On Mon, 6 Jul 2026 at 21:27, Philippe Mathieu-Daudé <[email protected]> wrote: > > On 6/7/26 19:53, Peter Maydell wrote: > > On Mon, 6 Jul 2026 at 18:29, Philippe Mathieu-Daudé > > <[email protected]> wrote: > >> - Bus capacity is odd. We have a generic Bus-class @max_dev field but > >> do not use it with PCI variants. If set, it limit the instances > >> capacity. The capacity is tracked in the instance state, and when > >> no @max_dev is set the only way to mark a bus full is using the > >> qbus_mark_full() helper. Then if we remove devices, the bus remains > >> full, and there is no helper to restore available slots. > > > > The qbus_mark_full() function is intended for the use case where > > a machine has a bus which in real hardware is entirely internal, > > so you have something like an I2C bus which has some on-board > > I2C devices connected to it, but there's no external socket > > that the user could add more to the bus for. The idea is that > > the machine model creates all the devices for this bus, then > > marks it as full. The devices never get removed from the bus, > > because they're hardwired in, not user-unpluggable. > > Are you saying, while this happens to work correctly for the > Ultrasparc Sabre PCI host bridge, this is not the appropriate > API to use? I understand the I2C example when the lines are > populated within a board but can't be accessed outside of it > (like our CLI -devices), but what about this Sabre case, the > bus is internal to the chipset; can we consider it a full bus > or should we use the BusClass::max_dev property instead?
If the bus is internal to the chipset then qbus_mark_full() sounds fine -- the only devices that will be put on the bus are the ones the machine itself creates, and the bus isn't intended to be available for users to dynamically add more. It looks from a quick grep like the only use we make of BusClass::max_dev is that we set it to 1 for: - PCI controllers that have a single root complex - virtio transports, because you can plug exactly one virtio backend into a transport Did I miss any other uses? -- PMM
