So really, I think this suggests that some devices shouldn't have
any requirement to sit on a bus. A UART16650A does not sit on bus.
It sits on a card and is wired to the ISA bus or is sometimes wired
directly to pins on a CPU on a SoC.
I don't think we want to model individual resistors on a serial card
as separate qdev objects. We want the serial card itself to be a
qdev (as it is a hotpluggable entity) and the individual serial
interfaces on that card (as they are duplicates of each other and of
interest to the user).
You're missing the fundamental problem which arises because we've
introduced an object model without thinking through how devices ought
to be modelled.
All devices should have a DeviceState associated with them.
Otherwise, there's really no point in having qdev at all.
We have lots of devices today that don't have DeviceState's associated
with them because the have a separate qdev representation with a
reference to the non-DeviceState object.
We have non-DeviceState objects because otherwise we end up with an
inheritance diamond. We have this problem because we want to have
relationships like: DeviceState <- SystemDeviceState <- ISADevice <-
ISASerialDevice.
But ISASerialDevice is not the only type of serial device. You can
also have a SystemSerialDevice that's directly attached to the System
bus. That means you'd have to have:
SerialDevice -> ISASerialDevice -> SystemDeviceState -> DeviceState
-> SystemSerialDevice -> SystemDeviceState ->
DeviceState
Which is a classic MI diamond. The only way to resolve this modelling
problem is to split out the common code and rely on a has-a
relationship instead of an is-a. That gives you:
ISASerialDevice->SystemDeviceState->DeviceState
SystemSerialDevice->SystemDeviceState->DeviceState
ISASerialDevice has-a SerialDevice
SystemSerialDevice has-a SerialDevice
And since we want SerialDevice inherit from a DeviceState (recall, all
devices should have DeviceStates):
SerialDevice->DeviceState
No more MI diamond and all devices have DeviceStates. Coincidentally,
it matches more closely how hardware works..