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[83.28.84.81]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48e3d42f884sm1327266f8f.57.2026.10.23.11.26.50 (version=TLS1_2 cipher=AES128-SHA bits=128/128); Fri, 23 Oct 2026 11:26:51 -0700 (PDT) Date: Fri, 23 Oct 2026 20:26:45 +0200 From: Michal Pecio To: Nicolai Buchwitz Cc: Greg Kroah-Hartman , Steve Bayless , Guido Kiener , linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, Mathias Nyman Subject: Re: [PATCH 0/4] usb: usbtmc: fix undersized high-speed bulk packets on xHCI Message-ID: <20261023202645.7bf4827a.michal.pecio@gmail.com> In-Reply-To: References: <20261020-fix-usbtmc-short-packets-v1-0-903f4d029193@tipi-net.de> <20261020184244.69b78d5e.michal.pecio@gmail.com> <20261021005654.23517441.michal.pecio@gmail.com> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Wed, 21 Oct 2026 11:34:19 +0200, Nicolai Buchwitz wrote: > I checked on Windows 11 (Intel xHCI, WinUSB + libusb): same > descriptors as on Linux (HS, bulk wMaxPacketSize 64), and a single > 4096 byte bulk IN read returns the complete 110 byte reply, so it > seems, that Windows programs the 64 bytes into the endpoint instead > of forcing 512. Yes, that's what it seems to be doing. I passed a fake storage device with weird endpoints to Windows 10 in QEMU and with some debug tracing (slot/ep context hexdumps are my addition) I see this: usb_xhci_fetch_trb addr 0x0000000139a3c450, CR_CONFIGURE_ENDPOINT, p 0x000000013a18d000, s 0x00000000, c 0x01003001 usb_xhci_slot_ctx addr 0x000000013a18d020, c 0x68000000 0x00000000 0x00000000 0x00000000 usb_xhci_ep_ctx addr 0x000000013a18d0a0, ep 4, c 0x00000000 0x00400016 0x39a3cc01 0x00000001 0x00000000 usb_xhci_ep_ctx addr 0x000000013a18d1c0, ep 13, c 0x00000000 0x00400036 0x375ff001 0x00000001 0x00000000 The 0x0040 in the second field is packet size, and it becomes 0x0400 when I change to 1024, or 0x0002 with 2050. Windows blindly copies those eleven bits here and only gets upset if they are all zero, then it resets and enumerates again a few times. 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. Regards, Michal