[PATCH] i915: Don't register backlight when max PWM value is unknown

[Grant Likely]

When a backlight isn't connected to the i915 it doesn't make any sense
to register the backlight device, but the driver currently tries to limp
along using a max brightness value of 1. Instead, this patch makes it so
that if the maximum PWM value cannot be determined, then the backlight
will not be registered.

Tested on MacbookPro8,3.

Signed-off-by: Grant Likely <grant.likely at secretlab.ca>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: David Airlie <airlied at linux.ie>
Cc: Matthew Garrett <mjg at redhat.com>
Cc: David Woodhouse <dwmw2 at infradead.org>
---
 drivers/gpu/drm/i915/intel_panel.c |   15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
index 3df4f5f..f410c6e 100644
--- a/drivers/gpu/drm/i915/intel_panel.c
+++ b/drivers/gpu/drm/i915/intel_panel.c
@@ -168,13 +168,8 @@ u32 intel_panel_get_max_backlight(struct drm_device *dev)
 	u32 max;
 
 	max = i915_read_blc_pwm_ctl(dev_priv);
-	if (max == 0) {
-		/* XXX add code here to query mode clock or hardware clock
-		 * and program max PWM appropriately.
-		 */
-		pr_warn_once("fixme: max PWM is zero\n");
-		return 1;
-	}
+	if (max == 0)
+		return 0; /* Cannot read max PWM. Assume no backlight */
 
 	if (HAS_PCH_SPLIT(dev)) {
 		max >>= 16;
@@ -413,6 +408,12 @@ int intel_panel_setup_backlight(struct drm_device *dev)
 	struct backlight_properties props;
 	struct drm_connector *connector;
 
+	/* Is there a backlight present? max will be zero if not */
+	if (intel_panel_get_max_backlight(dev) == 0) {
+		DRM_INFO("i915 doesn't seem to be connected to backlight\n");
+		return 0;
+	}
+
 	intel_panel_init_backlight(dev);
 
 	if (dev_priv->int_lvds_connector)
-- 
1.7.9.5