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Hans de Goede 81cc7e9947 ACPI: video: Check LCD flag on ACPI-reduced-hardware devices 
Starting with Windows 8, Windows no longer uses the ACPI-video interface
for backlight control by default. Instead backlight control is left up
to the GPU drivers and these are typically directly accessing the GPU
for this instead of going through ACPI.

This means that the ACPI video interface is no longer being tested by
many vendors, which leads to false-positive /sys/class/backlight entries
on devices which don't have a backlight at all such as desktops or
top-set boxes. These false-positives causes desktop environments to show
a non functional brightness slider in various places.

Checking the LCD flag greatly reduces the amount of false-positives,
so commit 5928c28152 ("ACPI / video: Default lcd_only to true on
Win8-ready and newer machines") enabled the checking of this flag
by default on all win8 BIOS-es. But this let to regressions on some
models, so the check was made stricter adding a DMI chassis-type check
to only enable the LCD flag checking on desktop/server chassis.

Unfortunately the chassis-type reported in the DMI strings is not always
reliable. One class of devices where this is a problem is Intel Bay Trail-T
based top-set boxes / mini PCs / HDMI sticks. These are based on reference
designs which were targetets and the reference design BIOS code
is often used without changing the chassis-type to something more
appropriate.

There are many, many Bay Trail-T based devices affected by this, so DMI
quirking our way out of this is a bad idea. This patch takes a different
approach, Bay Trail-T (unlike regular Bay Trail) is an ACPI-reduced-hw
platform and ACPI-reduced-hw platforms generally don't have
an embedded-controller and thus will use a native (GPU specific) backlight
interface. This patch enables Checking the LCD flag by default on
ACPI-reduced-hw platforms with a win8 BIOS independent of the reported
chassis-type, fixing the false positive /sys/class/backlight entries
on these devices.

Note in hindsight I should have never added the DMI chassis-type check
when the enabling of LCD flag checking on Windows 8 BIOS-es let to some
regressions. Instead I should have added DMI quirks for the (presumably
few) models where the LCD flag check let to issues. But I'm afraid that
it is too late to change this now, changing this now will likely lead to
a bunch of regressions.

This patch was tested on a Mele PCG03 mini PC.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
History 2021-04-08 19:50:58 +02:00
arch RISC-V Fixes for 5.12-rc6 2021-04-03 11:52:18 -07:00
block block: remove the unused RQF_ALLOCED flag 2021-04-02 11:18:31 -06:00
certs certs: Replace K{U,G}IDT_INIT() with GLOBAL_ROOT_{U,G}ID 2021-01-21 16:16:10 +00:00
crypto crypto: mips/poly1305 - enable for all MIPS processors 2021-03-08 11:52:17 +01:00
Documentation arm64 fixes for -rc5 2021-03-25 11:07:40 -07:00
drivers ACPI: video: Check LCD flag on ACPI-reduced-hardware devices 2021-04-08 19:50:58 +02:00
fs io_uring-5.12-2021-04-03 2021-04-03 14:26:47 -07:00
include ACPI: utils: Add acpi_reduced_hardware() helper 2021-04-08 19:50:12 +02:00
init Merge branch 'akpm' (patches from Andrew) 2021-03-14 12:23:34 -07:00
ipc fs: make helpers idmap mount aware 2021-01-24 14:27:20 +01:00
kernel Fix stack trace entry size to stop showing garbage 2021-04-02 08:39:00 -07:00
lib XArray: Fix splitting to non-zero orders 2021-03-30 13:42:33 -04:00
LICENSES LICENSES: Add the CC-BY-4.0 license 2020-12-08 10:33:27 -07:00
mm mm: fix race by making init_zero_pfn() early_initcall 2021-03-30 09:46:12 -07:00
net Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2021-03-24 18:16:04 -07:00
samples Merge git://git.kernel.org:/pub/scm/linux/kernel/git/netdev/net 2021-03-09 17:15:56 -08:00
scripts kbuild: lto: Merge module sections if and only if CONFIG_LTO_CLANG is enabled 2021-04-01 14:15:59 -07:00
security tomoyo: don't special case PF_IO_WORKER for PF_KTHREAD 2021-03-28 13:11:29 +09:00
sound sound fixes for 5.12-rc6 2021-04-01 10:09:31 -07:00
tools x86: 2021-04-01 12:42:55 -07:00
usr Kbuild updates for v5.12 2021-02-25 10:17:31 -08:00
virt KVM: x86/mmu: Consider the hva in mmu_notifier retry 2021-02-22 13:16:53 -05:00
.clang-format cxl for 5.12 2021-02-24 09:38:36 -08:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore clang-lto series for v5.12-rc1 2021-02-23 09:28:51 -08:00
.mailmap Merge branch 'akpm' (patches from Andrew) 2021-03-25 11:43:43 -07:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS treewide: Miguel has moved 2021-02-26 09:41:03 -08:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS Char/Misc driver fixes for 5.12-rc6 2021-04-03 10:05:16 -07:00
Makefile Linux 5.12-rc6 2021-04-04 14:15:36 -07:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.