drm/i915/dp: Tweak initial dpcd backlight.enabled value

In commit 79946723 ("drm/i915: Assume 100% brightness when not in
DPCD control mode"), we fixed the brightness level when DPCD control was
not active to max brightness. This is as good as we can guess since most
backlights go on full when uncontrolled.

However in doing so we changed the semantics of the initial
'backlight.enabled' value. At least on Pixelbooks, they  were relying
on the brightness level in DP_EDP_BACKLIGHT_BRIGHTNESS_MSB to be 0 on
boot such that enabled would be false. This causes the device to be
enabled when the brightness is set. Without this, brightness control
doesn't work. So by changing brightness to max, we also flipped enabled
to be true on boot.

To fix this, make enabled a function of brightness and backlight control
mechanism.

Fixes: 79946723 ("drm/i915: Assume 100% brightness when not in DPCD control mode")
Cc: Lyude Paul <lyude@redhat.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Cc: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Kevin Chowski <chowski@chromium.org>>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200918002845.32766-1-sean@poorly.run

drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c

@@ -52,17 +52,11 @@ static void set_aux_backlight_enable(struct intel_dp *intel_dp, bool enable)
 	}
 }
 
-/*
- * Read the current backlight value from DPCD register(s) based
- * on if 8-bit(MSB) or 16-bit(MSB and LSB) values are supported
- */
-static u32 intel_dp_aux_get_backlight(struct intel_connector *connector)
+static bool intel_dp_aux_backlight_dpcd_mode(struct intel_connector *connector)
 {
 	struct intel_dp *intel_dp = intel_attached_dp(connector);
 	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
-	u8 read_val[2] = { 0x0 };
 	u8 mode_reg;
-	u16 level = 0;
 
 	if (drm_dp_dpcd_readb(&intel_dp->aux,
 			      DP_EDP_BACKLIGHT_MODE_SET_REGISTER,
@@ -70,15 +64,29 @@ static u32 intel_dp_aux_get_backlight(struct intel_connector *connector)
 		drm_dbg_kms(&i915->drm,
 			    "Failed to read the DPCD register 0x%x\n",
 			    DP_EDP_BACKLIGHT_MODE_SET_REGISTER);
-		return 0;
+		return false;
 	}
 
+	return (mode_reg & DP_EDP_BACKLIGHT_CONTROL_MODE_MASK) ==
+	       DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD;
+}
+
+/*
+ * Read the current backlight value from DPCD register(s) based
+ * on if 8-bit(MSB) or 16-bit(MSB and LSB) values are supported
+ */
+static u32 intel_dp_aux_get_backlight(struct intel_connector *connector)
+{
+	struct intel_dp *intel_dp = intel_attached_dp(connector);
+	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
+	u8 read_val[2] = { 0x0 };
+	u16 level = 0;
+
 	/*
 	 * If we're not in DPCD control mode yet, the programmed brightness
 	 * value is meaningless and we should assume max brightness
 	 */
-	if ((mode_reg & DP_EDP_BACKLIGHT_CONTROL_MODE_MASK) !=
-	    DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD)
+	if (!intel_dp_aux_backlight_dpcd_mode(connector))
 		return connector->panel.backlight.max;
 
 	if (drm_dp_dpcd_read(&intel_dp->aux, DP_EDP_BACKLIGHT_BRIGHTNESS_MSB,
@@ -319,7 +327,8 @@ static int intel_dp_aux_setup_backlight(struct intel_connector *connector,
 
 	panel->backlight.min = 0;
 	panel->backlight.level = intel_dp_aux_get_backlight(connector);
-	panel->backlight.enabled = panel->backlight.level != 0;
+	panel->backlight.enabled = intel_dp_aux_backlight_dpcd_mode(connector) &&
+				   panel->backlight.level != 0;
 
 	return 0;
 }