On 06.06.2013 17:21, Greg KH wrote: > On Thu, Jun 06, 2013 at 11:01:12AM -0400, Dave Jones wrote: >> On Wed, Jun 05, 2013 at 10:54:26AM -0700, Greg KH wrote: >> > These patches make the USB to serial core have the ability to support up >> > to 3000 devices at once now. >> >> I'm curious how this works. What systems have 3000 usb ports ? Surely no one thought that a system could have 255 devices of that sort ~15 years ago. Isn't the idea always to support more that what is possible? >> Or are there some kind of multiplexer devices out there ? >> (If so, I'd like a link, I could use one of those). > > You don't need a multiplexer, you can have a bunch of different USB root > hubs in PCI slots, with USB hubs plugged into them, and lots of > multi-port USB devices connected to them. Tobias has such a system with > 272 single-port USB to serial devices connected, so it's not impossible > to create. Correct. Since you can only have 127 devices on a single USB root-hub, the trick is to spread the devices over many root-hubs. Most mainboards already come with 3-5 root-hubs on-board and you of course can expand that via pci/pcie cards, but be warned, for reasons beyond my imagination, the XHCI specification lets the root-hub vendor decice on how many devices it can support, so you could end up having XHCI root-hubs that support lets say 20 devices. > In thinking about it some more, I'll change that number to 500 for now, > as it's a static array within the tty layer, and then work on making > that dynamic so we don't have any limitations other than the number of > minor numbers for a single major. I also thought about it and I'd say 512 devices is about as high as I would go in any real-world application, otherwise the fallout of a broken system is slowly getting to big, depending on what you do over the serial ports. Also 512 is a nice number. ;) Tobias -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
