> USB is a well known standard and power saving in USB is also well understood and exercised. PCIe power saving was not even a thing until relatively recently

well i mean it's all PCIe at the end of the day - all the USB stacks run on top of PCIe lanes - principally anything USB is pure overhead.

> PCIe power saving was not even a thing until relatively recently, and anything you'd developed for this new "HCI-over-raw-PCIe" thing would have to be done almost from scratch.

as far as i can tell PCIe power saving is still not a thing (at least of any significance). in any case, the energy usage of the transport layer and what messages you're sending over it should be largely orthogonal - i don't know exactly what you think you'd need to implement here. i mean, we're not being hypothetical here: apple have literally done this in their new macs, and the blog post specifies what they had to do to make it work in asahi: send the bluetooth HCI messages over the PCIe transport directly!

> [...] and may even provide further physical power saving considering how dead simple USB is as an interface

again, it's all already on PCIe no matter which way you cut it..