[Bug 201795] [Regression] Wrong 4k resolution detected with DisplayPort to HDMI adapter on amdgpu

[bugzilla-daemon at bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=201795

--- Comment #20 from Nicholas Kazlauskas (nicholas.kazlauskas at amd.com) ---
Not every user will want the same scaling parameters for non-native signals.
The display itself will often offer a configurable mode itself for its scaler
for this reason.

I don't have the exact displays that you have, but I have a test setup with a
4096x2160 and 3840x2160 display and I'm able to mirror the desktop using Plasma
and have the image fullscreen on both displays (without black bars) because the
4096x2160 display defaults to fullscreen scaling.

But I can also reproduce your problem by changing the scaling mode in the
display's OSD to 1:1 scaling, which shows black bars. This is because the
driver doesn't do any scaling itself by default.

The driver can't query the monitor to understand what the display is going to
do, and it also can't guess what the user's intended preference is here.

So we expose the standard (ie. shared across other drivers as well) DRM
property for "scaling mode" to give more control over the behavior when
possible. As for why it's not configurable via GUI, that's more of a userspace
feature request for the Plasma / GNOME / etc developers.

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