[systemd-devel] systemd backlight:acpi_video0 fails, no such device

[Hans de Goede]

Hi,

On 02-09-19 07:17, Mantas Mikulėnas wrote:
> On Mon, Sep 2, 2019 at 7:34 AM Chris Murphy <lists at colorremedies.com <mailto:lists at colorremedies.com>> wrote:
> 
>     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 at fmac ~]$ sudo systemctl status
>     systemd-backlight at backlight:acpi_video0.service
>     ● systemd-backlight at backlight:acpi_video0.service - Load/Save Screen
>     Backlight Brightness of backlight:acpi_video0
>         Loaded: loaded (/usr/lib/systemd/system/systemd-backlight at .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 at .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 at backlight:acpi_video0.service: Main process exited,
>     code=exited, status=1/FAILURE
>     Sep 01 19:57:37 fmac.local systemd[1]:
>     systemd-backlight at 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 at 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
> 
> 
> Could it be that acpi_backlight is loaded at first, but gets replaced by intel_backlight before systemd could react?

Maybe, the gmux_backlight suggests that this is a macbook.

On normal machines, what you say will not happen because
linux/drivers/acpi/acpi_video.c has:

static int __init acpi_video_init(void)
{
	...
	if (intel_opregion_present())
		return 0;

And then later the i915 kms driver tells acpi_video it is ok
to actual register by calling acpi_video_register()

But on mac Books there also is the gmux, and I see that the gmux
code also calls acpi_video_register(), possibly before the i915
driver is setup, which would trigger the race you suggest where
acpi_video.c registers it backlight interface, which is only done
if there are no "native" preferred backlight interfaces and then
i915 comes along, registers its "native" backlight interface and
the acpi backlight code removes its acpi_video0 in response to
that.

So yes this seems like a race, which only gets seen on macbooks.

Regards,

Hans