drm/i915: use default 200 Hz backlight frequency

If the backlight modulation frequency can't be extracted from the
registers or from VBT, use 200 Hz as the default. This may enable
backlight on some machines that previously failed.
Reviewed-by: NImre Deak <imre.deak@intel.com>
Tested-by: NImre Deak <imre.deak@intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448958232-26520-2-git-send-email-jani.nikula@intel.com

drivers/gpu/drm/i915/intel_panel.c

@@ -1393,14 +1393,18 @@ static u32 get_backlight_max_vbt(struct intel_connector *connector)
 	u16 pwm_freq_hz = dev_priv->vbt.backlight.pwm_freq_hz;
 	u32 pwm;
 
-	if (!pwm_freq_hz) {
-		DRM_DEBUG_KMS("backlight frequency not specified in VBT\n");
+	if (!panel->backlight.hz_to_pwm) {
+		DRM_DEBUG_KMS("backlight frequency conversion not supported\n");
 		return 0;
 	}
 
-	if (!panel->backlight.hz_to_pwm) {
-		DRM_DEBUG_KMS("backlight frequency setting from VBT currently not supported on this platform\n");
-		return 0;
+	if (pwm_freq_hz) {
+		DRM_DEBUG_KMS("VBT defined backlight frequency %u Hz\n",
+			      pwm_freq_hz);
+	} else {
+		pwm_freq_hz = 200;
+		DRM_DEBUG_KMS("default backlight frequency %u Hz\n",
+			      pwm_freq_hz);
 	}
 
 	pwm = panel->backlight.hz_to_pwm(connector, pwm_freq_hz);
@@ -1409,8 +1413,6 @@ static u32 get_backlight_max_vbt(struct intel_connector *connector)
 		return 0;
 	}
 
-	DRM_DEBUG_KMS("backlight frequency %u Hz from VBT\n", pwm_freq_hz);
-
 	return pwm;
 }