I have a CAC-2504 connected to an Intel USB-C DP Alt Mode port. Video works at 3840×2160@60. Linux supports DP CEC tunneling and recognizes the CAC-2504 as an HDMI DP branch device, but the adapter reports DPCD register 0x3000 = 0x00. Linux therefore does not create /dev/cec0. The Linux kernel documentation lists CAC-2504 as a known-working CEC-tunneling adapter. Is there a firmware update for this adapter that enables/fixes DP CEC tunneling? I have seen references to firmware 11.116.007 for CAC-2504.

• CAC-2504 connected via USB-C DP Alt Mode.
• 3840×2160@60 works.
• Kernel 7.2.0.
• CONFIG_DRM_DISPLAY_DP_AUX_CEC=y.
• drm_dp_cec_attach() is definitely called.
• The adapter’s DPCD revision is 1.4.
• DPCD 0x3000 = 0x00, meaning it does not advertise DP CEC tunneling.
• Consequently Linux doesn’t create /dev/cec0.
• Linux documentation nevertheless lists CAC-2504 as a known-working DP-CEC adapter.