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

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

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On Wed, 3 Jun 2026 22:02:44 -0400, Alan Stern wrote:
> I used a bus analyzer to capture what happens when Windows 11 
> initializes and enumerates a USB-2 flash drive.  The short answer is 
> that yes, the initial Get-Configuration-Descriptor request is for 255 
> bytes.

Could you also try a few BIOSes, UEFIs and such?
Or anything from Apple?

It has occurred to me that if Linux enumeration switches Nikhil's
device more or less permanently to Android mode then his BIOS must
be using some other scheme too if the device to survives intact.

I also tried Windows 95 OSR 2.1 in QEMU, which AFAIK is the fist
Windows release to support USB in any form. It works like W10:

Get Descriptor DEVICE 64 bytes
Set Address
Get Descriptor DEVICE 18 bytes
Get Descriptor CONFIGURATION 255 bytes
Get Descriptor CONFIGURATION actual size, only if greater than 255

And an emulated SuperSpeed UAS device with W10, which is different:

<xHCI Address Device command *probably* goes here>
Get Descriptor DEVICE 18 bytes
Get Descriptor CONFIGURATION 255 bytes
Get Descriptor CONFIGURATION actual size, only if greater than 255
Get Descriptor BOS 255 bytes

I'd guess that MS tries to get whole descriptors in one go, but is
worried (or aware) of buggy devices crashing when wLength > 255.

Notable SuperSpeed differences vs Linux:
- doesn't bother with the DEVICE request before Set Address
- CONFIGURATION is queried before BOS
- first BOS request is for 255 bytes

Some of these may be responsible for the recent BOS problems with
some particular buggy video capture devices.

I'm attaching W95 enumerating a full-speed device with configuration
exceeding 255 bytes and W10 enumerating a SuperSpeed UAS device (the
first few packets are QEMU BIOS, which is similar to Linux).

Regards,
Michal

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