On Thu, May 28, 2020 at 02:16:21PM -0700, Alexandru M Stan wrote: > Hello, > > I'm currently helping to design the hardware for yet another chromebook. > > The particular SOC (and most other alternatives BTH) we're using has a > limited amount of USB controllers available, usually just 1. Being a > chromebook we have quite a few needs for USB though: 2 type C ports, > pogo pins for a connected keyboard, 2 cameras, a type A port. We're > probably going to have an internal hub no matter what, but it looks > like even a 4 port hub might not be enough, going to yet a bigger hub > (which might not be as power efficient) is not ideal, chaining hubs > that are builtin is also meh. > > I noticed that some of our cameras use the USB3.0 lines only. Other > cameras, since they're lower resolution ones use only USB2.0. I > wondered what would happen if the 2 types of cameras were to share a > port, since none of the data lines are common between them. And the hub has no problem with this? That's odd, I would think that it would not like this type of configuration. > I've built a little contraption to test this. It seems to work as > intended. Both usb cameras seem to enumerate, I can even stream from > both at the same time with no problem. A macbook seems to also kind of > work (I can't stream from both for some reason, but i can open either > I want). I can upload detailed lsusb outputs if needed. What host controller are you using for this? > My question here is: is this legal as far as Linux is concerned? Can 2 > devices be enumerated under one physical port (even though we're > talking about separate usb2.0 and 3.0 bus topologies). Is it "legal" as far as the USB spec is concerned? I would try to answer that first before worrying about if Linux can handle it or not :) > It seems to work so far with our linux 5.4 chrome os fork (which as > far as I know is identical to 5.4 LTS for matters concerning usb). But > the question is would this keep working? > > Is there any weird port reset interaction that might be annoying here? > I wouldn't want a reset on the usb 2.0 device to affect the 3.0 > device. Which is why I don't think the hub would like this type of configuration, as who controls the power connections? What happens if one device is suspended and the other isn't? > Is there any other higher level port management going on in linux, > where there might be plans to keep track of things, but it's not piped > enough to see it affect (negatively) this use case. > > If this is ok, was something like this ever done before? > > Bonus, maybe unrelated, question: I've wondered in the past if it's > legal to route 2.0 and 3.0 lines through different paths. > > For example, imagine I had one of each: > * 2+3.0 controller > * 2+3.0 device (eg a 3.0 type A port) > * 2.0 device (older usb device) > > Would it be legal to use only a 2.0 hub, with the 3.0 data lines for > the 3.0 device bypassing the hub. This would be cheaper than having a > fully fledged 2+3.0 hub with the second port's 3.0 data lines wasted. I think you need to talk to some USB hardware engineers for this type of question. good luck! greg k-h
