Looking at the enumerable buses in the kernel I don't see any which have real support for any kind of registration of devices prior to their enumeration. Similarly currently all the DT bindings in the kernel I've been able to notice cover only non-enumerable buses. This generally makes sense where enumerable buses are used in standard fashions since the binding would be at best redundant and at worst inaccurate. However there are often corner cases in embedded systems where hardware has been hooked up outside of the normal enumeration mechanisms, for example with software power control or extra signals wired. One example that's bugging me right now is that on the Insignal Arndale platform there's a USB hub connected to one of the USB ports on the SoC (not as a PHY, it seems we also need the internal PHY running to talk to the device). The hub needs to be "plugged" into the SoC after the SoC USB controller has started with some GPIOs so we need to tell the system that the hub exists and needs to be synchronised with the USB controller. Another case that's going to be problematic once it's in mainline is Slimbus - this is a bus used in some embedded audio subsystems which is enumerable in a similar manner to USB but where the devices on the bus are normally powered up only on demand (causing them to hotplug when used and unplug when idle) and have at least interrupt lines wired to the SoC using a normal interrupt outside the enumerable bus. I know there's been some discussion of this topic but do we have any general consensus on how to handle such things both from a Linux driver model point of view and from a DT/ACPI point of view?
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