[PATCH 2/2] drm/i915: do not setup backlight if not available according to VBT

[Jani Nikula <jani.nikula@intel.com>]

Some machines use an external EC for controlling the backlight. Info
about this is present in the VBT. Do not setup native backlight control
if no PWM backlight is available or supported according to VBT. The
acpi_backlight interface appears to work for the EC control.

In most cases there has been no harm done, but it looks like there are
machines out there that have both an EC and our PWM line connected to
the same wire. This, obviously, does not end well.

This should fix the regression caused by
commit bc0bb9fd1c7810407ab810d204bbaecb255fddde
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Thu Nov 14 12:14:29 2013 +0200

    drm/i915: remove QUIRK_NO_PCH_PWM_ENABLE

AFAICT the quirk removed by the above commit effectively resulted in
i915 not driving the backlight PWM output, thus not messing things up.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76276
Reference: https://bugzilla.kernel.org/show_bug.cgi?id=47941
CC: Aaron Lu <aaron.lu@intel.com>
CC: Kamal Mostafa <kamal@canonical.com>
CC: Eric Griffith <EGriffith92@gmail.com>
CC: Kent Baxley <kent.baxley@canonical.com>
CC: stable@vger.kernel.org [v3.14+]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
---
 drivers/gpu/drm/i915/intel_panel.c |    5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
index cb058408c70e..0eead16aeda7 100644
--- a/drivers/gpu/drm/i915/intel_panel.c
+++ b/drivers/gpu/drm/i915/intel_panel.c
@@ -1065,6 +1065,11 @@ int intel_panel_setup_backlight(struct drm_connector *connector)
 	unsigned long flags;
 	int ret;
 
+	if (!dev_priv->vbt.backlight.present) {
+		DRM_DEBUG_KMS("native backlight control not available per VBT\n");
+		return 0;
+	}
+
 	/* set level and max in panel struct */
 	spin_lock_irqsave(&dev_priv->backlight_lock, flags);
 	ret = dev_priv->display.setup_backlight(intel_connector);
-- 
1.7.9.5