drm/i915: Allow SSC parameter to override VBT value

Allow SSC to be enabled even when the BIOS disables it for testing SSC paths.
Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_drv.c

@@ -79,11 +79,11 @@ MODULE_PARM_DESC(lvds_downclock,
 		"Use panel (LVDS/eDP) downclocking for power savings "
 		"(default: false)");
 
-unsigned int i915_panel_use_ssc __read_mostly = 1;
+unsigned int i915_panel_use_ssc __read_mostly = -1;
 module_param_named(lvds_use_ssc, i915_panel_use_ssc, int, 0600);
 MODULE_PARM_DESC(lvds_use_ssc,
 		"Use Spread Spectrum Clock with panels [LVDS/eDP] "
-		"(default: true)");
+		"(default: auto from VBT)");
 
 int i915_vbt_sdvo_panel_type __read_mostly = -1;
 module_param_named(vbt_sdvo_panel_type, i915_vbt_sdvo_panel_type, int, 0600);

drivers/gpu/drm/i915/intel_display.c

@@ -4584,7 +4584,9 @@ static void intel_update_watermarks(struct drm_device *dev)
 
 static inline bool intel_panel_use_ssc(struct drm_i915_private *dev_priv)
 {
-	return dev_priv->lvds_use_ssc && i915_panel_use_ssc
+	if (i915_panel_use_ssc >= 0)
+		return i915_panel_use_ssc != 0;
+	return dev_priv->lvds_use_ssc
 		&& !(dev_priv->quirks & QUIRK_LVDS_SSC_DISABLE);
 }