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

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

On Sat, 6 Jun 2026 22:17:20 -0400, Alan Stern wrote:
> On Thu, Jun 04, 2026 at 12:53:23PM +0200, Michal Pecio wrote:
> > 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?  
> 
> Here's the information.  It may not clear things up as much as you
> would like.
> 
> Samsung UEFI BIOS:
> 	Get Device Desc		8
> 	Set Address
> 	Get Device Desc		18
> 	Get Config Desc		255
> 	Set Config
> 
> Lenovo UEFI BIOS:
> 	Set Address
> 	Get Device Desc		8
> 	Get Device Desc		8
> 	Get Device Desc		18
> 	Get Config Desc		8
> 	Get Config Desc		32
> 	Get String Desc 0	2
> 	Get String Desc 0	4
> 	Set Config
> 
> IpadOS 26.5:
> 	Set Address
> 	Get Device Desc		18
> 	Get String Desc 2	2
> 	Get String Desc 2	34
> 	Get String Desc 1	2
> 	Get String Desc 1	18
> 	Get String Desc 3	2
> 	Get String Desc 3	50
> 	Get Config Desc		9
> 	Get Config Desc		32
> 	Set Config
> 
> OSX 10.5:
> 	Get Device Desc		18
> 	Set Address
> 	Get Device Desc		18
> 	Get String Desc 2	2
> 	Get String Desc 2	34
> 	Get String Desc 1	2
> 	Get String Desc 1	18
> 	Get String Desc 3	2
> 	Get String Desc 3	50
> 	Get Config Desc		8
> 	Get Config Desc		32
> 	Set Config

Quite a variation.

Ipad is interesting because the single Device request must be a
high-speed optimization, or they hacked their xHC to ignore short
packets and keep reissuing IN tokens until the URB is filled.

Linux has been doing the initial 8 or 64 byte read at LS and HS since
forever and I found no explanation besides nobody daring to touch it.

Though I wonder if it wouldn't make sense to skip this at SS, because
Windows does (in my QEMU tests) and very likely Apple too.

> I suspect that non-storage devices (such as an Xbox clone) might have 
> more stringent requirements, because they don't need to be visible to
> a BIOS.  In general, imitating Windows is almost certainly the best 
> approach -- except perhaps for a few devices which are meant to be
> used exclusively with Macs.

Are you suggesting to make the 255 byte configuration the new default,
or are we still talking about a quirk exclusive to one device so far?

Regards,
Michal