On Mon, 29 Nov 2010, Sarah Sharp wrote: > Return the correct USB 3.0 hub descriptor when khubd (or usbfs) asks for > the roothub descriptor of the main usb_hcd. Fill in DeviceRemovable for > the USB 2.0 and USB 3.0 roothub descriptors, using the Device Removable > bit in the port status and control registers. I think xHCI is the first > host controller to actually properly set these bits (other hosts say all > devices are removable). Other host controllers don't have Device Removable bits in their port status and control registers. > When userspace asks for a USB 2.0-style hub descriptor for the USB 3.0 > roothub, stall the endpoint. This is what real external USB 3.0 hubs do. > We can't just fill in the buffer with a USB 3.0 hub descriptor, because > that descriptor can be longer than a USB 2.0 hub descriptor with a similar > number of ports, and we could cause a buffer overflow. Yes, you shouldn't return a USB-3 descriptor when asked for a USB-2 descriptor, but not for the reason you stated. The fact that the descriptor might be too long doesn't matter; the transfer will be truncated to fit in the buffer provided. After all, even now a program can use usbfs to ask for a hub descriptor for a non-USB-3 controller and pass a buffer that's too small. No, the real reason for not doing it is simply that you would be returning incorrect data to the caller -- not what was asked for. 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
