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Wed, 04 Nov 2026 21:06:27 -0800 (PST) Received: from rowland.harvard.edu ([2601:19b:d01:d210::66b0]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-91e7e01a3f5sm38061886d6.30.2026.11.04.21.06.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 04 Nov 2026 21:06:27 -0800 (PST) Date: Thu, 5 Nov 2026 00:06:24 -0500 From: Alan Stern To: Michal Pecio Cc: Pengfei Lv , gregkh@linuxfoundation.org, linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, xuhaiyang5@hisilicon.com, xuetao09@huawei.com, caiyadong@huawei.com, suzhuangluan@hisilicon.com Subject: Re: [PATCH] usb: core: hub: Re-guess ep0 maxpacket size on reset Message-ID: <59b0cc88-9ac2-453b-bd68-56aa356c0fc0@rowland.harvard.edu> References: <20261104124402.1269650-1-lvpengfei20@huawei.com> <20261104183603.17d663ce.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-Disposition: inline In-Reply-To: <20261104183603.17d663ce.michal.pecio@gmail.com> On Wed, Nov 04, 2026 at 06:36:03PM +0100, Michal Pecio wrote: > On Wed, 4 Nov 2026 09:59:24 -0500, Alan Stern wrote: > > On Wed, Nov 04, 2026 at 08:44:02PM +0800, Pengfei Lv wrote: > > > When the subsequent re-enumeration then uses the "new scheme" and > > > issues a 64-byte GET_DESCRIPTOR (get_bMaxPacketSize0 with size=64), > > > xhci_check_maxpacket() sees hw==sw==8 and does not issue an Evaluate > > > Context command. The xHC queues a Data TRB of length 64 but applies > > > ep0 max_packet=8 for short-packet detection. > > > > That's exactly what we want it to do. > > Should we want it, though? > > Were there known devices with bMaxPacketSize0 <= 16 which required the > whole transfer to complete before they are reset? > > That's not what happens in the initial iteration, so it seems any such > devices would always take two iterations to begin working. > > Is this what the only correct Proprietary Operating System does? I don't know offhand of any positive or negative examples. But if a device was going to hang if it was asked for the remaining part of the device descriptor after a non-initial reset, how would it ever get through initialization? The procedure does two resets, and after the second reset the device has to send the full descriptor. I don't know what other OSes do about this. Alan Stern