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>

1 files changed, 2 insertions, 1 deletions

diff --git a/drivers/gpu/drm/i915/intel_lvds.c b/drivers/gpu/drm/i915/intel_lvds.c
index 5041590dfdc6..aa74e59bec61 100644
--- a/drivers/gpu/drm/i915/intel_lvds.c
+++ b/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;