Backlight adjustment doesn't work on Google Pixel Slate

Bug #1895615 reported by satmandu on 2020-09-15
This bug affects 1 person
Affects Status Importance Assigned to Milestone
linux (Ubuntu)

Bug Description

The Pixel Slate/nocturne backlight does not work due to DPCD logic not identifying the DPCD backlight connection since there is no PWM on the nocturne or eve devices.

ChromeOS documentation of disconnected PWM:

Discussion of issue & solution:

This can be fixed with a simple patch which I have applied successfully to mainline 5.9-rc5:

Luckily the Ubuntu kernel patches set i915.enable_psr=0 which keeps Panel Self Refresh from dimming the backlight at boot, but that isn't even needed for this device once the backlight is hooked up correctly.

(As a bonus the keyboard backlight also starts working properly at boot.)

ProblemType: Bug
DistroRelease: Ubuntu 20.10
Package: linux-image-lowlatency
ProcVersionSignature: Ubuntu 5.8.0-18.19-lowlatency 5.8.4
Uname: Linux 5.8.0-18-lowlatency x86_64
ApportVersion: 2.20.11-0ubuntu45
Architecture: amd64
AudioDevicesInUse: Error: command ['fuser', '-v', '/dev/snd/seq', '/dev/snd/timer'] failed with exit code 1:
CasperMD5CheckResult: skip
Date: Mon Sep 14 23:52:45 2020
InstallationDate: Installed on 2020-08-24 (21 days ago)
InstallationMedia: Ubuntu 20.10 "Groovy Gorilla" - Alpha amd64 (20200728)
 Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
 Bus 001 Device 003: ID 18d1:5030 Google Inc. Hammer
 Bus 001 Device 002: ID 8087:0a2a Intel Corp. Bluetooth wireless interface
 Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
MachineType: Google Nocturne
 PATH=(custom, no user)
ProcFB: 0 i915drmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-5.8.0-18-lowlatency root=UUID=90e26b2a-aaad-4576-8e90-1962b97d4e96 ro fsck.mode=force
 Error: command ['pacmd', 'list'] failed with exit code 1: Home directory not accessible: Permission denied
 No PulseAudio daemon running, or not running as session daemon.
 linux-restricted-modules-5.8.0-18-lowlatency N/A
 linux-backports-modules-5.8.0-18-lowlatency N/A
 linux-firmware 1.190
SourcePackage: linux
StagingDrivers: ipu3_imgu
UpgradeStatus: No upgrade log present (probably fresh install) 06/04/2019
dmi.bios.release: 4.0
dmi.bios.vendor: coreboot
dmi.bios.version: Google_Nocturne.10984.90.0 Nocturne
dmi.board.vendor: Google
dmi.board.version: 1.0
dmi.chassis.type: 3
dmi.chassis.vendor: Google 0.0
dmi.modalias: dmi:bvncoreboot:bvrGoogle_Nocturne.10984.90.0:bd06/04/2019:br4.0:efr0.0:svnGoogle:pnNocturne:pvr1.0:rvnGoogle:rnNocturne:rvr1.0:cvnGoogle:ct3:cvr: Google_Nocturne Nocturne
dmi.product.version: 1.0
dmi.sys.vendor: Google

satmandu (satadru-umich) wrote :
tags: added: patch

This change was made by a bot.

Changed in linux (Ubuntu):
status: New → Confirmed
Alex Hung (alexhung) wrote :

It is a good idea to send your patch to below kernel's mailing lists so this can be reviewed by broader audiences to confirm it will work in all scenario.

INTEL DRM DRIVERS (excluding Poulsbo, Moorestown and derivative chipsets)
M: Jani Nikula <email address hidden>
M: Joonas Lahtinen <email address hidden>
M: Rodrigo Vivi <email address hidden>
L: <email address hidden>

Note: you can also run kernel script "./scripts/ dpcd59.patch" for all parties who will be interested in this patch.

You can also include "Cc: <email address hidden>" in your patch so it will be included to previous stable kernels (see

satmandu (satadru-umich) wrote :

Thanks! I've submitted a version to intel-gfx mailing list as [RFC PATCH 1/1] Remove DPCD Aux Backlight Control PWM_PIN check after discussions with Redhat's Lyude Paul.

Alex Hung (alexhung) wrote :

That's great. Let's wait for the good news!

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