From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ben Jencks Subject: Re: [PATCH] ACPI: Disable Windows 8 compatibility for some Lenovo ThinkPads Date: Wed, 03 Apr 2013 03:04:43 -0400 Message-ID: <515BD48B.7000901@bjencks.net> References: <1360599681-24781-1-git-send-email-seth.forshee@canonical.com> <5158E8A0.3050904@intel.com> <20130401130340.GA3269@thinkpad-t410> <515AA007.1050305@intel.com> <20130402130000.GA12381@thinkpad-t410> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Return-path: Received: from mail-yh0-f47.google.com ([209.85.213.47]:49198 "EHLO mail-yh0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1762521Ab3DCHKK (ORCPT ); Wed, 3 Apr 2013 03:10:10 -0400 Received: by mail-yh0-f47.google.com with SMTP id z12so188155yhz.34 for ; Wed, 03 Apr 2013 00:10:09 -0700 (PDT) In-Reply-To: <20130402130000.GA12381@thinkpad-t410> Sender: linux-acpi-owner@vger.kernel.org List-Id: linux-acpi@vger.kernel.org To: Seth Forshee Cc: Aaron Lu , Len Brown , "Rafael J. Wysocki" , linux-acpi@vger.kernel.org, joeyli , Matthew Garrett 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