linux-kernel - [PATCH 3.11 24/36] ACPI / video: Quirk initial backlight level 0

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Date:	Tue, 26 Nov 2013 10:12:34 -0800
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Qingshuai Tian <qingshuai.tian@...el.com>,
	Aaron Lu <aaron.lu@...el.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: [PATCH 3.11 24/36] ACPI / video: Quirk initial backlight level 0

3.11-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Aaron Lu <aaron.lu@...el.com>

commit 2c62333a408f5badd2d2ffd7177f95deeccc5ca4 upstream.

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.

References: https://bugzilla.kernel.org/show_bug.cgi?id=64031
References: https://bugzilla.kernel.org/show_bug.cgi?id=61231
References: https://bugzilla.kernel.org/show_bug.cgi?id=63111
Reported-by: Qingshuai Tian <qingshuai.tian@...el.com>
Tested-by: Aaron Lu <aaron.lu@...el.com> # on "Idealpad u330p"
Reported-and-tested-by: <erno@....fi> # on "Acer Aspire V5-573G"
Reported-and-tested-by: Kirill Tkhai <tkhai@...dex.ru> # on "HP 250 G1"
Signed-off-by: Aaron Lu <aaron.lu@...el.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 drivers/acpi/video.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/acpi/video.c
+++ b/drivers/acpi/video.c
@@ -848,7 +848,7 @@ acpi_video_init_brightness(struct acpi_v
 		for (i = 2; i < br->count; i++)
 			if (level_old == br->levels[i])
 				break;
-		if (i == br->count)
+		if (i == br->count || !level)
 			level = max_level;
 	}

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