On 04/02/2013 09:00 AM, Seth Forshee wrote: > On Tue, Apr 02, 2013 at 05:08:23PM +0800, Aaron Lu wrote: >> On 04/01/2013 09:03 PM, Seth Forshee wrote: >>> On Mon, Apr 01, 2013 at 09:53:36AM +0800, Aaron Lu wrote: >>>> On 02/12/2013 12:21 AM, Seth Forshee wrote: >>>>> The AML implementation for brightness control on several ThinkPads >>>>> contains a workaround to meet a Windows 8 requirement of 101 brightness >>>>> levels [1]. The implementation is flawed, as only 16 of the brighness >>>>> values reported by _BCL affect a change in brightness. _BCM silently >>>>> discards the rest of the values. Disabling Windows 8 compatibility on >>>>> these machines reverts them to the old behavior, making _BCL only report >>>>> the 16 brightness levels which actually work. Add a quirk to do this >>>>> along with a dmi callback to disable Win8 compatibility. >>>> >>>> If we disable the _BQC(i.e. set cap._BQC=0) for these systems, will the >>>> problem go away? If so, I think perhaps we can put these systems into a >>>> _BQC quirk table and set cap._BQC=0 for them. >>> >>> That helps a little, but we're still left with only 16 of the 101 >>> brightness levels causing any change in brightness. The firmware isn't >>> rounding the "bad" values or anything like that; it just silently >>> ignores them. >> >> I really wondered, how Windows handled this, it should have the same >> problem, unless they are not using the acpi video interface? > > I can only guess. > > I think I remember reading that Windows 8 does smooth backlight > transitions, so it may well hit every intermediate brightness value. > Lenovo could also be supplying a driver which rounds values to the > nearest working value or uses some other interface or something else. Just checked; Windows 8 doesn't use the ACPI interface. It seems to have access to at least 100 distinct brightness levels. I'd guess it's using the same interface as /sys/class/backlight/intel_backlight, which on my system has a max_brightness of 4438 and all the values seem to be actually distinct, if not necessarily discernible to the naked eye. -Ben -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
