drm/i915: Correctly enable backlight brightness adjustment via DPCD

intel_dp_aux_enable_backlight() assumed that the register
BACKLIGHT_BRIGHTNESS_CONTROL_MODE can only has value 01
(DP_EDP_BACKLIGHT_CONTROL_MODE_PRESET) when initialize.

This patch fixed that by handling all cases of that register.

Signed-off-by: Puthikorn Voravootivat <puthik@chromium.org>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170511230225.142870-3-puthik@chromium.org

drivers/gpu/drm/i915/intel_dp_aux_backlight.c

@@ -97,15 +97,36 @@ static void intel_dp_aux_enable_backlight(struct intel_connector *connector)
 {
 	struct intel_dp *intel_dp = enc_to_intel_dp(&connector->encoder->base);
 	uint8_t dpcd_buf = 0;
+	uint8_t edp_backlight_mode = 0;
 
 	set_aux_backlight_enable(intel_dp, true);
 
-	if ((drm_dp_dpcd_readb(&intel_dp->aux,
-			       DP_EDP_BACKLIGHT_MODE_SET_REGISTER, &dpcd_buf) == 1) &&
-	    ((dpcd_buf & DP_EDP_BACKLIGHT_CONTROL_MODE_MASK) ==
-	     DP_EDP_BACKLIGHT_CONTROL_MODE_PRESET))
-		drm_dp_dpcd_writeb(&intel_dp->aux, DP_EDP_BACKLIGHT_MODE_SET_REGISTER,
-				   (dpcd_buf | DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD));
+	if (drm_dp_dpcd_readb(&intel_dp->aux,
+			DP_EDP_BACKLIGHT_MODE_SET_REGISTER, &dpcd_buf) != 1) {
+		DRM_DEBUG_KMS("Failed to read DPCD register 0x%x\n",
+			      DP_EDP_BACKLIGHT_MODE_SET_REGISTER);
+		return;
+	}
+
+	edp_backlight_mode = dpcd_buf & DP_EDP_BACKLIGHT_CONTROL_MODE_MASK;
+
+	switch (edp_backlight_mode) {
+	case DP_EDP_BACKLIGHT_CONTROL_MODE_PWM:
+	case DP_EDP_BACKLIGHT_CONTROL_MODE_PRESET:
+	case DP_EDP_BACKLIGHT_CONTROL_MODE_PRODUCT:
+		dpcd_buf &= ~DP_EDP_BACKLIGHT_CONTROL_MODE_MASK;
+		dpcd_buf |= DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD;
+		if (drm_dp_dpcd_writeb(&intel_dp->aux,
+			DP_EDP_BACKLIGHT_MODE_SET_REGISTER, dpcd_buf) < 0) {
+			DRM_DEBUG_KMS("Failed to write aux backlight mode\n");
+		}
+		break;
+
+	/* Do nothing when it is already DPCD mode */
+	case DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD:
+	default:
+		break;
+	}
 }
 
 static void intel_dp_aux_disable_backlight(struct intel_connector *connector)