[Raion]

I'm making it a point to gather useful info on this. 

As everyone knows, IRIX's USB implementation is lacking. Very lacking. It only supports HID and USB Audio. On top of that, the O2, Octane, Onyx2/Origin2000 are not able to utilize USB at all due to the usb stack not getting compiled for/not supporting their kernels. 

So here's what I've been able to uncover:

1. If you plug in a USB stick or an unknown controller sometimes IRIX can "see" it as an unknown device in its hardware graph. 
2. It appears, from my guesstimate, that the reason it's only supported on Chimera systems is that the usb modules simply don't get compiled/linked into the IRIX sysgen kernel when certain IP platforms are selected. This is reinforced by the PLATFORM restriction symbols I see scattered around the development artifacts left behind out there. 
3. The USB header files included with IRIX don't specify additional classes beyond those already supported.

PymbleSoftware documented this years ago and I just have been sitting on the bits that he dug up. Part of what I'm doing, along with another dev, is documenting some parts of the IRIX kernel that aren't well documented. 

As people know, Rackable "SGI" appears in 2011 to have mass-shredded hundreds of programming manuals, internal documentation, engineering blueprints and engineer's notes by order of Eng Lim Goh (their CTO at the time) as that's what the SGI employees I went after were able to tell me when I contacted them about it. So we have really, no choice but to try and uncover/document these things ourselves. 

I can't make promises I will be able to uncover everything, but the devs of IRIX left us a ton of breadcrumbs around.