Steam Machine: HDMI 2.1 FRL (dcfeaturemask=0x402) works on kernel 7.2 but loses VRR; one-line amdgpu_dm fix missing from linux-neptune-72

Your system information

• SteamOS 3.9.0 (BUILD_ID 20260828.100), preview branch
• Kernel 7.2.0-valve1-1-neptune-72-gd39b4282853d (linux-neptune-72 7.2.0.valve1-1); also checked the 7.2.4-valve1 source, same code
• Steam client: current preview
• Hardware: Steam Machine (DMI "Fremont"), Navi 33 dGPU 1002:7481, DCN 3.2.1
• Display: LG OLED83C4 connected directly to the HDMI port (no dock or adapter)
• Opted into Steam client beta?: Yes
• Opted into SteamOS beta?: Yes (preview)
• Have you checked for updates in Settings > System?: Yes

Please describe your issue in as much detail as possible:

Kernel 7.2 brought native HDMI 2.1 FRL to amdgpu. The Steam Machine ships it disabled behind amdgpu.dcfeaturemask bit 0x400 (DC_FRL_MASK, Valve commit c3778921bf0d "drm/amd/display: Disable FRL and add module param to enable it"). Booting with amdgpu.dcfeaturemask=0x402 (FRL plus the default 0x2 bit) works well on the Steam Machine: the LG C4 now gets 3840x2160 at 144 Hz, 4:4:4, 10 bpc, HDR (PQ, BT.2020), where the stock TMDS path is limited to 4K120 4:2:0 8 bpc.

However on the FRL link VRR disappears entirely:

# TMDS (stock): /sys/kernel/debug/dri/0/HDMI-A-1/vrr_range
Min: 40
Max: 144
vrr_capable = 1

# FRL (dcfeaturemask=0x402):
Min: 0
Max: 0
vrr_capable = 0

Root cause (verified in the linux-neptune-72 source package): the Valve tree carries Tomasz Pakula's "drm/amd: VRR fixes, HDMI Gaming Features" series as [FROM-ML] commits, but not his later fix-up "Switch to signal type helper functions from DC" (https://github.com/Lawstorant/linux/commit/21d564d5a1, branch hdmi-7.2). In amdgpu_dm_update_freesync_caps() the HDMI branch is only entered for SIGNAL_TYPE_HDMI_TYPE_A. An FRL sink is SIGNAL_TYPE_HDMI_FRL, so it falls through every branch and freesync_capable stays false.

One-line fix, drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c (~line 14327 in 7.2.0-valve1):

 	/* HDMI */
-	} else if (sink->sink_signal == SIGNAL_TYPE_HDMI_TYPE_A) {
+	} else if (dc_is_hdmi_signal(sink->sink_signal)) {
 		/* Prefer HDMI VRR */
 		if (hdmi_vrr.supported) {

dc_is_hdmi_signal() already exists in drivers/gpu/drm/amd/display/include/signal_types.h and covers both TMDS and FRL.

Verification: I rebuilt only amdgpu.ko from the 7.2.0-valve1 tag with this change (out-of-tree build against linux-neptune-72-headers, installed under /lib/modules/<ver>/updates/). With FRL enabled the connector now reports vrr_range 40 to 144, vrr_capable=1, passive_vrr_capable=1, allm_capable=1, and gamescope enables VRR on the CRTC (VRR_ENABLED=1) at 3840x2160@143.99. The amdgpu_dm_dtn_log HPO row shows 4:4:4, 10 bpc, 4 lanes, DSC off. Several hours of play so far without link drops.

Patch, build script and install/rollback tooling for other Steam Machine owners: https://github.com/beaglemoo/steam-machine-hdmi21

Request: cherry-pick the one-line change into linux-neptune-72 (it is a strict subset of what Pakula's branch and the 7.4 AMD series do), and consider enabling DC_FRL_MASK by default on the Steam Machine once VRR works, as mainline 7.4 will.

Related: #2698 (4K120 over HDMI), #2740 (VRR on Steam Machine, different cause: TMDS path on 6.18).

Steps for reproducing this issue:

1. Steam Machine on SteamOS 3.9.0 connected by HDMI to an HDMI 2.1 TV with VRR (LG C4 here).
2. Add amdgpu.dcfeaturemask=0x402 to the kernel command line (/etc/default/grub.d/*.cfg + update-grub), reboot.
3. Observe 4K144 / 10 bpc modes appear, but cat /sys/kernel/debug/dri/0/HDMI-A-1/vrr_range reads 0/0 and modetest -M amdgpu -c shows vrr_capable = 0 on HDMI-A-1.
4. Apply the one-line change above to amdgpu, reboot: vrr_range 40-144, vrr_capable = 1, VRR active at 144 Hz.

Start in drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c at amdgpu_dm_update_freesync_caps(), then inspect dc_is_hdmi_signal() in drivers/gpu/drm/amd/display/include/signal_types.h. Build the amdgpu module and reproduce with amdgpu.dcfeaturemask=0x402, checking vrr_range and vrr_capable on HDMI-A-1. Done means FRL reports the display's VRR range and gamescope enables VRR at 4K144.