ACPI / video: improve quirk check in acpi_video_bqc_quirk()

If the _BCL package ordering is descending, the first level
(br->levels[2]) is likely to be 0, and if the number of levels
matches the number of steps, we might confuse a returned level to
mean the index.

For example:

  current_level = max_level = 100
  test_level = 0
  returned level = 100

In this case 100 means the level, not the index, and _BCM failed.
Still, if the _BCL package ordering is descending, the index of
level 0 is also 100, so we assume _BQC is indexed, when it's not.

This causes all _BQC calls to return bogus values causing weird
behavior from the user's perspective.  For example:

xbacklight -set 10; xbacklight -set 20;

would flash to 90% and then slowly down to the desired level (20).

The solution is simple; test anything other than the first level
(e.g. 1).

[rjw: Changelog]
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

diff --git a/drivers/acpi/video.c b/drivers/acpi/video.c
index 0ec434d..e1284b8 100644 (file)
--- a/drivers/acpi/video.c
+++ b/drivers/acpi/video.c
@@ -689,7 +689,7 @@ static int acpi_video_bqc_quirk(struct acpi_video_device *device,
         * Some systems always report current brightness level as maximum
         * through _BQC, we need to test another value for them.
         */
-       test_level = current_level == max_level ? br->levels[2] : max_level;
+       test_level = current_level == max_level ? br->levels[3] : max_level;
 
        result = acpi_video_device_lcd_set_level(device, test_level);
        if (result)