Re: USB: Request for guidance investigating configuration descriptor enumeration failure

[Michal Pecio <michal.pecio@gmail.com>]

[-- Attachment #1: Type: text/plain, Size: 2119 bytes --]

On Sun, 31 May 2026 10:12:50 -0400, Alan Stern wrote:
> On Sun, May 31, 2026 at 12:38:43PM +0200, Michal Pecio wrote:
> > I also found this article, which claims that Windows uses
> > wLength=255 for its config descriptor request "for compatibility
> > reasons". But your pcap doesn't show anything like that and so do
> > mine (Windows 10).
> > 
> > https://thewindowsupdate.com/2018/10/12/how-does-usb-stack-enumerate-a-device/
> > 
> > So I don't know what's true anymore. Running Windows under a Linux
> > host with individual device passthrough (e.g. qemu) could show what
> > really happens. Even better would be a USB bus analyzer. I don't
> > have one :(  
> 
> I do.  But I don't have one of those Xbox clone devices to test.  And 
> I don't think I have access to any computers with EHCI controllers 
> running Windows 10/11.

The thing to check would be how Windows enumerates any random device,
since I expect that Microsoft designs their own peripherals to work in
Windows out of the box without special quirks.

I have done a quick test with QEMU today. Launched it with

    -device qemu-xhci

booted Windows 10 22H2 and typed in QEMU monitor

    device_add usb-tablet,pcap=qemu-tablet-w10.pcap

The result is attached. The first device descriptor request uses
wLength=64, followed by proper length, and the first configuration
descriptor requests uses wLength=255.

Then there is some other stuff, and another request for the device
descriptor followed by a 9 byte request for configuration. These may
be the requests being logged by Wireshark, but I haven't installed
Wireshark in this VM yet to confirm and correlate.

So this is why I don't trust Wireshark on Windows very much.

And yes, if more people can confirm that Windows behaves like that,
I think that a quirk which mimics this exact behavior could prove more
useful than a wLength=10 one.

We don't know why the device behaves like that. Maybe it's a buffer
overflow in its FW, maybe it fingerprints the host to automagically
select "proper" mode of operation. Maybe the next such device will
check for 255 bytes exactly.

Regards,
Michal

[-- Attachment #2: qemu-tablet-w10.pcap --]
[-- Type: application/vnd.tcpdump.pcap, Size: 2843 bytes --]