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

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Message-ID: <CAAhV-H601B96D9rFrnARho4Lr9A+ah7Cx7eKiPr=epbG17ODHQ@mail.gmail.com>
Date: Sat, 3 Jan 2026 11:57:47 +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 Sat, Jan 3, 2026 at 11:33 AM Alan Stern <stern@...land.harvard.edu> wrote:
>
> On Sat, Jan 03, 2026 at 11:16:59AM +0800, Huacai Chen wrote:
> > 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?
>
> 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.
>
> Since these systems don't use PCI, the question I raised earlier still
> needs to be answered: How do they route connections between the ports
> and the two controllers?
>
> There may be some exceptions, but for the most part, the code in
> ehci-hcd was written assuming that only PCI-based controllers will have
> companions.  If you want to make an exception for loongson-2k0500, you
> will need to figure out how to get it to work.
Loongson-2K0500 use EHCI/OHCI with platform bus, while
Loongson-2K1000/2000 use EHCI/OHCI with PCI bus. They use the same USB
IP cores, so the route connections are probably the same.

Huacai
>
> Have you tested any of those systems to see how they behave if a USB-1
> device is already plugged in and running when the ehci-hcd driver gets
> loaded?
>
> Alan Stern