Re: "Windows 2012" OSI causes backlight problems on current ThinkPad firmware

[Ben Jencks <ben@bjencks.net>]

On 12/24/2012 06:05 PM, joeyli wrote:
> 於 一，2012-12-24 於 16:32 -0500，Ben Jencks 提到：
>> The addition of "Windows 2012" to the OSI strings in the 3.7 cycle has
>> caused backlight brightness adjustment to break on my ThinkPad T530.
>>
>> Without "Windows 2012" _BCL returns the actual 16 brightness levels:
>>
>> 5, 10, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 80, 90, 100
>>
>> When "Windows 2012" is present, it returns 101 levels 0-100, but _BCM
>> rounds any value up to one of the real values above.
>>
>> This interacts badly with gnome-settings-daemon, which attempts to
>> change brightness in increments of 5 (1/20 of 100). If brightness is
>> currently 100, it attempts to decrement it to 95, but it gets rounded
>> back up to 100, so it's impossible to decrease the brightness.
>>
>> Booting with acpi_osi="!Windows 2012" fixes the problem, though I'm
>> not
>> sure if it introduces others; I see several other things depend on
>> WIN8
>> in DSDT.
>>
>> Perhaps this should be fixed in gnome-settings-daemon, but since it
>> appears on the surface to be a kernel regression (3.6 works, 3.7
>> broken)
>> I thought I'd report it here first. 
> 
> Yes, this is g-s-d's problem, g-s-d should adapt to more levels of
> brightness:
> 
> plugins/power/gsd-power-manager.c
> 
> /* on ACPI machines we have 4-16 levels, on others it's ~150 */
> #define BRIGHTNESS_STEP_AMOUNT(max) ((max) < 20 ? 1 : (max) / 20)

Actually, thinking about this some more, I'm not sure what g-s-d could
do about this. It could behave better if brightness changes don't
"stick", but as long as the hardware/kernel is lying to userspace about
how many brightness levels are available the experience can't be
completely right.

If the exact brightness levels aren't exposed, pressing the brightness
up/down keys will skip a real level or not change the real level at all
in some cases.

Would it be possible to add a quirk in to exclude "Windows 2012" on this
hardware/bios?

-Ben
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