On Wed, Apr 03, 2013 at 03:27:46PM +0800, Aaron Lu wrote: > On 04/03/2013 03:04 PM, Ben Jencks wrote: > > 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. > > Thanks for the information. > > If all these affected systems have gpu driver's interface, I think we > can simply add them to the video_detect_dmi_table so that no acpi > backlight interface will be created for them, and gpu's interface will > be used by user space and hotkey processing can still be handled by acpi > video driver. I've got two machines here with discrete graphics, one with nvidia and one with AMD (neither one is a Lenovo). With the nvidia machine neither nouveau nor the proprietary driver registers a backlight device. On the AMD machine radeon does register a backlight but the proprietary driver does not. So I think quirking away the acpi_video backlight interface has potential to leave some users without any backlight control at all. 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
