Internal display backlight not working on a Intel Apollolake based tablet

[Mikko Kovanen]

Hi,

Problem:
With current kernels (both Ubuntu kernels and mainline kernels) the display backlight in one of our tablet variants does not work because i915 GFX driver lacks support for I2C transfers from display initialization VBT sequences and the backlight driver is left uninitialized.

Ubuntu version:
Ubuntu 20.04 daily build (downloaded on 2020-01-20)

Device:
Aava Mobile Inari10B tablet (Intel Apollolake platform)

Existing fixes:
Intel has implement a kernel fix for this problem and it is currently on its way to 5.6 kernel (https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/drivers/gpu/drm/i915/display/intel_dsi_vbt.c?h=next-20200129&id=8cbf89db294166cc13d90a89422605e0c9f8bbc2). Unfortunately that change is not working correctly as-is, further fix is currently in drm-intel-next-queued (https://cgit.freedesktop.org/drm/drm-intel/commit/?id=de409661c4c90d63cfc64579edbad0a6b10bd50d), which should also end up in 5.6 kernel because it has a fixes tag referring to the initial fix commit.

Background information:
Our company produces Intel Apollolake based tablets and some of our customers would like to use Ubuntu as the OS. Optimal situation would be that we could recommend Ubuntu 20.04 LTS release to our customers.
Since I was unable to find conclusive information regarding which kernel version is going to end up in Ubuntu 20.04 LTS release, I decided to open this bug to resolve if and how the fix for backlight problem could get included in Ubuntu 20.04 LTS release in it's first version.

BR,
Mikko Kovanen

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: linux-image-5.4.0-9-generic 5.4.0-9.12
ProcVersionSignature: Ubuntu 5.4.0-9.12-generic 5.4.3
Uname: Linux 5.4.0-9-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu15
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: ubuntu 1708 F.... pulseaudio
CasperVersion: 1.438
CurrentDesktop: ubuntu:GNOME
Date: Thu Jan 30 07:30:24 2020
LiveMediaBuild: Ubuntu 20.04 LTS "Focal Fossa" - Alpha amd64 (20200124)
MachineType: OEM ApolloLake
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=C.UTF-8
 SHELL=/bin/bash
ProcFB: 0 i915drmfb
ProcKernelCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/username.seed quiet splash ---
RelatedPackageVersions:
 linux-restricted-modules-5.4.0-9-generic N/A
 linux-backports-modules-5.4.0-9-generic N/A
 linux-firmware 1.184
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 12/11/2019
dmi.bios.vendor: INSYDE Corp.
dmi.bios.version: W-02.52
dmi.board.asset.tag: NA
dmi.board.name: NA
dmi.board.vendor: OEM
dmi.board.version: NA
dmi.chassis.asset.tag: NA
dmi.chassis.type: 10
dmi.chassis.vendor: OEM
dmi.chassis.version: NA
dmi.modalias: dmi:bvnINSYDECorp.:bvrW-02.52:bd12/11/2019:svnOEM:pnApolloLake:pvrNA:rvnOEM:rnNA:rvrNA:cvnOEM:ct10:cvrNA:
dmi.product.family: Apollolake
dmi.product.name: ApolloLake
dmi.product.sku: 0
dmi.product.version: NA
dmi.sys.vendor: OEM

[Ubuntu Kernel Bot (ubuntu-kernel-bot)]

This change was made by a bot.

[Mikko Kovanen (mikkokovanen)]

Hi,

since it now appears that Focal will have 5.4 kernel (1), I tested integrating the upstream kernel patches (2) related to this issue in current Focal kernel master and master-next branches (3). Patches applied smoothly to both branches and with both kernels the display backlight works correctly in my test devices (Intel Apollolake based tablets).

BR,
Mikko Kovanen

1) https://lists.ubuntu.com/archives/kernel-team/2020-February/107227.html
2) https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit?id=8cbf89db294166cc13d90a89422605e0c9f8bbc2
   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit?id=6f4261fa86dfe08c34ad99eba66368f43e9dd4c3
   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit?id=1788fdf14e518e363bae9d18345d93102f4ee5ad
3) https://kernel.ubuntu.com/git/ubuntu/ubuntu-focal.git/