[REQUEST] Add support for Intel DPST (Display Power Saving Technology)

[José Relvas]

On Tue, Apr 16, 2024 at 11:00:12AM +0300, Jani Nikula wrote:
> On Fri, 05 Apr 2024, José Relvas <josemonsantorelvas at gmail.com> wrote:
> > The best approach here would probably be to expose a similar attribute to amdgpu's 
> > "panel_power_savings", with a scale that controls the feature's aggressiveness,
> > then update userspace tools, including power-profiles-daemon, to set the value
> > based on the intended energy scheme.
> 
> I don't really know what panel_power_savings does or how it works, but
> clearly it's not how this particular thing works on Intel
> hardware. There isn't a trivial knob you could adjust.

I've investigated this further. AMD's marketing name for it
is "Vari-Bright" and, on Windows, can be controlled through their gfx
control panel, with a 4-level slider. Internally, it seems to be called
"Adaptive Backlight Modulation" (ABM). It increases the brightness of the
image to account for a lower backlight level.

I had a cursory look at the amdgpu driver, but I'm not sure if the algorithm
is there, or if its handled in firmware.

> Where you have to add an API for reading the image histogram, add an API
> to set the image enhancement parameters, and how that function works is
> userspace policy that needs to be implemented in userspace.

Hmm... this could be handled by a daemon. Said daemon could hook into
PPD's state to check if it should be trying to save power.

-

One more question. Some documentation refers to 
"OPST (OLED power saving technology)". It's my understanding that this
uses the same hardware blocks and APIs as DPST - only the algorithm
in software is different. Userspace would need a way to distinguish
OLED panels from LCD panels so it can use the right algorithm. Is
there a reliable way of doing this? 
On my Thinkpad P1 gen 6, the OLED exposes a standard intel_backlight,
which actually controls the PWM. Afaik, there's no other way of
telling OLED and LCD apart in userspace.

José Relvas