On Fri, 23 Sep 2011, Steve Calfee wrote: > On 09/23/11 10:17, Alan Stern wrote: > > On Fri, 23 Sep 2011, Steve Calfee wrote: > > > >> On 09/23/11 07:27, Alan Stern wrote: > >>> On Thu, 22 Sep 2011, Iain Barker wrote: > >>> > >>>> Hi all, > >>>> > >>>> Is it possible to limit the maximum speed at which a USB 2.0 HUB will connect to USB devices? > >>>> e.g. to make USB 2 devices connect at FULL_SPEED instead of HIGH_SPEED > >>> > >>> The only way to do this is to force the hub itself to connect at full > >>> speed. There's no way to tell a hub to degrade just one of its ports. > >>> If the hub is plugged directly into an EHCI controller and there's no > >>> companion USB-1.1 controller (as is the case with Intel's > >>> "rate-matching" hubs), there's no way to do it at all. > >>> > >> Hi Iain and Alan, > >> > >> Yes, there is a way to do it. Connect a FS only hub in front of the one > >> you want to downshift. That will shift the entire downstream bus to FS only. > > > > That's what I said above: Force the hub itself to connect at full > > speed. > > > > In this case it's not possible. The hub is built into the system; > > there's no way to interpose another hub between it and the controller, > > and even if there were, the EHCI controller wouldn't be able to > > communicate with a full-speed-only hub. More and more we're seeing > > systems that have only an EHCI controller with no companion > > controllers; all communication has to take place through the built-in > > hub. > > > Hi Alan, > > I see your point. However, how can this be true? How would you connect a > mouse to that port? I don't know of any HS mice. By "that port", I assume you mean the port through which the internal hub connects to the EHCI controller. You can't connect a mouse to that port. The port is internal to the motherboard and it is hardwired to the hub. All the exposed ports lead to the hub. Note that there's no problem plugging a low-speed mouse into a high-speed hub. > An EHCI controller does not have to use a companion, it can have built > in TT hardware (like the ARC/TRANSDIMENSION/chipidea/MIPS/... ehci > core). That one can change speeds and not have a companion. Or it can have a dedicated hub plugged into its one and only port, which is the strategy Intel has been using. I don't know of any server-type systems (or desktop or laptop, for that matter) using an EHCI controller with an embedded TT. > If Iain has an external USB drive, it plugs in somewhere. If he plugs > that into a FS hub, which is plugged into a USB port, then I think he > will have the usb drive running at FS. Whether the USB drive could be considered "external" isn't clear. Iain's exact words were: "HP Proliant DL120 G7 with ATP industrial-grade USB 2.0 storage modules hcd is connected to a hub on the motherboard, nothing is external (no cables etc)". It's not obvious whether that "no cables etc" applies to the storage modules or just to the internal hub. If there is a plug that can be removed, then yes, he could do as you suggest. It's not what he asked, though, and it might not solve his underlying problem -- although it certainly would slow down his servers considerably. Alan Stern -- 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
