ACPI: video: Fix reversed brightness behavior on ThinkPad SL series

Section B.6.2 of ACPI 3.0b specification that defines _BCL method
doesn't require the brightness levels returned to be sorted.
At least ThinkPad SL300 (and probably all IdeaPads) returns the
array reversed (i.e. bightest levels have lowest indexes), which
causes the brightness management behave in completely reversed
manner on these machines (brightness increases when the laptop is
idle, while the display dims when used).

Sorting the array by brightness level values after reading the list
fixes the issue.

http://bugzilla.kernel.org/show_bug.cgi?id=12037

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Tested-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Len Brown <len.brown@intel.com>

drivers/acpi/video.c

@@ -36,6 +36,7 @@
 #include <linux/backlight.h>
 #include <linux/thermal.h>
 #include <linux/video_output.h>
+#include <linux/sort.h>
 #include <asm/uaccess.h>
 
 #include <acpi/acpi_bus.h>
@@ -625,6 +626,16 @@ acpi_video_bus_DOS(struct acpi_video_bus *video, int bios_flag, int lcd_flag)
 	return status;
 }
 
+/*
+ * Simple comparison function used to sort backlight levels.
+ */
+
+static int
+acpi_video_cmp_level(const void *a, const void *b)
+{
+	return *(int *)a - *(int *)b;
+}
+
 /*
  *  Arg:	
  *  	device	: video output device (LCD, CRT, ..)
@@ -676,6 +687,10 @@ acpi_video_init_brightness(struct acpi_video_device *device)
 		count++;
 	}
 
+	/* don't sort the first two brightness levels */
+	sort(&br->levels[2], count - 2, sizeof(br->levels[2]),
+		acpi_video_cmp_level, NULL);
+
 	if (count < 2)
 		goto out_free_levels;