[Bug 263032] backlight(8) can't query the device

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=263032

            Bug ID: 263032
           Summary: backlight(8) can't query the device
           Product: Base System
           Version: 13.0-RELEASE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: bin
          Assignee: bugs@FreeBSD.org
          Reporter: artur.brzozowski@protonmail.com

Created attachment 232927
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=232927&action=edit
<pciconf/kldstat/sysctl/dmesg> details

On a FreeBSD 13.0-RELEASE-p8 installation, backlight(8) does not work despite
proper initialisation of the backlight device.

Hardware affected:
vendor     = 'Advanced Micro Devices, Inc. [AMD/ATI]'
device     = 'RV730/M96 [Mobility Radeon HD 4650/5165]'

Available backlight device:
crw-rw----   1 root  video  0x1f3  4 Apr. 10:22 backlight0
lrwxr-xr-x   1 root  wheel     23  4 Apr. 10:22 radeon_bl00@ ->
../backlight/backlight0

Use of backlight(8) as non-root user (video group):
backlight: Cannot query the backlight device
Use of backlight(8) as root:
backlight: Cannot update the backlight device
(subsequent uses by a non-root also report the failure to update, not query,
the backlight device)

Workaround: loading 'acpi_video' module and using sysctl(8) to manipulate
'hw.acpi.video.lcd0.brightness'.

Two other laptops with Intel chips do not have this problem.

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