On 04/03/2013 09:45 PM, Seth Forshee wrote: > 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. Then let's try to work around this problem in acpi video driver. Thanks, Aaron > > 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 > -- 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
