On Fri, 12 Jul 2013, Peter Chen wrote: > On Thu, Jul 11, 2013 at 02:58:04PM -0400, Alan Stern wrote: > > The hub driver was recently changed to use "global" suspend for system > > suspend transitions on non-SuperSpeed buses. This means that we don't > > suspend devices individually by setting the suspend feature on the > > upstream hub port; instead devices all go into suspend automatically > > when the root hub stops transmitting packets. The idea was to save > > time and to avoid certain kinds of wakeup races. > > > > Now it turns out that many hubs are buggy; they don't relay wakeup > > requests from a downstream port to their upstream port if the > > downstream port's suspend feature is not set (depending on the speed > > of the downstream port, whether or not the hub is enabled for remote > > wakeup, and possibly other factors). > > > > We can't have hubs dropping wakeup requests. Therefore this patch > > goes partway back to the old policy: It sets the suspend feature for a > > port if the device attached to that port or any of its descendants is > > enabled for wakeup. People will still be able to benefit from the > > time savings if they don't care about wakeup and leave it disabled on > > all their devices. > > > > I have a question what kinds of cases we can get the time saving? > > - For no hub cases (like most embedded devices), it just moves > set suspendM at portsc from usb_port_suspend to ehci_bus_suspend Right. It has always worked this way, even before the CONFIG_USB_SUSPEND option was removed. > - For hub cases, since you said it must send suspend to hub at > global suspend case (system suspend procedure), this suspend > request procedure can't be skipped. No -- it is necessary to send the suspend request to the hub _only_ if one of the devices beneath the hub is enabled for remote wakeup. Otherwise the suspend request can be skipped. For example, if you had a USB flash drive plugged into a hub, it wouldn't be enabled for remote wakeup. Therefore its port would not need a selective suspend. 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
