Message-ID: <CAAhV-H7j=cD9dkaB5bWxNdPtoVR4NUFvFs=n46TaNte1zGqoOA@mail.gmail.com>
Date: Sat, 3 Jan 2026 11:16:59 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Alan Stern <stern@...land.harvard.edu>
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 2, 2026 at 11:13 PM Alan Stern <stern@...land.harvard.edu> wrote:
>
> 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.
>
Thank you for your explanation, so it means there are two methods: The
old one is EHCI with a companion OHCI; the new one is EHCI with a
USB-2.0 hub. Right?

> > 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?
Unfortunately, in 2026 there are really "EHCI with a companion OHCI"
for non-PCI systems, please see
arch/loongarch/boot/dts/loongson-2k0500.dtsi.

Huacai

>
> Alan Stern

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