# K1436–K1440 PCI bridge bus-tree contract

The root host owns an inclusive ECAM bus range. Each bridge describes the bus
where it resides (`primary`), the first bus behind it (`secondary`), and the
last bus behind it (`subordinate`). A root bridge must reside on the root bus;
a nested bridge must reside on its parent's secondary bus. In every bridge,
`secondary` is strictly greater than `primary` and no greater than
`subordinate`. The child's complete downstream range must fit within its
parent's downstream range.

Every secondary bus is claimed once. Sibling downstream windows must not
overlap. A child using its parent's secondary bus as its own secondary bus is
rejected as `SecondaryCollision`; invalid hierarchy, duplicate bus claims,
and full inventory errors leave the existing tree and resource count intact.
Publication occurs only after validating the entire new node.

The positive local fixture has a root bridge from bus 0 to buses 1–2 and a
nested bridge on bus 1 leading to bus 2. The negative fixture gives that
nested bridge secondary bus 1, colliding with the parent. K1440 must verify
real PCI bridge configuration and one-time device discovery in an isolated
guest before development guest acceptance can be recorded.