Comment 7 for bug 1861521

I noticed that upstream v5.4.18 allowed me to adjust my screen brightness while Ubuntu-5.4-5.4.0-14.17 does not, which indicates an Ubuntu SAUCE patch as the culprit. I bisected between the two kernels and this was the result:

$ git bisect good
3269788061d24e316633165608259de1c110b801 is the first bad commit
commit 3269788061d24e316633165608259de1c110b801
Author: AceLan Kao <email address hidden>
Date: Thu Dec 12 17:07:44 2019 +0800

    USUNTU: SAUCE: drm/i915: Force DPCD backlight mode on Dell Precision 4K sku

    BugLink: https://bugs.launchpad.net/bugs/1856134

    This platform using DPCD aux to control backlight,
    so adding the first 3 OUI bytes to the quirk.

    Signed-off-by: AceLan Kao <email address hidden>
    Signed-off-by: Seth Forshee <email address hidden>

:040000 040000 5eb20635c698b49ae34aece26e3ee9f24631ca72 0a3389d202d1306745004ff8837731f2ceda317e M drivers

I'm no expert in this area but it looks to me like that commit is being too generic and a more specific device_id is needed to target that Dell device.

Here's the information on my panel:

$ edid-decode /sys/class/drm/card0-eDP-1/edid
edid-decode (hex):

00 ff ff ff ff ff ff 00 26 cf 7f 05 00 00 00 00
00 1b 01 04 a5 1f 11 78 0a 1d 39 a7 50 47 97 27
13 4f 54 00 00 00 01 01 01 01 01 01 01 01 01 01
01 01 01 01 01 01 60 6d 80 c8 70 38 14 40 18 30
a5 00 35 ae 10 00 00 19 00 00 00 fe 00 4d 31 34
30 4e 56 46 37 20 52 30 20 0a 00 00 00 03 00 04
30 ff 05 3c 9d 2b 14 39 9d 00 00 00 00 00 00 03
00 06 18 ff 0f 3c 3b 2a 0b 39 3b 01 01 00 00 ad

----------------

EDID version: 1.4
Manufacturer: IVO Model 1407 Serial Number 0
Made in year 2017
Digital display
8 bits per primary color channel
DisplayPort interface
Maximum image size: 31 cm x 17 cm
Gamma: 2.20
Supported color formats: RGB 4:4:4, YCrCb 4:4:4
First detailed timing includes the native pixel format and preferred refresh rate
Color Characteristics
  Red: 0.6523, 0.3134
  Green: 0.2802, 0.5908
  Blue: 0.1523, 0.0771
  White: 0.3105, 0.3291
Established Timings I & II: none
Standard Timings: none
Detailed mode: Clock 280.000 MHz, 309 mm x 174 mm
               1920 1944 1992 2120 ( 24 48 128)
               1080 1090 1095 1100 ( 10 5 5)
               -hsync -vsync
               VertFreq: 120.069 Hz, HorFreq: 132.075 kHz
Alphanumeric Data String: M140NVF7 R0
Manufacturer-Specified Display Descriptor (0x03): 00 03 00 04 30 ff 05 3c 9d 2b 14 39 9d 00 00 00 ....0..<.+.9....
Manufacturer-Specified Display Descriptor (0x03): 00 03 00 06 18 ff 0f 3c 3b 2a 0b 39 3b 01 01 00 .......<;*.9;...
Checksum: 0xad