linux-kernel - Re: [PATCH] USB: OHCI/UHCI: Add soft dependencies on ehci

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Message-ID: <34c7edd0-3c0c-4a57-b0ea-71e4cba2ef26@rowland.harvard.edu>
Date: Fri, 2 Jan 2026 10:13:50 -0500
From: Alan Stern <stern@...land.harvard.edu>
To: Huacai Chen <chenhuacai@...nel.org>
Cc: Diederik de Haas <diederik@...ow-tech.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Huacai Chen <chenhuacai@...ngson.cn>, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	Shengwen Xiao <atzlinux@...a.com>,
	linux-rockchip@...ts.infradead.org
Subject: Re: [PATCH] USB: OHCI/UHCI: Add soft dependencies on ehci_hcd

On Fri, Jan 02, 2026 at 10:36:35AM +0800, Huacai Chen wrote:
> On Wed, Dec 31, 2025 at 11:21 PM Alan Stern <stern@...land.harvard.edu> 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