On Mon, 22 Feb 2010, Andrey Borzenkov wrote:
> This is in 2.6.33-rc8 currently but has been true for a long time.
>
> Ever since my distribution completely switched to (hot-/cold-)plugging
> all needed modules I get this:
>
> ehci_hcd: USB 2.0 'Enhanced' Host Controller (EHCI) Driver
> Warning! ehci_hcd should always be loaded before uhci_hcd and ohci_hcd,
> not after
>
> During cold plugging uhci comes before ehci simply by virtue of
> directory traversal order:
>
> 00:1d.0 USB Controller: Intel Corporation 82801H (ICH8 Family) USB UHCI
> Controller #1 (rev 02)
> 00:1d.1 USB Controller: Intel Corporation 82801H (ICH8 Family) USB UHCI
> Controller #2 (rev 02)
> 00:1d.2 USB Controller: Intel Corporation 82801H (ICH8 Family) USB UHCI
> Controller #3 (rev 02)
> 00:1d.7 USB Controller: Intel Corporation 82801H (ICH8 Family) USB2 EHCI
> Controller #1 (rev 02)
>
>
> {pts/1}% ls -f1 /sys/devices/pci0000:00
> [...]
> 0000:00:1d.0/
> 0000:00:1d.1/
> 0000:00:1d.2/
> 0000:00:1d.7/
> [...]
>
> So my question is - is the warning still true? How serious problems can
> result if uhci is loaded before ehci?
The warning is accurate, but it isn't terribly serious. When uhci-hcd
is loaded first, devices will initially connect through the UHCI
controllers. Then when ehci-hcd is loaded later, the devices will be
disconnected and they will reconnect through the EHCI controller.
Those devices which aren't high-speed will then be routed back to the
UHCI controllers.
Usually there's no harm done, but the disconnections will generate some
bad-looking log messages, and it's possible that some devices will get
confused by being disconnected in the middle of an operation. If
ehci-hcd is loaded first, none of these disconnection issues arise.
Alan Stern
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