ACPI / i915: Update the condition to ignore firmware backlight change request

Some of the Thinkpads' firmware will issue a backlight change request
through i915 operation region unconditionally on AC plug/unplug, the
backlight level used is arbitrary and thus should be ignored. This is
handled by commit 0b9f7d93 (ACPI / i915: ignore firmware requests
for backlight change). Then there is a Dell laptop whose vendor backlight
interface also makes use of operation region to change backlight level
and with the above commit, that interface no long works. The condition
used to ignore the backlight change request from firmware is thus
changed to: if the vendor backlight interface is not in use and the ACPI
backlight interface is broken, we ignore the requests; oterwise, we keep
processing them.

Fixes: 0b9f7d93 (ACPI / i915: ignore firmware requests for backlight change)
Link: https://lkml.org/lkml/2014/9/23/854Reported-and-tested-by: Pali Rohár <pali.rohar@gmail.com>
Cc: 3.16+ <stable@vger.kernel.org> # 3.16+
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/gpu/drm/i915/intel_opregion.c

@@ -396,6 +396,16 @@ int intel_opregion_notify_adapter(struct drm_device *dev, pci_power_t state)
 	return -EINVAL;
 }
 
+/*
+ * If the vendor backlight interface is not in use and ACPI backlight interface
+ * is broken, do not bother processing backlight change requests from firmware.
+ */
+static bool should_ignore_backlight_request(void)
+{
+	return acpi_video_backlight_support() &&
+	       !acpi_video_verify_backlight_support();
+}
+
 static u32 asle_set_backlight(struct drm_device *dev, u32 bclp)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
@@ -404,11 +414,7 @@ static u32 asle_set_backlight(struct drm_device *dev, u32 bclp)
 
 	DRM_DEBUG_DRIVER("bclp = 0x%08x\n", bclp);
 
-	/*
-	 * If the acpi_video interface is not supposed to be used, don't
-	 * bother processing backlight level change requests from firmware.
-	 */
-	if (!acpi_video_verify_backlight_support()) {
+	if (should_ignore_backlight_request()) {
 		DRM_DEBUG_KMS("opregion backlight request ignored\n");
 		return 0;
 	}