Re: [PATCH 0/4] usb: usbtmc: fix undersized high-speed bulk packets on xHCI

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

On Sat, 24 Oct 2026 13:17:11 +0200, Nicolai Buchwitz wrote:
> On 23.10.2026 20:26, Michal Pecio wrote:
> > I don't have any out of spec device, so I tunned a Cypress FX2 into
> > one - it exactly replays every received packet back to the host. I
> > found that power of two sizes from 1 to 512 work, except on Etron
> > HCs which only take 512. Guess we could have an exception for those.
> > 
> > So this seems very viable. I will turn it into a proper patch and
> > see what the maintainers think. Seems better than patching class
> > drivers.  
> 
> Thanks! If you need something testing with the devices I have at
> hand, please let me know.

Hmm, I blamed existing code and found that it actually solved some
reported compatibility issues. Devices that specify bogus packet size
(that they don't really care about) do exist. Somebody had a problem
similar to what I see with Etron (maybe it was Etron), somebody's AMD
chip didn't like packet size of 4. Using legal sizes helped them.

I think I will restrict this to plausible values and default to 512.

Testing which could help is to check various xHCI controllers if they
support those 64 byte endpoints or not. Just because Microsoft and
Chinese TE vendors do something doesn't necessarily mean it's good ;)

The Etron issue is real and Windows seems to have no solution for it
(instant yellow exclamation mark on connection). Better to avoid such
drama with devices that don't care. Hopefully it's unique to Etron.

Regards,
Michal