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SubjectRe: [linux-usb] Re: USB device allocation (fwd)
On Wed, Oct 06, 1999, Matthew Dharm <mdharm@one-eyed-alien.net> wrote:
>
> My understanding (and maybe I'm smoking crack here) is that there are only
> 127 devices allowed on a USB tree, and minor numbers go all the way up to
> 255. So, why not simply have devices accessed by their device number?
>
> That is to say, the first device on a USB tree is /dev/usb0. The next is
> /dev/usb1. Our little user-space daemon will simply provide symbolic
> links between, say, /dev/mouse and /dev/usb0 (if that's proper). Minor
> numbers are assigned by some sensible system (i.e. first-come
> first-serve, or perhaps some topology based thing).

Not smoking crack, but there are 127 device id's. This means up to 127
physically different devices can be connected to one device. However, a
device doesn't mean one function. USB devices have things called
interfaces which operate independately.

I have a keyboard with a mouse port on it (legacy ps/2 style mouse),
it's 1 USB device id, but 2 functions, and thusly 2 devices the way unix
traditionally see them.

You can theoretically have an unlimited amount of interfaces on one
device, but practically, about 15 (16 endpoints - 1 default).

Think of a High Density modem, with 8 (or up to 15) modems in one box,
kinda like a USR Total Control card or something similar. Definately
feasible.

That also doesn't take into account multiple busses.

JE


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