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

include/dev/kdev.h

@@ -40,5 +40,7 @@ int  kdev_path(int i, char *buf, int cap);
 
 // Announce a device on the uevent socket: action is "add", "remove", "change".
 void kdev_uevent(int i, const char *action);
+// The next SEQNUM: events from every source (kdev, PCI, USB) share one count.
+uint kdev_seqnum(void);
 // What reading a device's `uevent` file says.
 int  kdev_uevent_text(int i, char *buf, int cap);