Bug#971953: linux-image-5.8: Intel Bay Trail PMIC, laptop screen backlight cannot be adjusted

On Sat, 10 Oct 2020 16:25:56 +0200 /dev/fra <devfra@inventati.org> wrote:

> On the ASUS T100TA convertible laptop [1], and probably on similar
> devices with the same Intel Bay Trail chipset as well, the screen

> backlight cannot be adjusted and it is set to the maximum brightness.

I'm experiencing this same issue on Acer Aspire Switch 10 convertible laptop which has Intel Atom Z3735F processor. At boot it shows "i915 0000:00:02.0: [drm] *ERROR* Failed to get the SoC PWM chip" error message.

Here's the relevant log from journalctl -xb:

```

Nov 28 11:23:16 debian kernel: fb0: switching to inteldrmfb from EFI VGA
Nov 28 11:23:16 debian kernel: Console: switching to colour dummy device 80x25
Nov 28 11:23:16 debian kernel: i915 0000:00:02.0: vgaarb: deactivate vga console
Nov 28 11:23:16 debian kernel: [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
Nov 28 11:23:16 debian kernel: i915 0000:00:02.0: vgaarb: changed VGA decodes: olddecodes=io+mem,decodes=io+mem:owns=io+mem
Nov 28 11:23:16 debian kernel:  mmcblk1: p1 p2 p3
Nov 28 11:23:16 debian kernel: i915 0000:00:02.0: [drm] *ERROR* Failed to get the SoC PWM chip
Nov 28 11:23:16 debian kernel: [drm] Initialized i915 1.6.0 20200515 for 0000:00:02.0 on minor 0
Nov 28 11:23:16 debian kernel: ACPI: Video Device [GFX0] (multi-head: yes  rom: no  post: no)
Nov 28 11:23:16 debian kernel: acpi device:21: registered as cooling_device4
Nov 28 11:23:16 debian kernel: input: Video Bus as /devices/LNXSYSTM:00/LNXSYBUS:00/PNP0A08:00/LNXVIDEO:00/input/input2
Nov 28 11:23:16 debian kernel: i915 0000:00:02.0: [drm] HDaudio controller not detected, using LPE audio instead
Nov 28 11:23:16 debian kernel: fbcon: i915drmfb (fb0) is primary device
Nov 28 11:23:16 debian kernel: Console: switching to colour frame buffer device 160x50
Nov 28 11:23:16 debian kernel: i915 0000:00:02.0: fb0: i915drmfb frame buffer device

```

Here is the dmesg: https://pastebin.com/raw/2uGUYVkE

```

$ uname -a
Linux debian 5.8.0-0.bpo.2-amd64 #1 SMP Debian 5.8.10-1~bpo10+1 (2020-09-26) x86_64 GNU/Linux

$ lsusb
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 001 Device 003: ID 06cb:2968 Synaptics, Inc. 
Bus 001 Device 002: ID 05e3:0608 Genesys Logic, Inc. Hub
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
$ lspci
00:00.0 Host bridge: Intel Corporation Atom Processor Z36xxx/Z37xxx Series SoC Transaction Register (rev 0f)
00:02.0 VGA compatible controller: Intel Corporation Atom Processor Z36xxx/Z37xxx Series Graphics & Display (rev 0f)
00:14.0 USB controller: Intel Corporation Atom Processor Z36xxx/Z37xxx, Celeron N2000 Series USB xHCI (rev 0f)
00:1a.0 Encryption controller: Intel Corporation Atom Processor Z36xxx/Z37xxx Series Trusted Execution Engine (rev 0f)
00:1f.0 ISA bridge: Intel Corporation Atom Processor Z36xxx/Z37xxx Series Power Control Unit (rev 0f)
$
```

For time being, I'm using xrandr to change perceived brightness as suggested at https://wiki.archlinux.org/index.php/backlight#Xorg:_adjust_perceived_brightness_with_xrandr