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

[Sean Paul]

From: Sean Paul <seanpaul at chromium.org>

In commit 79946723092b ("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: 79946723092b ("drm/i915: Assume 100% brightness when not in DPCD control mode")
Cc: Lyude Paul <lyude at redhat.com>
Cc: Jani Nikula <jani.nikula at intel.com>
Cc: Juha-Pekka Heikkila <juhapekka.heikkila at gmail.com>
Cc: "Ville Syrjälä" <ville.syrjala at linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi at intel.com>
Cc: Kevin Chowski <chowski at chromium.org>>
Signed-off-by: Sean Paul <seanpaul at chromium.org>
---
 .../drm/i915/display/intel_dp_aux_backlight.c | 31 ++++++++++++-------
 1 file changed, 20 insertions(+), 11 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
index acbd7eb66cbe..036f504ac7db 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
+++ b/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;
 }
-- 
Sean Paul, Software Engineer, Google / Chromium OS