Re: [PATCH] usb: core: add USB_QUIRK_NO_BOS for devices that hang on BOS descriptor

[Greg KH <gregkh@linuxfoundation.org>]

On Sun, Dec 07, 2025 at 01:37:34AM +0100, Michal Pecio wrote:
> On Sun,  7 Dec 2025 01:00:07 +0100, Johannes Brüderl wrote:
> > Add USB_QUIRK_NO_BOS quirk flag to skip requesting the BOS descriptor
> > for devices that cannot handle it.
> > 
> > Add Elgato 4K X (0fd9:009b) to the quirk table. This device hangs when
> > the BOS descriptor is requested at SuperSpeed Plus (10Gbps).
> > 
> > Link: https://bugzilla.kernel.org/show_bug.cgi?id=220027
> > Signed-off-by: Johannes Brüderl <johannes.bruederl@gmail.com>
> 
> IIRC what we found in this bug is that the device will happily respond
> to BOS descriptor requests issued by lsusb after it's enumerated.

That's very odd.

> So it appears that (only when running at 10G, wtf) the device expects
> something to happen before it is able to produce this descriptor. And
> apparently Linux is doing things in different order than certain other
> operating system supported by the vendor.

Any ideas what "order" is not the same here?  Should we just not be
reading the BOS descriptor at early enumeration and just wait until
someone actually wants it?

> But the reporter simply applied a patch similar to yours and lost
> interest in debugging any further.

Understood, but I'd like to get to the root of this if possible so that
we don't end up just adding a bunch of devices to this quirk because we
are doing something "odd" that normal USB-IF testing isn't catching.
That feels like a bad overall decision to make.

> And unfortunately WinPCAP fails to capture most of enumeration process,
> so I don't know how that other software is doing it.

usbmon with windows in a virtual machine might catch this?

thanks,

greg k-h