On Tue, 31 Jul 2012, Sarah Sharp wrote: > > It should also apply to ports of hubs in compound devices. > > The port power off behavior is different for those hubs, Are you sure about that? > so I want to > address that later. For USB 2.0 hubs, doesn't it detect connect events > if you power off a port? No. At least, not according the USB-2.0 spec, section 11.24.2.7.1.1. Some hubs may not obey the spec in this regard. > Also, few USB 2.0 hubs actually power gate the > ports (or all ports are ganged together), so I don't think this would > actually save power. Maybe not, but some hubs do actually turn off power to the ports. Besides, the kernel should do the right thing even if the devices don't. Particularly since the kernel can't tell how much power a hub is actually providing to its ports. > > It is not obvious that external ports should be handled like internal ports. Why not? The important criterion is whether the port is connectable, not whether it is internal. > > It might be sensible to apply auto-powerdown to a connected and opened > > device, but to leave external ports on, so that hotplugs are detected. > > Sure. That's why I was proposing that external ports have the port > power sysfs file set to "on" by default. Then userspace can set it to > "auto" when it feels that external ports are ok to be powered off, > because the user isn't interacting with the system, and it's ok to delay > hotplug events. The kernel should do this for all connectable ports, internal or external. 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
