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ACPI / video: improve quirk check in acpi_video_bqc_quirk()

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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>
This commit is contained in:
Felipe Contreras 2013-08-03 23:00:25 +02:00 committed by Rafael J. Wysocki
parent c095ba7224
commit b3b301c5fe
1 changed files with 1 additions and 1 deletions
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2
drivers/acpi/video.c
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@ -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)