On Thu, Mar 28, 2024 at 03:20:10AM +0000, Cui Alan wrote:
> If we can query the capability of the root hub among the whole system, to indicate if kernel should
> notice userspace that the device is pluged into a inappropriate port. When a port with higher speed and/or more capabilities the
> device required is available. eg a dp display device on a normal typec even usb 2.0 only, or a superspeed device on a usb 2.0 port.
> (Windows DID that)
>
> Also some non-standard device only provide a usb superspeed without usb 2.0 even the specifaction says
>
> > 11.3 USB 3.2 Device Support for USB 2.0
> >USB 3.2 device certification requirements require support for USB 2.0 for all user attached devices.
>
> I looked up the USB BOS descriptor and found that the field describing the device's speed capabilities is one-hot coded.
> So also the device can put a billboard or BOS on usb2.0 to indicate it cannot run on such a port.
>
> struct _SUPER_SPEED_USB_DEVICE_CAPABILITY_DESCRIPTOR
> {
> BYTE bLength;
> BYTE bDescriptorType;
> BYTE bDevCapabilityType; // ONE HOT BIT CODING!
> BYTE bmAttributes;
> WORD wSpeedsSupported;
> BYTE bFunctionalitySupport;
> BYTE bU1DevExitLat;
> WORD wU2DevExitLat;
> }SUPER_SPEED_USB_DEVICE_CAPABILITY_DESCRIPTOR
>
> I search the kernel source, but nothing shows that kernel trying to notice user or even a pr_info when situations above happened.
There is a message that gets logged when a high-speed-capable device is
plugged into a USB-1.1 port, but no message when a SuperSpeed-only
device is plugged into a USB-2 port.
Would you like to write a patch that produces such a message?
Alan Stern
