underscan

When enabling underscan for hdmi monitors, attempt to detect
whether we are driving a TV or a monitor.  The should hopefully
prevent underscan from being enabled on monitors attached via
hdmi that do not overscan the image.  Only enable underscan
if the mode is a common hdtv mode (480p, 720p, etc.).

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/radeon_display.c

@@ -1094,6 +1094,18 @@ void radeon_modeset_fini(struct radeon_device *rdev)
 	radeon_i2c_fini(rdev);
 }
 
+static bool is_hdtv_mode(struct drm_display_mode *mode)
+{
+	/* try and guess if this is a tv or a monitor */
+	if ((mode->vdisplay == 480 && mode->hdisplay == 720) || /* 480p */
+	    (mode->vdisplay == 576) || /* 576p */
+	    (mode->vdisplay == 720) || /* 720p */
+	    (mode->vdisplay == 1080)) /* 1080p */
+		return true;
+	else
+		return false;
+}
+
 bool radeon_crtc_scaling_mode_fixup(struct drm_crtc *crtc,
 				struct drm_display_mode *mode,
 				struct drm_display_mode *adjusted_mode)
@@ -1141,7 +1153,8 @@ bool radeon_crtc_scaling_mode_fixup(struct drm_crtc *crtc,
 			if (ASIC_IS_AVIVO(rdev) &&
 			    ((radeon_encoder->underscan_type == UNDERSCAN_ON) ||
 			     ((radeon_encoder->underscan_type == UNDERSCAN_AUTO) &&
-			      drm_detect_hdmi_monitor(radeon_connector->edid)))) {
+			      drm_detect_hdmi_monitor(radeon_connector->edid) &&
+			      is_hdtv_mode(mode)))) {
 				radeon_crtc->h_border = (mode->hdisplay >> 5) + 16;
 				radeon_crtc->v_border = (mode->vdisplay >> 5) + 16;
 				radeon_crtc->rmx_type = RMX_FULL;