drm/i915: Add support for panels with VESA backlights with PWM enable/disable

This simply adds proper support for panel backlights that can be controlled
via VESA's backlight control protocol, but which also require that we
enable and disable the backlight via PWM instead of via the DPCD interface.
We also enable this by default, in order to fix some people's backlights
that were broken by not having this enabled.

For reference, backlights that require this and use VESA's backlight
interface tend to be laptops with hybrid GPUs, but this very well may
change in the future.

v4:
* Make sure that we call intel_backlight_level_to_pwm() in
  intel_dp_aux_vesa_enable_backlight() - vsyrjala
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://gitlab.freedesktop.org/drm/intel/-/issues/3680
Fixes: fe7d52bc ("drm/i915/dp: Don't use DPCD backlights that need PWM enable/disable")
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: <stable@vger.kernel.org> # v5.12+
Link: https://patchwork.freedesktop.org/patch/msgid/20211105183342.130810-2-lyude@redhat.com

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

@@ -293,6 +293,13 @@ intel_dp_aux_vesa_enable_backlight(const struct intel_crtc_state *crtc_state,
 	struct intel_panel *panel = &connector->panel;
 	struct intel_dp *intel_dp = enc_to_intel_dp(connector->encoder);
 
+	if (!panel->backlight.edp.vesa.info.aux_enable) {
+		u32 pwm_level = intel_backlight_invert_pwm_level(connector,
+								 panel->backlight.pwm_level_max);
+
+		panel->backlight.pwm_funcs->enable(crtc_state, conn_state, pwm_level);
+	}
+
 	drm_edp_backlight_enable(&intel_dp->aux, &panel->backlight.edp.vesa.info, level);
 }
 
@@ -304,6 +311,10 @@ static void intel_dp_aux_vesa_disable_backlight(const struct drm_connector_state
 	struct intel_dp *intel_dp = enc_to_intel_dp(connector->encoder);
 
 	drm_edp_backlight_disable(&intel_dp->aux, &panel->backlight.edp.vesa.info);
+
+	if (!panel->backlight.edp.vesa.info.aux_enable)
+		panel->backlight.pwm_funcs->disable(old_conn_state,
+						    intel_backlight_invert_pwm_level(connector, 0));
 }
 
 static int intel_dp_aux_vesa_setup_backlight(struct intel_connector *connector, enum pipe pipe)
@@ -321,6 +332,15 @@ static int intel_dp_aux_vesa_setup_backlight(struct intel_connector *connector,
 	if (ret < 0)
 		return ret;
 
+	if (!panel->backlight.edp.vesa.info.aux_enable) {
+		ret = panel->backlight.pwm_funcs->setup(connector, pipe);
+		if (ret < 0) {
+			drm_err(&i915->drm,
+				"Failed to setup PWM backlight controls for eDP backlight: %d\n",
+				ret);
+			return ret;
+		}
+	}
 	panel->backlight.max = panel->backlight.edp.vesa.info.max;
 	panel->backlight.min = 0;
 	if (current_mode == DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD) {
@@ -340,12 +360,7 @@ intel_dp_aux_supports_vesa_backlight(struct intel_connector *connector)
 	struct intel_dp *intel_dp = intel_attached_dp(connector);
 	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
 
-	/* TODO: We currently only support AUX only backlight configurations, not backlights which
-	 * require a mix of PWM and AUX controls to work. In the mean time, these machines typically
-	 * work just fine using normal PWM controls anyway.
-	 */
-	if ((intel_dp->edp_dpcd[1] & DP_EDP_BACKLIGHT_AUX_ENABLE_CAP) &&
-	    drm_edp_backlight_supported(intel_dp->edp_dpcd)) {
+	if (drm_edp_backlight_supported(intel_dp->edp_dpcd)) {
 		drm_dbg_kms(&i915->drm, "AUX Backlight Control Supported!\n");
 		return true;
 	}