[jcrowthe]

On the Mac Studio (M2 Ultra, J475d) no display controller is wired to the four rear USB-C ports, so a USB-C DisplayPort display there stays dark. The dcpext1 routes in t602x-j474-j475.dtsi sit inside the APPLE_T602X_NON_ULTRA block together with the USB4/PCIe-C wiring, and t6022 does not define it.

The die-0 half of the t6022 keeps the plain labels and the same rear-port wiring as the M2 Max Studio, so this narrows the guard instead of adding a J475d copy:

• t602x-j474-j475.dtsi: only the USB4 aliases, the PCIe-C tunnels and the non-Ultra &display iommus stay under APPLE_T602X_NON_ULTRA. The dcpext1 display routes, dpaudio2, the ATC crossbars and the always-on ATC common power domains apply to J475d too.
• t6022-j475d.dts: t6022-jxxxd.dtsi points the display subsystem at the die-1 HDMI DART only, so add dispext1_dart there for the framebuffers of the new pipe.

Why dcpext1

From macOS on the same J475d (ioreg, IODeviceTree and service planes):

• display-crossbar0 (display-crossbar,t602x) has ufp-endpoints dispext0–7 and dfp-endpoints atc0–7 plus lpdptx-phy0/1. The routing is chosen at runtime, so the ADT alone doesn't fix a controller per port.
• With an LG UltraFine 4K on a USB-C to DisplayPort cable, macOS drives it from DCPEXT1 (dcpext1@15C00000, die 0) on rear port 2 (hpm2 / Port-USB-C@3, usb-pd@3b) and again on rear port 1 (hpm3 / Port-USB-C@4, usb-pd@3c). The display-crossbar0 NVRAM state changes in exactly one byte between the two (0x20 → 0x30).
• HDMI is DCPEXT5 under macOS. Linux keeps using dcpext0_die1 (dcpext4) for HDMI, which is unchanged here.

Device tree check

Built with the kernel's own scripts/dtc (the t602x trees use floating-point literals):

• On bed544b without this change, t6022-j475d.dtb is byte-identical to the one in linux-aurora 7.2.11.aurora16-1.
• With it, t6020-j474s and t6021-j475c keep the same nodes and properties. Sorted dtc -I dtb -O dts -s output is identical; only the position of the dcpext1 alias within /aliases changes.
• On t6022-j475d, the only changes are the dcpext1 alias, dcp@315c00000 enabled with phys/mux-controls/typec-routes for usb-pd@38, @3f, @3b and @3c, dpaudio2 enabled, the four atc*-dpxbar muxes enabled, apple,always-on on ps_atc0..3_common, and the second iommus entry on display-subsystem.

Testing

Mac Studio (M2 Ultra, 2023), apple,j475d/apple,t6022. Stock linux-aurora 7.2.11.aurora16-1 kernel and modules, with only this device tree swapped into boot.bin. OS firmware 13.5, system firmware 26.6.2. m1n1 stage 2 is ba130ffb + omacom/m1n1#46 + the T602x DART patch from #379, which the HDMI DCP needs on this machine (#428); dcpext1 does not depend on it. Hyprland 0.57.2. LG UltraFine 4K over a USB-C to DisplayPort cable, Dell U2718Q on HDMI at the same time.

dcpext1 boots cold and joins the aggregate next to the HDMI DCP:

apple-dcp 315c00000.dcp: DCP index:2 dptx target phy: 0 dptx die: 0
apple-drm soc@200000000:display-subsystem: bound 315c00000.dcp (ops dcp_comp_ops [appledrm])
apple-dcp 315c00000.dcp: DCP booted
apple-display-crossbar f0304c000.mux: Switched dpphy to dispext1,0
apple-dcp 315c00000.dcp: allocated Type-C DPTX PHY 2
apple-dcp 315c00000.dcp: set_digital_out_mode() "3840x2160": 60 ... depth:10

Results (rear ports counted from the left, looking at the back):

Action	Port	Connector	Result
Coldplug	rear 2 (typec2)	DP-3	pass, 3840x2160@60, 10 bpc
Hotplug	rear 1 (typec3)	DP-4	pass, 3840x2160@60 (see note 2)
Hotplug	rear 3 (typec1)	DP-2	pass, 3840x2160@60, about 0.4 s to picture
Hotplug	rear 4 (typec0)	DP-1	pass, 3840x2160@60, about 0.4 s to picture
DPMS off, then wake on input	rear 4	DP-1	pass, stayed off for 75 s, woke on input
HDMI alongside DP	HDMI-A-1		pass, both at 4K60
Front USB-C ports (die 1)			not tested, not wired by this change
Thunderbolt DP tunnel			not tested, USB4/PCIe-C stays non-Ultra
DP audio			not tested; av_service_connect starts the audio service
Suspend/resume			not tested

No WARNING, BUG or oops in any of these boots.

Notes, none of which keeps the display from coming up:

1. On unplug from rear 2, a modeset already in flight was refused: ERROR: Modeset done, but pipe not enabled: fSoftPowerState=1, fDisplayPowerState=0, fHardPowerState=1 / set_digital_out_mode returned 80000104.
2. The first connect on rear 1 timed out once (dcp_dptx_connect: timed out waiting for port 0 link configuration) and came up on its own about a second later. Rear 3 and rear 4 connected without it. The HDMI DCP on this machine logs the same timeout at every boot (j475d (Mac Studio M2 Ultra): dcpext RTKit boot times out (-62), apple-drm never binds — HDMI and Thunderbolt displays all stuck on simpledrm #428), where it does not retry by itself.
3. While locked, the screens repeatedly turn back on. The Dell drops HPD about 7 s into standby and reasserts it 0.5 s later, and Hyprland's backend then re-enables the DPMS-off outputs. That is a compositor issue, reported as DPMS-off outputs are turned back on when another connector's hotplug triggers a CRTC recheck hyprwm/aquamarine#495, and it also happens with the LG switched off.

Related: #428 (J475d HDMI), #379 (m1n1 DART fix), and fc4add9 / 4ef8124, which added these routes for the non-Ultra boards.

[jcrowthe]

Follow-up stacked on this one: #463 enables Thunderbolt DP tunnels on the J475d (t6022 qualification in thunderbolt_apple and phy-apple-atc, plus dual-stream DT wiring with die-0 dcpext0). It includes this commit unchanged as its base, so merging #462 first keeps #463's diff to its own three commits.