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[Po-Hsu Lin <po-hsu.lin@xxxxxxxxxxxxx>]

** Changed in: linux-hwe (Ubuntu)
       Status: Fix Committed => Fix Released

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https://bugs.launchpad.net/bugs/1749420

Title:
  [regression] Colour banding and artefacts appear system-wide on an
  Asus Zenbook UX303LA with Intel HD 4400 graphics

Status in linux package in Ubuntu:
  Fix Released
Status in linux-hwe package in Ubuntu:
  Fix Released
Status in linux source package in Xenial:
  Fix Released
Status in linux-hwe source package in Xenial:
  Fix Released
Status in linux source package in Artful:
  Fix Released
Status in linux source package in Bionic:
  Fix Released

Bug description:
  === SRU Justification ===
  [Impact]
  Color dithering not working correctly because the graphics driver
  assumes the panel support 8bpc, but the panel only supports 6bpc.

  [Test]
  User reports the issue start appearing from v4.8-rc2.
  The issue is fixed with the patch.

  [Fix]
  Add a quirk to tell DRM that this panel only supports 6bpc.

  [Regression Potential]
  Low. This fix matches a very specific panel.

  === Original Bug Report ===
  - Ubuntu 18.04 (with updates as of 14 Feb 2018).
  - Laptop: Asus Zenbook UX303
  - CPU: Intel i5 4210U
  - Graphics: Intel HD 4400

  Problem description: Colour artefacts and banding in all apps.
  Graphics, photos and videos display as though the colour range is
  improperly configured.

  See this photo of a 1080p video taken with a camera (poor quality, but
  illustrates the issue): https://ubuntucommunity.s3-us-
  east-2.amazonaws.com/original/2X/c/c8f266715226953c6a77db6367ff9b7895784e35.jpg

  Look at the sky’s light colours. They display with smooth gradients on
  Ubuntu 16.04, but as the photo illustrates 18.04 seems to display the
  video with a smaller range of white colours. This is the most apparent
  with either very dark or very light colours. Even on window titlebars
  the banding is visible (displayed as lines with different shades of
  grey/black).

  Compared with a PrtSc shot of the same image:
  https://i.imgur.com/VkOSCaQ.png

  The artefacts are not present on PrtSc shots that are taken on Ubuntu
  18.04 but viewed on a different system (16.04 and Windows).

  Installing a previous kernel version fixes the issue completely. I
  used UKUU Update Utility to install kernel 4.4 (the same that I use in
  Ubuntu 16.04 without issues) and upon a reboot with 4.4 running on
  18.04 everything displayed smoothly. So this indicates that the system
  might be using a sub-optimal colour depth with the newer kernel?

  ProblemType: Bug
  DistroRelease: Ubuntu 18.04
  Package: linux-image-4.13.0-32-generic 4.13.0-32.35 [modified: boot/vmlinuz-4.13.0-32-generic]
  ProcVersionSignature: Ubuntu 4.13.0-32.35-generic 4.13.13
  Uname: Linux 4.13.0-32-generic x86_64
  ApportVersion: 2.20.8-0ubuntu10
  Architecture: amd64
  AudioDevicesInUse:
   USER        PID ACCESS COMMAND
   /dev/snd/controlC0:  ystein     1344 F.... pulseaudio
   /dev/snd/controlC1:  ystein     1344 F.... pulseaudio
  CurrentDesktop: ubuntu:GNOME
  Date: Wed Feb 14 11:25:43 2018
  EcryptfsInUse: Yes
  HibernationDevice: RESUME=UUID=d0bae648-a4d8-454c-b9fc-4cc04874d6c1
  InstallationDate: Installed on 2018-02-12 (1 days ago)
  InstallationMedia: Ubuntu 18.04 LTS "Bionic Beaver" - Alpha amd64 (20180210)
  Lsusb:
   Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
   Bus 001 Device 004: ID 8087:07dc Intel Corp.
   Bus 001 Device 003: ID 064e:9700 Suyin Corp. Asus Integrated Webcam
   Bus 001 Device 002: ID 046d:c52b Logitech, Inc. Unifying Receiver
   Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
  MachineType: ASUSTeK COMPUTER INC. UX303LA
  ProcFB: 0 inteldrmfb
  ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.13.0-32-generic.efi.signed root=UUID=25279d7b-cc8a-4fee-9088-cfc69607cbca ro quiet splash vt.handoff=1
  RelatedPackageVersions:
   linux-restricted-modules-4.13.0-32-generic N/A
   linux-backports-modules-4.13.0-32-generic  N/A
   linux-firmware                             1.170
  SourcePackage: linux
  UpgradeStatus: No upgrade log present (probably fresh install)
  dmi.bios.date: 09/01/2014
  dmi.bios.vendor: American Megatrends Inc.
  dmi.bios.version: UX303LA.204
  dmi.board.asset.tag: ATN12345678901234567
  dmi.board.name: UX303LA
  dmi.board.vendor: ASUSTeK COMPUTER INC.
  dmi.board.version: 1.0
  dmi.chassis.asset.tag: No Asset Tag
  dmi.chassis.type: 10
  dmi.chassis.vendor: ASUSTeK COMPUTER INC.
  dmi.chassis.version: 1.0
  dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvrUX303LA.204:bd09/01/2014:svnASUSTeKCOMPUTERINC.:pnUX303LA:pvr1.0:rvnASUSTeKCOMPUTERINC.:rnUX303LA:rvr1.0:cvnASUSTeKCOMPUTERINC.:ct10:cvr1.0:
  dmi.product.family: UX
  dmi.product.name: UX303LA
  dmi.product.version: 1.0
  dmi.sys.vendor: ASUSTeK COMPUTER INC.

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