Round 34: ROOT CAUSE - mtu3 node never bound; simple-mfd fix; payload 9c55f520

RESEARCH.md

@@ -1297,3 +1297,47 @@ from pumpkin minus all USB/PHY configs.
 | prompt but keys do nothing | keyboard not enumerated (pogo not seated?) or stdin mux — re-seat keyboard and power-cycle |
 
 Awaiting user reboot report.
+
+## Round 34 — [ROOT CAUSE] mtu3 node never bound; "No USB controllers found"
+
+Round 33 report: "no usb controller detected" — matches
+usb-uclass.c:467 `No USB controllers found`, printed when
+`controllers_initialized == 0`, i.e. NO UCLASS_USB bus device was even
+probed (a probed-but-failed controller would have printed
+"Bus usb@11200000: probe failed, error N"). So the xHCI node was never
+BOUND. Not a driver/pin problem: xhci-mtk.o was linked, the dtb had
+mac+ippc+phys, and pinctrl is irrelevant to the controller.
+
+### Root cause (found in source, drivers/core/root.c)
+
+`dm_scan_fdt_node()` walks ONLY the immediate subnodes of a bound node.
+Recursion into a node's children happens solely when some driver binds
+that node (bus drivers recurse via dm_scan_fdt_dev, e.g. the generic
+simple-bus driver's uclass post_bind, drivers/core/simple-bus.c).
+`usb@11200000` is a child of `ssusb@11201000` ("mediatek,mtu3"), which
+has NO U-Boot driver ⇒ its subtree was never scanned ⇒ usb_host never
+bound ⇒ zero UCLASS_USB devices. MMC/video bind because they are
+direct children of /soc (simple-bus).
+
+### Fix (u-boot `75fe5bc9`, payload `9c55f520…`)
+
+The R33 overlay now also sets
+`compatible = "mediatek,mt8183-mtu3", "mediatek,mtu3", "simple-mfd"`
+on ssusb: the generic simple-bus driver binds the node (CONFIG_SIMPLE_BUS=y)
+and its post_bind scans the xhci child. No MMIO touched by that driver
+(empty `ranges` → plat read skipped harmlessly).
+
+Build OK, `_start` still 0x4C001000, dtb verified (simple-mfd present).
+Flashed `9c55f520…`, cmp OK, vbutil body verification succeeded.
+
+### Decision tree for the reboot
+
+| observation | meaning |
+|---|---|
+| banner → `Bus usb@11200000: ...` lines → `N USB Device(s) found` → prompt; typing works | WORKING |
+| banner → `Bus usb@11200000: probe failed, error -N` | xhci/phy/clk probe failed at stage N — transcribe |
+| banner → `Scanning bus ... failed, error N` | root-hub enumeration failed — transcribe |
+| `Bus usb@11200000: No USB Device found` then prompt | controller OK, nothing on the bus — re-seat the keyboard on the pogo pins, power-cycle |
+| `No USB controllers found` again | binding still failing — next step: krane_fb_log in lists_bind_fdt path |
+
+Awaiting user reboot report.