How does one figure out what to use for "32-bit BAR Space Preferred Size" to allow for live-adding of devices?

[markmi]

How does one figure out what to use for "32-bit BAR Space Preferred Size" to allow for live-adding of devices after booting?

Could there be a way to see the size for an example "all possible pre-connected" boot to later enter for planned live connections of some subset of that configuration?

I'm guessing rule(s) of thumb are needed that will generally over-allocate by some amount instead of being minimal.

(Note: I'm not likely to connect PCIe devices after booting for the foreseeable future. But the question just seemed to stand out on seeing the field and its description.)

[mariobalanica]

RPi's PCIe does not support hot-plugging, but you can force a bus rescan in Windows (refresh Device Manager) and Linux (echo 1 > /sys/bus/pci/rescan), that's the only reason behind this option. It tends to go wrong when removing devices, so it should be avoided.

I'm guessing rule(s) of thumb are needed that will generally over-allocate by some amount instead of being minimal.

Yeah, that's the idea. If (hypothetically) you connect a device after boot, rescan, and it fails to allocate 32-bit BARs, then the space can be increased.

UEFI will reserve 5 MB at all times since this is what RP1 wants. Generally 32-bit BARs tend to be small and well below that size (unless you use switches with multiple endpoints..), so realistically there's no need to ever touch that setting.