systemd backlight:acpi_video0 fails, no such device

[Chris Murphy <lists@xxxxxxxxxxxxxxxxx>]

systemd-243~rc2-2.fc31.x86_64
kernel-5.3.0-0.rc6.git1.1.fc32.x86_64

This might be a regression, at least I don't remember this happening
before. I can use the expected keys for built-in display brightness,
and built-in keyboard brightness. But the service unit fails with an
out of the box installation.


[chris@fmac ~]$ sudo systemctl status
systemd-backlight@backlight:acpi_video0.service
● systemd-backlight@backlight:acpi_video0.service - Load/Save Screen
Backlight Brightness of backlight:acpi_video0
   Loaded: loaded (/usr/lib/systemd/system/systemd-backlight@.service;
static; vendor preset: disabled)
   Active: failed (Result: exit-code) since Sun 2019-09-01 19:57:37
MDT; 8min ago
     Docs: man:systemd-backlight@.service(8)
  Process: 667 ExecStart=/usr/lib/systemd/systemd-backlight load
backlight:acpi_video0 (code=exited, status=1/FAILURE)
 Main PID: 667 (code=exited, status=1/FAILURE)

Sep 01 19:57:37 fmac.local systemd[1]: Starting Load/Save Screen
Backlight Brightness of backlight:acpi_video0...
Sep 01 19:57:37 fmac.local systemd-backlight[667]: Failed to get
backlight or LED device 'backlight:acpi_video0': No such device
Sep 01 19:57:37 fmac.local systemd[1]:
systemd-backlight@backlight:acpi_video0.service: Main process exited,
code=exited, status=1/FAILURE
Sep 01 19:57:37 fmac.local systemd[1]:
systemd-backlight@backlight:acpi_video0.service: Failed with result
'exit-code'.
Sep 01 19:57:37 fmac.local systemd[1]: Failed to start Load/Save
Screen Backlight Brightness of backlight:acpi_video0.
[chris@fmac ~]$

# find /sys -name "*video0*"
/sys/class/video4linux/video0
/sys/devices/pci0000:00/0000:00:1a.7/usb1/1-2/1-2:1.0/video4linux/video0
# ls -l /sys/class/backlight/
total 0
lrwxrwxrwx. 1 root root 0 Sep  1 19:57 gmux_backlight ->
../../devices/pnp0/00:03/backlight/gmux_backlight
lrwxrwxrwx. 1 root root 0 Sep  1 19:57 intel_backlight ->
../../devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight
# find /sys -name "*acpi*"
/sys/kernel/debug/acpi
/sys/bus/platform/drivers/acpi-fan
/sys/bus/platform/drivers/axp288_pmic_acpi
/sys/bus/acpi
/sys/bus/acpi/drivers/acpi_als
/sys/firmware/acpi
/sys/module/rtc_cmos/parameters/use_acpi_alarm
/sys/module/acpi_als
/sys/module/industrialio/holders/acpi_als
/sys/module/pci_hotplug/parameters/debug_acpi
/sys/module/kfifo_buf/holders/acpi_als
/sys/module/acpiphp
/sys/module/libata/parameters/noacpi
/sys/module/libata/parameters/acpi_gtf_filter
/sys/module/acpi
/sys/module/acpi/parameters/acpica_version

OK so maybe no expected hook to discover the brightness to save it or
load it is all? *shrug*


-- 
Chris Murphy
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