[Bug 92866] intel_backlight doesn't work with i915 Haswell

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=92866

--- Comment #12 from Holger Schurig <holgerschurig at gmail.com> ---
Jani, there is no dmesg output.

I actually have two different systems with 8086:0a16 devices, one is running an
i5-4300U and an Advantech BIOS, the other is running some slower CPU (forgot
which one) on has a congatec BIOS (derived from AMI BIOS, I think).

Both have the same issue with vbetool, it's just the 8086:a011 (also i915) that
worked (and various device with even other graphics card).


I'm writing this because both Haswell devices export different backlight
directories in /sys. This is the congatec BIOS one:

root at hwtester:/sys/class/backlight/acpi_video0# grep -d skip . *
actual_brightness:0
bl_power:0
brightness:0
max_brightness:100
type:firmware

When (still with drm.debug=14) I "echo 0 >bl_power" or when I run "echo 0
>brightness" ...

* nothing visible changes on the display and
* nothing new will be emitted to dmesg


Aaaaaaand this is i915 with the Advantech BIOS:

root at hwtester:/sys/class/backlight/intel_backlight# grep -d skip . *
actual_brightness:937
bl_power:1
brightness:937
max_brightness:937
type:raw

First, it exports a different backlight interfaces. But (still with
drm.debug=14) I can "echo 0 >bl_power" and won't see anything at all in dmesg.

But when I run "echo 1 >bl_power", I see this:

[   82.253378] [drm:intel_backlight_device_update_status] updating
intel_backlight, brightness=937/937
[   82.253381] [drm:intel_panel_actually_set_backlight] set backlight PWM = 937
[   82.253388] [drm:intel_edp_backlight_power] panel power control backlight
disable

Only after this will "echo 0 >bl_power" emit this:

[  157.801385] [drm:intel_backlight_device_update_status] updating
intel_backlight, brightness=937/937
[  157.801388] [drm:intel_panel_actually_set_backlight] set backlight PWM = 937
[  157.801395] [drm:intel_edp_backlight_power] panel power control backlight
enable

That is, the first "echo 0" was silent, the 2nd not. And visible is nothing, if
I enable or disable anything.

I also think that the dmesg output doesn't reflect the intent (despite from not
working). I emits "disable" when I send a 1 (enable) and vica verca.

Funny fact: the opaque OS called "Windows 10" can turn the backlight on and off
on both i915 devices.

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