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Fails to set HDR on builtin laptop monitor

I wanted to work on implementing HDR support in GTK using gnome-shell's new HDR support and got this shiny new laptop, but it seems HDR support doesn't work. We (read: me with the help of the gnome-shell developers) made sure that the kernel calls are actually succeeding, but I could not see any visual difference from switch HDR on and off.

Steps to reproduce:

  1. Get a ThinkPad X1 Carbon Gen 9 20XXS95H00 containing "PCI 8086:9a49 Intel Corporation TigerLake-LP GT2 [Iris Xe Graphics]".
  2. Make sure it contains the fancy HDR monitor with this edid:
Block 0, Base EDID:
  EDID Structure Version & Revision: 1.4
  Vendor & Product Identification:
    Manufacturer: CSO
    Model: 5123
    Made in: 2020
  Basic Display Parameters & Features:
    Digital display
    Bits per primary color channel: 10
    DisplayPort interface
    Maximum image size: 30 cm x 19 cm
    Gamma: 2.20
    Supported color formats: RGB 4:4:4
    First detailed timing includes the native pixel format and preferred refresh rate
    Display is continuous frequency
  Color Characteristics:
    Red  : 0.6650, 0.3359
    Green: 0.2539, 0.6875
    Blue : 0.1386, 0.0527
    White: 0.3066, 0.3408
  Established Timings I & II: none
  Standard Timings: none
  Detailed Timing Descriptors:
    DTD 1:  3840x2400   60.000000 Hz  16:10   148.800 kHz    595.200000 MHz (302 mm x 189 mm)
                 Hfront   48 Hsync  32 Hback   80 Hpol N
                 Vfront    3 Vsync   6 Vback   71 Vpol N
    Display Range Limits:
      Monitor ranges (Bare Limits): 48-60 Hz V, 149-149 kHz H, max dotclock 600 MHz
    Alphanumeric Data String: 'CSOT T3'
    Alphanumeric Data String: 'MNE007ZA1-2'
  Extension blocks: 1
Checksum: 0x71

----------------

Block 1, CTA-861 Extension Block:
  Revision: 3
  Native detailed modes: 0
  Colorimetry Data Block:
    BT2020RGB
  HDR Static Metadata Data Block:
    Electro optical transfer functions:
      Traditional gamma - SDR luminance range
      SMPTE ST2084
    Supported static metadata descriptors:
      Static metadata type 1
    Desired content max luminance: 106 (496.743 cd/m^2)
    Desired content max frame-average luminance: 106 (496.743 cd/m^2)
    Desired content min luminance: 36 (0.099 cd/m^2)
Checksum: 0x9a  Unused space in Extension Block: 112 bytes
  1. Get stock Fedora 38
  2. (optional) Install drm-tip kernel
  3. run gnome-shell
  4. follow instructions outlined in the merge request adding HDR modes and enable HDR mode in gnome-shell.
  5. observe the screen blanking when changing that setting to "on" (or back to "off").
  6. ensure that when the setting is "on", drm_info reports:
...
│   ├───Connector 0
│   │   ├───Object ID: 308
│   │   ├───Type: eDP
│   │   ├───Status: connected
│   │   ├───Physical size: 300x190 mm
│   │   ├───Subpixel: unknown
│   │   ├───Encoders: {0}
│   │   ├───Modes
│   │   │   └───3840x2400@60.00 preferred driver nhsync nvsync 
│   │   └───Properties
│   │       ├───"EDID" (immutable): blob = 368
│   │       ├───"DPMS": enum {On, Standby, Suspend, Off} = On
│   │       ├───"link-status": enum {Good, Bad} = Good
│   │       ├───"non-desktop" (immutable): range [0, 1] = 0
│   │       ├───"TILE" (immutable): blob = 0
│   │       ├───"CRTC_ID" (atomic): object CRTC = 98
│   │       ├───"scaling mode": enum {Full, Center, Full aspect} = Full aspect
│   │       ├───"panel orientation" (immutable): enum {Normal, Upside Down, Left Side Up, Right Side Up} = Normal
│   │       ├───"Broadcast RGB": enum {Automatic, Full, Limited 16:235} = Automatic
│   │       ├───"max bpc": range [6, 12] = 12
│   │       ├───"Colorspace": enum {Default, RGB_Wide_Gamut_Fixed_Point, RGB_Wide_Gamut_Floating_Point, opRGB, DCI-P3_RGB_D65, BT2020_RGB, BT601_YCC, BT709_YCC, XVYCC_601, XVYCC_709, SYCC_601, opYCC_601, BT2020_CYCC, BT2020_YCC} = BT2020_RGB
│   │       ├───"HDR_OUTPUT_METADATA": blob = 375
│   │       └───"vrr_capable" (immutable): range [0, 1] = 1
...
  1. fail to see any visual changes.
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