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Fri, 02 Jan 2026 07:13:53 -0800 (PST) Date: Fri, 2 Jan 2026 10:13:50 -0500 From: Alan Stern To: Huacai Chen Cc: Diederik de Haas , Greg Kroah-Hartman , Huacai Chen , linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, Shengwen Xiao , linux-rockchip@lists.infradead.org Subject: Re: [PATCH] USB: OHCI/UHCI: Add soft dependencies on ehci_hcd Message-ID: <34c7edd0-3c0c-4a57-b0ea-71e4cba2ef26@rowland.harvard.edu> References: <20251230080014.3934590-1-chenhuacai@loongson.cn> <2025123049-cadillac-straggler-d2fb@gregkh> <73d472ea-e660-474c-b319-b0e8758406c0@rowland.harvard.edu> <0c85d288-405f-4aaf-944e-b1d452d0f275@rowland.harvard.edu> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Fri, Jan 02, 2026 at 10:36:35AM +0800, Huacai Chen wrote: > On Wed, Dec 31, 2025 at 11:21 PM Alan Stern wrote: > > > > On Wed, Dec 31, 2025 at 05:38:05PM +0800, Huacai Chen wrote: > > > From your long explanation I think the order is still important. "New > > > connection" may be harmless for USB keyboard/mouse, but really > > > unacceptable for USB storage. > > > > > > If we revert 05c92da0c524 and 9beeee6584b9, the real problem doesn't > > > disappear. Then we go back to pre-2008 to rely on distributions > > > providing a correct modprobe.conf? > > > > The warning message in 9beeee6584b9 was written a long time ago; back > > then I didn't realize that the real dependency was between the -pci > > drivers rather than the -hcd ones (and I wasn't aware of softdeps). The > > soft dependency in 05c92da0c524 is between the -pci drivers, so it is > > correct. > > > > To put it another way, on PCI-based systems it is not a problem if the > > modules are loaded in this order: uhci-hcd, ohci-hcd, ehci-hcd, > > ehci-pci, ohci-pci, uhci-pci. Even though the warning message would be > > logged, the message would be wrong. > Correct me if I'm wrong. > > I found XHCI is compatible with USB1.0/2.0 devices, Yes. > but EHCI isn't > compatible with USB1.0. Instead, EHCI usually has an OHCI together, > this is not only in the PCI case. It's more complicated than that. For quite a long time now, most systems using EHCI have not included a companion OHCI or UHCI controller. Instead they include a built-in USB-2.0 hub; the hub is wired directly into the EHCI controller and the external ports are connected to the hub. USB-2.0 hubs include transaction translators that relay packets between high-speed and low- or full-speed connections, so they can talk to both USB-1 and USB-2 devices. Hence no companion controller is needed. I don't remember when Intel starting selling chipsets like this, but it was probably around 2000 or earlier. (Some non-Intel-based systems included a transaction translator directly in the root hub, so they didn't even need to have an additional USB-2.0 hub.) Before that, systems did include companion controllers along with an EHCI controller. I don't know of any non-PCI systems that did this, but of course some may exist. However, the EHCI-1.0 specification says this in section 4.2 "Port Routing and Control" (p. 54): The USB 2.0 host controller must be implemented as a multi-function PCI device if the implementation includes companion controllers. > So I guess OHCI/UHCI have an EHCI dependency in order to avoid "new > connection", not only in the PCI case. Do you know of any non-PCI systems that do this? Alan Stern