Maybe it's because of the following line?

kwin_wayland_drm: Failed to create gamma blob! Invalid argument

No, that's because NVidia doesn't support setting gamma ramps on Wayland. It won't have a negative effect, except that color correction and night color won't work.

I see that you force OpenGl 2.0. Can you try again without any kwin specific environment variables?

(In reply to Zamundaaa from comment #2)

> I see that you force OpenGl 2.0. Can you try again without any kwin specific
> environment variables?

Yes, here is the log removing KWIN_COMPOSE=O2:

kwin_wayland_drm: drmModeAddFB2WithModifiers is supported on GPU "/dev/dri/card1"
kwin_wayland_drm: Using Atomic Mode Setting on gpu "/dev/dri/card1"
kwin_wayland_drm: Number of planes on GPU "/dev/dri/card1" : 12
kwin_wayland_drm: Plane 33 has properties "type"="Primary", "SRC_X"=0, "SRC_Y"=0, "SRC_W"=0, "SRC_H"=0, "CRTC_X"=0, "CRTC_Y"=0, "CRTC_W"=0, "CRTC_H"=0, "FB_ID"=0, "CRTC_ID"=0, "rotation"=invalid value: 1, "IN_FORMATS"=34
kwin_wayland_drm: Plane 36 has properties "type"="Cursor", "SRC_X"=0, "SRC_Y"=0, "SRC_W"=0, "SRC_H"=0, "CRTC_X"=0, "CRTC_Y"=0, "CRTC_W"=0, "CRTC_H"=0, "FB_ID"=0, "CRTC_ID"=0, "rotation" not found, "IN_FORMATS"=37
kwin_wayland_drm: Plane 41 has properties "type"="Overlay", "SRC_X"=0, "SRC_Y"=0, "SRC_W"=0, "SRC_H"=0, "CRTC_X"=0, "CRTC_Y"=0, "CRTC_W"=0, "CRTC_H"=0, "FB_ID"=0, "CRTC_ID"=0, "rotation"=invalid value: 1, "IN_FORMATS"=42
kwin_wayland_drm: Plane 46 has properties "type"="Primary", "SRC_X"=0, "SRC_Y"=0, "SRC_W"=0, "SRC_H"=0, "CRTC_X"=0, "CRTC_Y"=0, "CRTC_W"=0, "CRTC_H"=0, "FB_ID"=0, "CRTC_ID"=0, "rotation"=invalid value: 1, "IN_FORMATS"=47
kwin_wayland_drm: Plane 49 has properties "type"="Cursor", "SRC_X"=0, "SRC_Y"=0, "SRC_W"=0, "SRC_H"=0, "CRTC_X"=0, "CRTC_Y"=0, "CRTC_W"=0, "CRTC_H"=0, "FB_ID"=0, "CRTC_ID"=0, "rotation" not found, "IN_FORMATS"=50
kwin_wayland_drm: Plane 54 has properties "type"="Overlay", "SRC_X"=0, "SRC_Y"=0, "SRC_W"=0, "SRC_H"=0, "CRTC_X"=0, "CRTC_Y"=0, "CRTC_W"=0, "CRTC_H"=0, "FB_ID"=0, "CRTC_ID"=0, "rotation"=invalid value: 1, "IN_FORMATS"=55
kwin_wayland_drm: Plane 59 has properties "type"="Primary", "SRC_X"=0, "SRC_Y"=0, "SRC_W"=0, "SRC_H"=0, "CRTC_X"=0, "CRTC_Y"=0, "CRTC_W"=0, "CRTC_H"=0, "FB_ID"=0, "CRTC_ID"=0, "rotation"=invalid value: 1, "IN_FORMATS"=60
kwin_wayland_drm: Plane 62 has properties "type"="Cursor", "SRC_X"=0, "SRC_Y"=0, "SRC_W"=0, "SRC_H"=0, "CRTC_X"=0, "CRTC_Y"=0, "CRTC_W"=0, "CRTC_H"=0, "FB_ID"=0, "CRTC_ID"=0, "rotation" not found, "IN_FORMATS"=63
kwin_wayland_drm: Plane 67 has properties "type"="Overlay", "SRC_X"=0, "SRC_Y"=0, "SRC_W"=0, "SRC_H"=0, "CRTC_X"=0, "CRTC_Y"=0, "CRTC_W"=0, "CRTC_H"=0, "FB_ID"=0, "CRTC_ID"=0, "rotation"=invalid value: 1, "IN_FORMATS"=68
kwin_wayland_drm: Plane 72 has properties "type"="Primary", "SRC_X"=0, "SRC_Y"=0, "SRC_W"=0, "SRC_H"=0, "CRTC_X"=0, "CRTC_Y"=0, "CRTC_W"=0, "CRTC_H"=0, "FB_ID"=0, "CRTC_ID"=0, "rotation"=invalid value: 1, "IN_FORMATS"=73
kwin_wayland_drm: Plane 75 has properties "type"="Cursor", "SRC_X"=0, "SRC_Y"=0, "SRC_W"=0, "SRC_H"=0, "CRTC_X"=0, "CRTC_Y"=0, "CRTC_W"=0, "CRTC_H"=0, "FB_ID"=0, "CRTC_ID"=0, "rotation" not found, "IN_FORMATS"=76
kwin_wayland_drm: Plane 80 has properties "type"="Overlay", "SRC_X"=0, "SRC_Y"=0, "SRC_W"=0, "SRC_H"=0, "CRTC_X"=0, "CRTC_Y"=0, "CRTC_W"=0, "CRTC_H"=0, "FB_ID"=0, "CRTC_ID"=0, "rotation"=invalid value: 1, "IN_FORMATS"=81
kwin_wayland_drm: Crtc 40 has properties "MODE_ID"=0, "ACTIVE"=0, "VRR_ENABLED"=0, "GAMMA_LUT" not found, "GAMMA_LUT_SIZE" not found
kwin_wayland_drm: Crtc 53 has properties "MODE_ID"=0, "ACTIVE"=0, "VRR_ENABLED"=0, "GAMMA_LUT" not found, "GAMMA_LUT_SIZE" not found
kwin_wayland_drm: Crtc 66 has properties "MODE_ID"=0, "ACTIVE"=0, "VRR_ENABLED"=0, "GAMMA_LUT" not found, "GAMMA_LUT_SIZE" not found
kwin_wayland_drm: Crtc 79 has properties "MODE_ID"=0, "ACTIVE"=0, "VRR_ENABLED"=0, "GAMMA_LUT" not found, "GAMMA_LUT_SIZE" not found
kwin_xkbcommon: XKB: created compose table from locale en_US.UTF-8 with path /usr/xorg/share/X11/locale/en_US.UTF-8/Compose
kf.globalaccel.kglobalacceld: Failed to register service org.kde.kglobalaccel
kwin_core: Init of kglobalaccel failed
kwin_core: Attempting to load the OpenGL scene
kwin_scene_opengl: Egl Initialize succeeded
kwin_scene_opengl: EGL version:  1 . 5
kwin_scene_opengl: Created EGL context with attributes: 
Version requested:      true
Version:        3.1
Robust: true
Reset on video memory purge:    true
Forward compatible:     false
Core profile:   false
Compatibility profile:  false
High priority:  true
kwin_scene_opengl: Created EGL context with attributes: 
Version requested:      true
Version:        3.1
Robust: true
Reset on video memory purge:    true
Forward compatible:     false
Core profile:   false
Compatibility profile:  false
High priority:  true
OpenGL vendor string:                   NVIDIA Corporation
OpenGL renderer string:                 NVIDIA GeForce RTX 2070 with Max-Q Design/PCIe/SSE2
OpenGL version string:                  3.1.0 NVIDIA 515.57
OpenGL shading language version string: 1.40 NVIDIA via Cg compiler
Driver:                                 NVIDIA
Driver version:                         515.57
GPU class:                              Unknown
OpenGL version:                         3.1
GLSL version:                           1.40
Linux kernel version:                   5.18.9
Requires strict binding:                no
GLSL shaders:                           yes
Texture NPOT support:                   yes
Virtual Machine:                        no
kwin_scene_opengl: Filter multi-plane format 842093913
kwin_scene_opengl: Filter multi-plane format 842094158
kwin_scene_opengl: Filter multi-plane format 825382478
kwin_scene_opengl: Filter multi-plane format 909203022
kwin_scene_opengl: Filter multi-plane format 875714126
kwin_scene_opengl: EGL driver advertises 22 supported dmabuf formats with modifiers
kwin_core: OpenGL compositing has been successfully initialized
kwin_wayland_drm: Could not find edid for connector DrmConnector(id=86, gpu=KWin::DrmGpu(0x645f50), name="DP-1-unknown", connection="Disconnected", countMode=0)
kwin_wayland_drm: Connector 86 has properties "CRTC_ID"=0, "non-desktop"=0, "DPMS"=0, "EDID"=0, "overscan" not found, "vrr_capable" not found, "underscan" not found, "underscan vborder" not found, "underscan hborder" not found, "Broadcast RGB" not found, "max bpc" not found, "link-status"="Good"
kwin_wayland_drm: Connector 89 has properties "CRTC_ID"=0, "non-desktop"=0, "DPMS"=0, "EDID"=93, "overscan" not found, "vrr_capable" not found, "underscan" not found, "underscan vborder" not found, "underscan hborder" not found, "Broadcast RGB" not found, "max bpc" not found, "link-status"="Good"
kwin_wayland_drm: New output on GPU /dev/dri/card1: DELL U2718Q/4K8X78AG078L
kwin_wayland_drm: Could not find edid for connector DrmConnector(id=92, gpu=KWin::DrmGpu(0x645f50), name="HDMI-A-1-unknown", connection="Disconnected", countMode=0)
kwin_wayland_drm: Connector 92 has properties "CRTC_ID"=0, "non-desktop"=0, "DPMS"=0, "EDID"=0, "overscan" not found, "vrr_capable" not found, "underscan" not found, "underscan vborder" not found, "underscan hborder" not found, "Broadcast RGB" not found, "max bpc" not found, "link-status"="Good"
kwin_wayland_drm: Reading output configuration for  KWin::DrmOutput(0x662790, name="DP-2", geometry=QRect(0,0 3840x2160), scale=1)
kwin_decorations: Trying to load decoration plugin:  "/usr/local/kde5/lib/plugins/org.kde.kdecoration2/breezedecoration.so"
Couldn't start kglobalaccel from org.kde.kglobalaccel.service: QDBusError("org.freedesktop.DBus.Error.Disconnected", "Not connected to D-Bus server")
kwin_xkbcommon: XKB: Compiling from RMLVO: rules 'evdev', model 'pc105', layout 'us', variant 'intl', options '(null)'
kwin_xkbcommon: XKB: Compiling from KcCGST: keycodes 'evdev+aliases(qwerty)', types 'complete', compat 'complete', symbols 'pc+us(intl)+inet(evdev)'
kwin_xkbcommon: XKB: Compiling xkb_keycodes "(unnamed)"
kwin_xkbcommon: XKB: Compiling xkb_types "(unnamed)"
kwin_xkbcommon: XKB: Compiling xkb_compatibility "(unnamed)"
kwin_xkbcommon: XKB: The "group" statement in compat is unsupported; Ignored
kwin_xkbcommon: XKB: The "group" statement in compat is unsupported; Ignored
kwin_xkbcommon: XKB: The "group" statement in compat is unsupported; Ignored
kwin_xkbcommon: XKB: The "allowExplicit" field in indicator statements is unsupported; Ignored
kwin_xkbcommon: XKB: The "allowExplicit" field in indicator statements is unsupported; Ignored
kwin_xkbcommon: XKB: The "allowExplicit" field in indicator statements is unsupported; Ignored
kwin_xkbcommon: XKB: The "allowExplicit" field in indicator statements is unsupported; Ignored
kwin_xkbcommon: XKB: The "indicatorDrivesKeyboard" field in indicator statements is unsupported; Ignored
kwin_xkbcommon: XKB: The "allowExplicit" field in indicator statements is unsupported; Ignored
kwin_xkbcommon: XKB: Indicator name "Shift Lock" was not declared in the keycodes section; Adding new indicator
kwin_xkbcommon: XKB: Indicator name "Group 2" was not declared in the keycodes section; Adding new indicator
kwin_xkbcommon: XKB: Indicator name "Mouse Keys" was not declared in the keycodes section; Adding new indicator
Couldn't start kglobalaccel from org.kde.kglobalaccel.service: QDBusError("org.freedesktop.DBus.Error.Disconnected", "Not connected to D-Bus server")
kwin_xkbcommon: XKB: Compiling from RMLVO: rules 'evdev', model 'pc105', layout 'us', variant 'intl', options '(null)'
kwin_xkbcommon: XKB: Compiling from KcCGST: keycodes 'evdev+aliases(qwerty)', types 'complete', compat 'complete', symbols 'pc+us(intl)+inet(evdev)'
kwin_xkbcommon: XKB: Compiling xkb_keycodes "(unnamed)"
kwin_xkbcommon: XKB: Compiling xkb_types "(unnamed)"
kwin_xkbcommon: XKB: Compiling xkb_compatibility "(unnamed)"
kwin_xkbcommon: XKB: The "group" statement in compat is unsupported; Ignored
kwin_xkbcommon: XKB: The "group" statement in compat is unsupported; Ignored
kwin_xkbcommon: XKB: The "group" statement in compat is unsupported; Ignored
kwin_xkbcommon: XKB: The "allowExplicit" field in indicator statements is unsupported; Ignored
kwin_xkbcommon: XKB: The "allowExplicit" field in indicator statements is unsupported; Ignored
kwin_xkbcommon: XKB: The "allowExplicit" field in indicator statements is unsupported; Ignored
kwin_xkbcommon: XKB: The "allowExplicit" field in indicator statements is unsupported; Ignored
kwin_xkbcommon: XKB: The "indicatorDrivesKeyboard" field in indicator statements is unsupported; Ignored
kwin_xkbcommon: XKB: The "allowExplicit" field in indicator statements is unsupported; Ignored
kwin_xkbcommon: XKB: Indicator name "Shift Lock" was not declared in the keycodes section; Adding new indicator
kwin_xkbcommon: XKB: Indicator name "Group 2" was not declared in the keycodes section; Adding new indicator
kwin_xkbcommon: XKB: Indicator name "Mouse Keys" was not declared in the keycodes section; Adding new indicator
kwin_xkbcommon: XKB: Compiling xkb_symbols "(unnamed)"
kwin_virtualkeyboard: Registering the DBus interface
kwin_core: Successfully loaded plugin effect:  "wobblywindows"
kwin_core: Successfully loaded plugin effect:  "startupfeedback"
kwin_core: Successfully loaded plugin effect:  "screenshot"
kwin_core: Successfully loaded plugin effect:  "blendchanges"
kwin_core: Successfully loaded plugin effect:  "zoom"
kwin_core: Successfully loaded plugin effect:  "windowview"
kwin_core: Successfully loaded plugin effect:  "screenedge"
kwin_core: Successfully loaded plugin effect:  "screentransform"
kwin_core: Successfully loaded plugin effect:  "kscreen"
kwin_core: Successfully loaded plugin effect:  "highlightwindow"
kwin_core: Successfully loaded plugin effect:  "colorpicker"
kwin_wayland_drm: Failed to create gamma blob! Invalid argument
kwin_core: Successfully loaded scripted effect:  "kwin4_effect_sessionquit"
kwin_core: Successfully loaded scripted effect:  "kwin4_effect_translucency"
kwin_wayland_drm: Removing output KWin::DrmOutput(0x662790, name="DP-2", geometry=QRect(0,0 3840x2160), scale=1)

(In reply to Zamundaaa from comment #2)
> No, that's because NVidia doesn't support setting gamma ramps on Wayland. It
> won't have a negative effect, except that color correction and night color
> won't work.

Ok, so I changed the bug report title, since the main issue is the black screen (with the backlight activated). Thanks.

There're 2 more things I'd like to add when trying to run directly from console:

1) if I run "kwin_wayland --drm --lockscreen" it shows the lockscreen instead of a black screen and if I "login", it will go to the black screen

2) if I run "dbus-run-session startplasma-wayland", it starts the following and stalls at klauncher:

 1182 tty1     Sl+    0:00 /usr/kde5.25/bin/plasma_session --no-lockscreen
 1185 tty1     Sl+    0:00 /usr/kde/bin/kwin_wayland_wrapper --xwayland
 1186 tty1     Sl+    0:00 /usr/kde/bin/kwin_wayland --wayland-fd 7 --socket wayland-0 --xwayland
 1189 ?        S      0:00 /usr/kde/bin/ksplashqml
 1190 tty1     S+     0:00 /usr/kde5.25/bin/kdeinit5 --oom-pipe 4
 1191 tty1     Z+     0:00 [start_kdeinit_w] <defunct>
 1192 tty1     S+     0:00 /usr/kde5.25/lib/libexec/kf5/start_kdeinit
 1193 ?        Ss     0:00 /usr/kde5.25/bin/kdeinit5 --oom-pipe 4
 1194 ?        S      0:00 /usr/kde5.25/lib/libexec/kf5/klauncher --fd=9

And it stays at the console... So I wonder why it is stuck at klauncher...

In the end it was the DISPLAY environment variable that was preventing wayland from starting. If DISPLAY or WAYLAND_DISPLAY is set, Kwin will not use the DRM backend. I found it after looking at main_wayland.cpp:

static QString automaticBackendSelection()
{
    if (qEnvironmentVariableIsSet("WAYLAND_DISPLAY")) {
        return s_waylandPlugin;
    }
    if (qEnvironmentVariableIsSet("DISPLAY")) {
        return s_x11Plugin;
    }
    return s_drmPlugin;
}

Maybe it would help if it was more verbose? Since many users are reporting getting the "black screen", I wonder how many of them have DISPLAY set before trying to login into Wayland... After the "No backend specified through command line argument, trying auto resolution" message, Kwin could log something like "DISPLAY environment variable detected. Using X11 backend". ;-)

So I'm closing this as "NOT A BUG". I hope it helps other users. Thanks everyone!

Being more verbose about this would make tons of sense. Can you please file a new bug report about that? Thanks a lot!

(In reply to Nate Graham from comment #7)
> Being more verbose about this would make tons of sense. Can you please file
> a new bug report about that? Thanks a lot!

Done: bug 457491.

Thank you Nate!