PCI and USB devices in the /sys/devices tree, with uevents and USB hot-plug

PCI devices live at /sys/devices/pci0000:00/<addr> and USB devices under the
xHCI controller that carries their bus, below that bus's root hub usbN;
/sys/bus/pci/devices and /sys/bus/usb/devices are symlinks into it, and every
device has its subsystem and driver links, which is how udev learns SUBSYSTEM
and DRIVER. Writing to a device's uevent now announces it with MODALIAS,
PCI_ID, PRODUCT and the rest, so udevadm trigger covers them, and the USB core
sends add when a device is attached and remove as it goes. The root hub is
announced when the controller comes up, before anything is plugged in.

Checked under QEMU: lspci and udevadm info through the links, udevadm monitor
showing the kernel and udev add event for device_add usb-kbd/usb-mouse and
the remove event for device_del, and a triggered add for usb1 and 1-5.

Known limits: the bus-to-controller mapping is a convention (bus N is the Nth
xHCI in PCI order); PCI bridges are not parents; USB interfaces and
/dev/bus/usb nodes are absent; busybox's lsusb reads /dev/bus/usb and prints
nothing.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EVRhxi9zpdW2uDUN4FVcao

kernel/drivers/bus/xhci/xhci.c

@@ -32,6 +32,7 @@
 #include "fs/file.h"
 #include "dev/pci.h"
 #include "dev/usb.h"
+#include "core/hwinfo.h"
 #include "xhci.h"
 
 #define RING_TRBS   64                  // TRBs per ring, including the link
@@ -1673,6 +1674,11 @@ xhci_probe(struct pci_dev *d)
     kinfo("xhci: no devices attached\n");
 
 
+  // Number the bus and announce its root hub before the first port can be
+  // scanned, so `usbN` is in /sys, and udev knows of it, ahead of anything
+  // plugged into it.
+  usb_bus_add(x);
+
   // Now, and only now, turn interrupts on. The order is: claim the line
   // (which unmasks it and sets level/active-low), clear anything the
   // controller already considers pending so that it drives INTx low, and then