Re: Splitting 1 USB port between 2 devices

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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