Revert "drm/i915: Add option to support dynamic backlight via DPCD"

This reverts commit ae25ecea.

The DPCD backlight commits regress a Thinkpad X1 Carbon 4th Gen and a
BXT-P (in CI). Enabling dynamic backlight boots to a black screen, and
enabling DPCD backlight leads to a black screen after suspend/resume.

References: http://mid.mail-archive.com/20170706135349.6tu3lz7uehazlnnn@boom
References: http://mid.mail-archive.com/20170627132326.f2q3yn4bh5flji4q@boom
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101619Reported-by: David Weinehall <david.weinehall@linux.intel.com>
Fixes: ae25ecea ("drm/i915: Add option to support dynamic backlight via DPCD")
Cc: Jenny TC <jenny.tc@intel.com>
Cc: David Weinehall <david.weinehall@linux.intel.com>
Cc: Puthikorn Voravootivat <puthik@chromium.org>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: intel-gfx@lists.freedesktop.org
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/051eeb592361985d2d06333c61c220dd92253b09.1500542254.git.jani.nikula@intel.com

drivers/gpu/drm/i915/i915_params.c

@@ -65,7 +65,6 @@ struct i915_params i915 __read_mostly = {
 	.inject_load_failure = 0,
 	.enable_dpcd_backlight = -1,
 	.enable_gvt = false,
-	.enable_dbc = true,
 };
 
 module_param_named(modeset, i915.modeset, int, 0400);
@@ -255,7 +254,3 @@ MODULE_PARM_DESC(enable_dpcd_backlight,
 module_param_named(enable_gvt, i915.enable_gvt, bool, 0400);
 MODULE_PARM_DESC(enable_gvt,
 	"Enable support for Intel GVT-g graphics virtualization host support(default:false)");
-
-module_param_named_unsafe(enable_dbc, i915.enable_dbc, bool, 0600);
-MODULE_PARM_DESC(enable_dbc,
-	"Enable support for dynamic backlight control (default:true)");

drivers/gpu/drm/i915/i915_params.h

@@ -67,8 +67,7 @@
 	func(bool, verbose_state_checks); \
 	func(bool, nuclear_pageflip); \
 	func(bool, enable_dp_mst); \
-	func(bool, enable_gvt); \
-	func(bool, enable_dbc)
+	func(bool, enable_gvt)
 
 #define MEMBER(T, member) T member
 struct i915_params {

drivers/gpu/drm/i915/intel_dp_aux_backlight.c

@@ -173,24 +173,6 @@ static bool intel_dp_aux_set_pwm_freq(struct intel_connector *connector)
 	return true;
 }
 
-/*
-* Set minimum / maximum dynamic brightness percentage. This value is expressed
-* as the percentage of normal brightness in 5% increments.
-*/
-static bool
-intel_dp_aux_set_dynamic_backlight_percent(struct intel_dp *intel_dp,
-					   u32 min, u32 max)
-{
-	u8 dbc[] = { DIV_ROUND_CLOSEST(min, 5), DIV_ROUND_CLOSEST(max, 5) };
-
-	if (drm_dp_dpcd_write(&intel_dp->aux, DP_EDP_DBC_MINIMUM_BRIGHTNESS_SET,
-			  dbc, sizeof(dbc)) < 0) {
-		DRM_DEBUG_KMS("Failed to write aux DBC brightness level\n");
-		return false;
-	}
-	return true;
-}
-
 static void intel_dp_aux_enable_backlight(const struct intel_crtc_state *crtc_state,
 					  const struct drm_connector_state *conn_state)
 {
@@ -226,14 +208,6 @@ static void intel_dp_aux_enable_backlight(const struct intel_crtc_state *crtc_st
 		if (intel_dp_aux_set_pwm_freq(connector))
 			new_dpcd_buf |= DP_EDP_BACKLIGHT_FREQ_AUX_SET_ENABLE;
 
-	if (i915.enable_dbc &&
-	    (intel_dp->edp_dpcd[2] & DP_EDP_DYNAMIC_BACKLIGHT_CAP)) {
-		if(intel_dp_aux_set_dynamic_backlight_percent(intel_dp, 0, 100)) {
-			new_dpcd_buf |= DP_EDP_DYNAMIC_BACKLIGHT_ENABLE;
-			DRM_DEBUG_KMS("Enable dynamic brightness.\n");
-		}
-	}
-
 	if (new_dpcd_buf != dpcd_buf) {
 		if (drm_dp_dpcd_writeb(&intel_dp->aux,
 			DP_EDP_BACKLIGHT_MODE_SET_REGISTER, new_dpcd_buf) < 0) {