On Tue, Dec 23, 2014 at 03:15:45PM -0500, Alan Stern wrote: > On Tue, 23 Dec 2014, Peter Chen wrote: > > > > The Linux USB stack supports turning off port power only under a very limited > > > set of conditions. For example, if the port is hard-wired or not connected at all, > > > and if remote wakeup is not required. > > > > > > > Alan, any reasons/limitations we do not support it (by libusb)? > > For the same reason we don't allow userspace to interfere with any > device: When a kernel driver is in charge of a device, any changes the > user wants to make must go through the driver. If users were allowed > to make changes to a device without telling the driver, then the driver > would not be able to do its job properly. Why we can't turn port power off without unbinding driver? I see we can reset device at devio, why we can reset a device, but can't turn its port off? > > In fact, it's only a coincidence that Deepak's libusb call is able to > succeed. Hub control messages use USB_RECIP_OTHER instead of > USB_RECIP_INTERFACE, even though they are always meant to go to the hub > interface. If the messages used USB_RECIP_INTERFACE then the kernel > would prevent libusb from sending the message unless the user program > first claimed the hub interface (which would mean unbinding the > kernel's hub driver). > Some laptops turns off vbus during the system suspend, does the driver is aware of it? -- Best Regards, Peter Chen -- 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
