drm/i915: Check error return when converting pipe to connector

An error from intel_get_pipe_from_connector() would mean a bug somewhere
else, but we still should check for it to prevent some other more
obscure bug later.

v2:
- Fall back to a reasonable default instead of bailing out in case of
  error. (Jani)
v3:
- Fix s/PIPE_INVALID/INVALID_PIPE/ typo. (Jani)
v4:
- Fix bogus bracing around WARN() condition. (Ville)

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1494408113-379-5-git-send-email-imre.deak@intel.com

drivers/gpu/drm/i915/intel_panel.c

@@ -888,10 +888,14 @@ static void pch_enable_backlight(struct intel_connector *connector)
 	struct drm_i915_private *dev_priv = to_i915(connector->base.dev);
 	struct intel_panel *panel = &connector->panel;
 	enum pipe pipe = intel_get_pipe_from_connector(connector);
-	enum transcoder cpu_transcoder =
-		intel_pipe_to_cpu_transcoder(dev_priv, pipe);
+	enum transcoder cpu_transcoder;
 	u32 cpu_ctl2, pch_ctl1, pch_ctl2;
 
+	if (!WARN_ON_ONCE(pipe == INVALID_PIPE))
+		cpu_transcoder = intel_pipe_to_cpu_transcoder(dev_priv, pipe);
+	else
+		cpu_transcoder = TRANSCODER_EDP;
+
 	cpu_ctl2 = I915_READ(BLC_PWM_CPU_CTL2);
 	if (cpu_ctl2 & BLM_PWM_ENABLE) {
 		DRM_DEBUG_KMS("cpu backlight already enabled\n");
@@ -973,6 +977,9 @@ static void i965_enable_backlight(struct intel_connector *connector)
 	enum pipe pipe = intel_get_pipe_from_connector(connector);
 	u32 ctl, ctl2, freq;
 
+	if (WARN_ON_ONCE(pipe == INVALID_PIPE))
+		pipe = PIPE_A;
+
 	ctl2 = I915_READ(BLC_PWM_CTL2);
 	if (ctl2 & BLM_PWM_ENABLE) {
 		DRM_DEBUG_KMS("backlight already enabled\n");
@@ -1037,6 +1044,9 @@ static void bxt_enable_backlight(struct intel_connector *connector)
 	enum pipe pipe = intel_get_pipe_from_connector(connector);
 	u32 pwm_ctl, val;
 
+	if (WARN_ON_ONCE(pipe == INVALID_PIPE))
+		pipe = PIPE_A;
+
 	/* Controller 1 uses the utility pin. */
 	if (panel->backlight.controller == 1) {
 		val = I915_READ(UTIL_PIN_CTL);
@@ -1093,6 +1103,7 @@ void intel_panel_enable_backlight(struct intel_connector *connector)
 	if (!panel->backlight.present)
 		return;
 
+	if (!WARN_ON_ONCE(pipe == INVALID_PIPE))
 		DRM_DEBUG_KMS("pipe %c\n", pipe_name(pipe));
 
 	mutex_lock(&dev_priv->backlight_lock);