Re: EHCI and OHCI port numbering?

Alan Stern <[email protected]> Mon, 30 Jul 2007 13:54:48 -0400 (EDT)
Newsgroups	gmane.linux.usb.devel
Message-ID	<[email protected]>
On Mon, 30 Jul 2007, Alex Chiang wrote:

> Hello,
> 
> How are ports numbered/ordered in the case of an EHCI controller
> with a companion controller?
> 
> Example:  N_PORTS = 6
> 
> AND
>           N_CC = 2
>           N_PCC = 3
> 
> OR        PORTROUTE[] = 1,1,1,2,2,2
> 
> Would give a routing configuration of:
> 
>    EHCI port 1 = OHCI1 port 1 (not 2 or 3)
>    EHCI port 2 = OHCI1 port 2 (not 1 or 3)
>    EHCI port 3 = OHCI1 port 3 (not 1 or 2)
>    EHCI port 4 = OHCI2 port 1    etc.
>    EHCI port 5 = OHCI2 port 2
>    EHCI port 6 = OHCI2 port 3
> 
> Is this assumption correct?

Yes.  Except that the companion controllers might not be OHCI-1 and
OHCI-2 -- they might be OHCI-3 and OHCI-4.  In fact, something like
that is certain to happen if you have more than one EHCI controller.

> The broader context of this question is, on ia64 systems, certain
> EFI drivers only support 1.1. So imagine a scenario where one
> might have two USB cdroms attached to the system, but only want
> to boot off of one (or maybe something more plausible, a USB hard
> drive and a USB cdrom, and are booting off the CD).
> 
> When the bootloader hands off to the kernel and says "use the
> device attached to OHCI controller N, port M" will the value of M
> change when the EHCI driver takes over?

There's no way for the bootloader to tell the kernel that.  The most it 
can do is something like "root=/dev/sr0".  It's up to the udev scripts 
in the kernel's initramfs to make sure that the correct device is 
mapped by /dev/sr0.

Alan Stern


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