On Fri, 13 Jan 2012, Sarah Sharp wrote: > On Fri, Jan 13, 2012 at 10:45:24AM -0500, Alan Stern wrote: > > [CC: list trimmed a little; I hope nobody objects] > > > > On Thu, 12 Jan 2012, Sarah Sharp wrote: > > > > > It would be a big help if it was added. It matters to Linux server > > > distributions in particular because several server vendors have started > > > to integrate emulated USB devices onto the server. A "BMC" device from > > > an outside vendor attaches to a USB host port, and emulates a hub with a > > > USB mouse, keyboard, and maybe a USB CD drive attached. Then a sys > > > admin can remotely share a .iso file, and on the server side the CD just > > > appears in the emulated USB CD drive. > > > > The impact will be minimal, because of drive polling. > > Yes, I know. I had hoped that if the auto-suspend timeout was set to > zero, the mass storage driver would suspend the device immediately after > any SCSI command, but you've said that's not true. Well, it makes sense for usb-storage devices always to have their autosuspend timeout set to 0. This means relying on the timeouts of the underlying SCSI devices. I don't know if you would want to set _those_ to 0, but you might set them to something like 100 ms. And hopefully the CD polling rate would be set pretty low -- but that's probably not under direct user control. But the emulated hub will still have an autosuspend timeout of 2 seconds, unless that is changed as well. > > Well, one thing they could do right away is unbind the hub driver from > > the BMC's emulated hub. Then the emulated hub would be suspended and > > wouldn't affect the EHCI controller. > > > > Of course, it would then be necessary to rebind the hub driver before > > using the BMC's integrated components, which might be an issue. > > Yes, I suppose that could work, assuming they're actually installing any > software on the server side for their BMC. With the set up they have > now, the server only has to understand about USB devices in order to > have the admin control the system via mouse, keyboard, and mass storage > device. Only the admin's box needs to have special software to start > the session. (I'm just guessing how this all works, of course, so I may > be wrong.) The problem is that rebinding the hub driver couldn't be done via the BMC's keyboard. It would have to happen some other way, such as over the network. Ah -- another solution is to unbind the emulated CD drive from usb-storage. Then it would get suspended automatically, and the mouse and keyboard would still be usable. 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
