This laptop has an BIOS option "Switchable Graphics". When the options is selected, it behaves like a normal Optimus laptop. In this case, brightness sysfs works since it's driven by Intel graphics.
When the options is deselected, the Intel graphics device disappears, Nvidia is the only graphics device. The issue happens in this case.
Changing /sys/class/backlight/nv_backlight/brightness, the brightness level remains the same - almost no backlight.
Using proprietary driver, brightness gets changed correctly by /sys/class/backlight/nvidia_0/brightness.
No trace while using MMIOTRACE, under proprietary driver.
When in non-switchable mode, are there other things in /sys/class/backlight? e.g. acpi_video0?
Also, presumably this is on v5.0 or later? Prior to that, there was no backlight support for Pascal.
(In reply to Ilia Mirkin from comment #1)
> When in non-switchable mode, are there other things in /sys/class/backlight?
> e.g. acpi_video0?
No, nv_backlight (or nvidia_0) is the only interface.
I also tried to call _BCM directly via acpidbg, but both Intel's and Nvidia's _BCM method changes nothing.
Please refer to the original bug report .
> Also, presumably this is on v5.0 or later? Prior to that, there was no
> backlight support for Pascal.
Yes, on both v5.0 and linux-next.