[PATCH 2/2] ACPI / video: Quirk initial backlight level 0

[Aaron Lu <aaron.lu@intel.com>]

Some firmware doesn't initialize initial backlight level to a proper
value and _BQC will return 0 on first time evaluation. We used to be
able to detect such incorrect value with our code logic, as value 0
normally isn't a valid value in _BCL. But with the introduction of Win8,
firmware begins to fill _BCL with values from 0 to 100, now 0 becomes
a valid value but that value will make user's screen black. This patch
test initial _BQC for value 0, if such a value is returned, do not use
it.

Reference: https://bugzilla.kernel.org/show_bug.cgi?id=64031
Reference: https://bugzilla.kernel.org/show_bug.cgi?id=61231
Reference: https://bugzilla.kernel.org/show_bug.cgi?id=63111
Reported-by: Qingshuai Tian <qingshuai.tian@intel.com>
Tested-by: Aaron Lu <aaron.lu@intel.com> on "Idealpad u330p"
Reported-and-tested-by: <erno@iki.fi> on "Acer Aspire V5-573G"
Reported-and-tested-by: Kirill Tkhai <tkhai@yandex.ru> on "HP 250 G1"
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Cc: stable <stable@vger.kernel.org>
---
 drivers/acpi/video.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/acpi/video.c b/drivers/acpi/video.c
index bf521b36c2f9..a049fa9360d0 100644
--- a/drivers/acpi/video.c
+++ b/drivers/acpi/video.c
@@ -858,7 +858,7 @@ acpi_video_init_brightness(struct acpi_video_device *device)
 		for (i = 2; i < br->count; i++)
 			if (level == br->levels[i])
 				break;
-		if (i == br->count)
+		if (i == br->count || !level)
 			level = max_level;
 	}
 
-- 
1.8.4.39.ga0d3f10