From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.kernel.org ([198.145.29.99]:38384 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750960AbeDOWyL (ORCPT ); Sun, 15 Apr 2018 18:54:11 -0400 Date: Sun, 15 Apr 2018 23:54:06 +0100 From: James Hogan To: Hans de Goede Cc: Zhang Rui , "Rafael J . Wysocki" , Len Brown , linux-acpi@vger.kernel.org, stable@vger.kernel.org, Sasha Levin , Greg Kroah-Hartman Subject: Re: [REGRESSION] [v2] ACPI / video: Default lcd_only to true on Win8 ready and newer machines Message-ID: <20180415225403.GA6373@jamesdev> References: <20171223184147.12016-1-hdegoede@redhat.com> <20180414230746.GA29537@saruman> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8BIT In-Reply-To: Sender: stable-owner@vger.kernel.org List-ID: On Sun, Apr 15, 2018 at 02:04:26PM +0200, Hans de Goede wrote: > Hi, > > On 15-04-18 01:07, James Hogan wrote: > > On Sat, Dec 23, 2017 at 07:41:47PM +0100, Hans de Goede wrote: > > > We're seeing a lot of bogus backlight interfaces on newer machines without > > > a LCD such as desktops, servers and HDMI sticks. This causes userspace to > > > show a non-functional brightness slider in e.g. the GNOME3 system menu, > > > which is undesirable. More in general we should simply just not register > > > a non functional backlight interface. > > > > > > Checking the lcd flag causes the bogus acpi_video backlight interfaces to > > > go away (on the machines this was tested on). > > > > > > This commit enables the lcd_only option by default on any machines which > > > are win8 ready, fixing this. > > > > > > This is not entirely without risk of regressions, but video_detect.c > > > already prefers native-backlight interfaces over the acpi_video one > > > on win8 ready machines, calling acpi_video_unregister_backlight() as soon > > > as a native interface shows up. This is done because the acpi backlight > > > interface often is broken on win8 ready machines, because win8 does not > > > seem to actually use it. > > > > This patch (in the form of commit 965736ee654d ("ACPI / video: Default > > lcd_only to true on Win8-ready and newer machines") in stable v4.15.17), > > breaks backlight control on my 2013 XPS13 laptop. > > > > It normally uses the acpi backlight device, but after this patch that > > device no longer shows up in sysfs. > > > > This isn't the first time the backlight has gotton broken on this > > system, though I think last time it was because the intel backlight > > driver got used instead of the ACPI one and that didn't work properly > > with it, so it needed a quirk to make it use ACPI instead. > > > > Is some other quirk needed around here too? > > Yes looks like it (sigh), but I cannot find an existing quirk for > your laptop. I did at one point have to revert commit bc0bb9fd1c78 ("drm/i915: remove QUIRK_NO_PCH_PWM_ENABLE") for a few kernel releases until 4.3, presumably fixed by commit b029e66fa8e3 ("drm/i915: Backlight control using CRC PMIC based PWM driver"). > When you say quirk do you mean you add something to > the kernel commandline to work around this? No, it was automatic, in the source. I don't know this code well enough to know how they got it working. > Normally we would add > a dmi based quirk so that future versions automatically do the > right thing. I was looking for the existing quirk so that I > could re-use the dmi strings, but I don't see an existing quirk. > > Can you run: > > sudo dmidecode > dell-xps13-2013-dmi.log > > And then send me a *personal* mail with that file attached. > It is best to not send this to the list as it will contain you > machines serial number and other unique information. Will do. Thanks James