drm/i915: disable LVDS downclock by default

Many platform support this feature, and it can provide significant
power savings when the reduced refresh rate is low.  However, on some
platforms a secondary (reduced) timing is provided but not actually
supported by the hardware.  This results in undesirable flicker at
runtime.

So disable the feature by default, but allow users to opt-in to the
reduced clock behavior with a new module parameter, lvds_downclock,
that can be set to 1 to enable the feature.
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/i915_drv.c

@@ -45,6 +45,9 @@ module_param_named(fbpercrtc, i915_fbpercrtc, int, 0400);
 unsigned int i915_powersave = 1;
 module_param_named(powersave, i915_powersave, int, 0400);
 
+unsigned int i915_lvds_downclock = 0;
+module_param_named(lvds_downclock, i915_lvds_downclock, int, 0400);
+
 static struct drm_driver driver;
 
 #define INTEL_VGA_DEVICE(id, info) {		\

drivers/gpu/drm/i915/i915_drv.h

@@ -722,6 +722,7 @@ extern struct drm_ioctl_desc i915_ioctls[];
 extern int i915_max_ioctl;
 extern unsigned int i915_fbpercrtc;
 extern unsigned int i915_powersave;
+extern unsigned int i915_lvds_downclock;
 
 extern void i915_save_display(struct drm_device *dev);
 extern void i915_restore_display(struct drm_device *dev);

drivers/gpu/drm/i915/intel_bios.c

@@ -197,7 +197,8 @@ parse_lfp_panel_data(struct drm_i915_private *dev_priv,
 		memset(temp_mode, 0, sizeof(*temp_mode));
 	}
 	kfree(temp_mode);
-	if (temp_downclock < panel_fixed_mode->clock) {
+	if (temp_downclock < panel_fixed_mode->clock &&
+	    i915_lvds_downclock) {
 		dev_priv->lvds_downclock_avail = 1;
 		dev_priv->lvds_downclock = temp_downclock;
 		DRM_DEBUG_KMS("LVDS downclock is found in VBT. ",

drivers/gpu/drm/i915/intel_lvds.c

@@ -926,7 +926,8 @@ static void intel_find_lvds_downclock(struct drm_device *dev,
 		}
 	}
 	mutex_unlock(&dev->mode_config.mutex);
-	if (temp_downclock < panel_fixed_mode->clock) {
+	if (temp_downclock < panel_fixed_mode->clock &&
+	    i915_lvds_downclock) {
 		/* We found the downclock for LVDS. */
 		dev_priv->lvds_downclock_avail = 1;
 		dev_priv->lvds_downclock = temp_downclock;