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. > > OK, and if GUI tool like g-s-d decides to go some more steps when > adjusting backlight level, it may always choose the non-functional > values. Hmm, doesn't seem to be an usable way then. Exactly. > 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. > > I submitted a second set of patches [1] which writes all intermediate > > values between the old and new brightness values and disables _BQC for > > these machines (empirically rather than using a quirk table), though no > > one seems to be interested in reviewing them. > > Suppose we are at level 100, and the user sets the target level to be > 20, then we will need to call _BCM 80 times? Yes. And on machines where _BQC actually returns legitimate values it will get called 80 times as well. On these Lenovo's we'd quickly determine that _BQC doesn't work and stop trying to use it however. Seth -- 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
