3 files changed, 63 insertions, 7 deletions

diff --git a/drivers/gpu/drm/i915/i915_params.c b/drivers/gpu/drm/i915/i915_params.c
index 14e2c2e57f96..5b5ab15d191f 100644
--- a/drivers/gpu/drm/i915/i915_params.c
+++ b/drivers/gpu/drm/i915/i915_params.c
@@ -63,7 +63,7 @@ struct i915_params i915 __read_mostly = {
 	.huc_firmware_path = NULL,
 	.enable_dp_mst = true,
 	.inject_load_failure = 0,
-	.enable_dpcd_backlight = false,
+	.enable_dpcd_backlight = -1,
 	.enable_gvt = false,
 };
 
@@ -246,9 +246,10 @@ MODULE_PARM_DESC(enable_dp_mst,
 module_param_named_unsafe(inject_load_failure, i915.inject_load_failure, uint, 0400);
 MODULE_PARM_DESC(inject_load_failure,
 	"Force an error after a number of failure check points (0:disabled (default), N:force failure at the Nth failure check point)");
-module_param_named(enable_dpcd_backlight, i915.enable_dpcd_backlight, bool, 0600);
+module_param_named_unsafe(enable_dpcd_backlight, i915.enable_dpcd_backlight, int, 0600);
 MODULE_PARM_DESC(enable_dpcd_backlight,
-	"Enable support for DPCD backlight control (default:false)");
+	"Enable support for DPCD backlight control "
+	"(-1:auto (default), 0:force disable, 1:force enabled if supported");
 
 module_param_named(enable_gvt, i915.enable_gvt, bool, 0400);
 MODULE_PARM_DESC(enable_gvt,
diff --git a/drivers/gpu/drm/i915/i915_params.h b/drivers/gpu/drm/i915/i915_params.h
index febbfdbd30bd..0d6cf9138dc4 100644
--- a/drivers/gpu/drm/i915/i915_params.h
+++ b/drivers/gpu/drm/i915/i915_params.h
@@ -53,6 +53,7 @@
 	func(int, edp_vswing); \
 	func(int, reset); \
 	func(unsigned int, inject_load_failure); \
+	func(int, enable_dpcd_backlight); \
 	/* leave bools at the end to not create holes */ \
 	func(bool, alpha_support); \
 	func(bool, enable_cmd_parser); \
@@ -66,7 +67,6 @@
 	func(bool, verbose_state_checks); \
 	func(bool, nuclear_pageflip); \
 	func(bool, enable_dp_mst); \
-	func(bool, enable_dpcd_backlight); \
 	func(bool, enable_gvt)
 
 #define MEMBER(T, member) T member
diff --git a/drivers/gpu/drm/i915/intel_dp_aux_backlight.c b/drivers/gpu/drm/i915/intel_dp_aux_backlight.c
index d2830ba3162e..fea161727c6e 100644
--- a/drivers/gpu/drm/i915/intel_dp_aux_backlight.c
+++ b/drivers/gpu/drm/i915/intel_dp_aux_backlight.c
@@ -251,15 +251,66 @@ intel_dp_aux_display_control_capable(struct intel_connector *connector)
 	/* Check the eDP Display control capabilities registers to determine if
 	 * the panel can support backlight control over the aux channel
 	 */
-	if (intel_dp->edp_dpcd[1] & DP_EDP_TCON_BACKLIGHT_ADJUSTMENT_CAP &&
-	    (intel_dp->edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_AUX_SET_CAP) &&
-	    !(intel_dp->edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_PWM_PIN_CAP)) {
+	if ((intel_dp->edp_dpcd[1] & DP_EDP_TCON_BACKLIGHT_ADJUSTMENT_CAP) &&
+	    (intel_dp->edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_AUX_SET_CAP)) {
 		DRM_DEBUG_KMS("AUX Backlight Control Supported!\n");
 		return true;
 	}
 	return false;
 }
 
+/*
+ * Heuristic function whether we should use AUX for backlight adjustment or not.
+ *
+ * We should use AUX for backlight brightness adjustment if panel doesn't this
+ * via PWM pin or using AUX is better than using PWM pin.
+ *
+ * The heuristic to determine that using AUX pin is better than using PWM pin is
+ * that the panel support any of the feature list here.
+ * - Regional backlight brightness adjustment
+ * - Backlight PWM frequency set
+ * - More than 8 bits resolution of brightness level
+ * - Backlight enablement via AUX and not by BL_ENABLE pin
+ *
+ * If all above are not true, assume that using PWM pin is better.
+ */
+static bool
+intel_dp_aux_display_control_heuristic(struct intel_connector *connector)
+{
+	struct intel_dp *intel_dp = enc_to_intel_dp(&connector->encoder->base);
+	uint8_t reg_val;
+
+	/* Panel doesn't support adjusting backlight brightness via PWN pin */
+	if (!(intel_dp->edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_PWM_PIN_CAP))
+		return true;
+
+	/* Panel supports regional backlight brightness adjustment */
+	if (drm_dp_dpcd_readb(&intel_dp->aux, DP_EDP_GENERAL_CAP_3,
+			      &reg_val) != 1) {
+		DRM_DEBUG_KMS("Failed to read DPCD register 0x%x\n",
+			       DP_EDP_GENERAL_CAP_3);
+		return false;
+	}
+	if (reg_val > 0)
+		return true;
+
+	/* Panel supports backlight PWM frequency set */
+	if (intel_dp->edp_dpcd[2] & DP_EDP_BACKLIGHT_FREQ_AUX_SET_CAP)
+		return true;
+
+	/* Panel supports more than 8 bits resolution of brightness level */
+	if (intel_dp->edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_BYTE_COUNT)
+		return true;
+
+	/* Panel supports enabling backlight via AUX but not by BL_ENABLE pin */
+	if ((intel_dp->edp_dpcd[1] & DP_EDP_BACKLIGHT_AUX_ENABLE_CAP) &&
+	    !(intel_dp->edp_dpcd[1] & DP_EDP_BACKLIGHT_PIN_ENABLE_CAP))
+		return true;
+
+	return false;
+
+}
+
 int intel_dp_aux_init_backlight_funcs(struct intel_connector *intel_connector)
 {
 	struct intel_panel *panel = &intel_connector->panel;
@@ -270,6 +321,10 @@ int intel_dp_aux_init_backlight_funcs(struct intel_connector *intel_connector)
 	if (!intel_dp_aux_display_control_capable(intel_connector))
 		return -ENODEV;
 
+	if (i915.enable_dpcd_backlight == -1 &&
+	    !intel_dp_aux_display_control_heuristic(intel_connector))
+		return -ENODEV;
+
 	panel->backlight.setup = intel_dp_aux_setup_backlight;
 	panel->backlight.enable = intel_dp_aux_enable_backlight;
 	panel->backlight.disable = intel_dp_aux_disable_backlight;