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Subject[PATCH] acpi: video: improve quirk check
If the _BCL package is descending, the first level (br->levels[2]) will
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. But if
the _BCL package 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

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

Signed-off-by: Felipe Contreras <>

On top of this we might want to test yet another value, because br->levels[3]
might be the current value (although very unlikely).

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 0ec434d..e1284b8 100644
--- 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)

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