[Removed linux-pm from the CC: list, as this part of the discussion has managed to drift away from issues of power management.] On Fri, 11 Nov 2011, Sarah Sharp wrote: > > > I've had some slides forwarded to me that have some interesting > > > electrical captures that show that if a host controller is too fast in > > > enabling SOFs after resume signaling ends (where too fast is < 1us), the > > > device doesn't have time to enable its high speed terminations. > > > > Section 7.1.7.7 seems to say that devices have 1.3 us to enable their > > high-speed terminations. (Actually it says "they must transition back > > to high-speed operation, without arbitration, within two low-speed bit > > times of the K to SE0 transition". That's an odd way of putting it, > > but since a low-speed bit lasts for 2/3 us, the meaning is clear.) If > > the host controller starts sending SOFs before this then presumably > > it's not compliant -- although the spec doesn't say that the upstream > > port must wait at least this long. A peculiar omission. > > It may be that they just didn't think about the effect of SOFs. I know > the USB-IF certification folks don't have a test for selective suspend, > since that requires Windows drivers to support it. They do have a test > for system suspend and resume, but devices would probably get reset > after being resumed and the SOF issue would have been missed. It's not just SOFs; any packet would have the same effect. The host should not start sending any data until the downstream device has had its full chance to switch back to high-speed connectivity. This applies to external hubs as well as to host controllers, by the way. Has anybody measured their characteristics? > > Was anyone able to find out if some brands of controllers were > > consistently better or worse than others? > > It actually seemed to be system specific. Some laptops one host > controller version would have random delays, while other laptops with > the same host controller would have a consistent 6us delay before > starting SOFs. So I don't think we can draw any conclusions about one > brand of controllers being better than another. Weird. I can't imagine what would cause that sort of change in behavior. 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
