linux-kernel - Re: [RFC]Is usb port number fixed?

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Date:	Mon, 23 Apr 2012 11:13:06 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Wang YanQing <udknight@...il.com>
cc:	Anil Nair <anilcoll90@...il.com>, Greg KH <greg@...ah.com>,
	lkml <linux-kernel@...r.kernel.org>, <linux-usb@...r.kernel.org>
Subject: Re: [RFC]Is usb port number fixed?

On Mon, 23 Apr 2012, Wang YanQing wrote:

> On Sun, Apr 22, 2012 at 08:47:24AM -0400, Alan Stern wrote:
> > On Sun, 22 Apr 2012, Anil Nair wrote:
> > 
> > > The port number comes from root hub managed by the host controller. Its
> > > fixed dynamically during enumeration, and can change.
> > > Please correct me if i am wrong.
> > 
> > You are indeed wrong.  Port numbers are not fixed dynamically and they 
> > cannot change.
> Right?

Right.

> Could you pin out the codes in kernel?

I don't understand the question.  What code in the kernel do you want 
to see?

> > On the other hand, bus numbers _are_ assigned dynamically and they can 
> > (and do) change.
> > 
> Indeed, I never meat the situation lspci output different bus number on the same machine.
> Any more comment?

Try unloading ehci-hcd, uhci-hcd, and ohci-hcd, and then reloading them 
in different orders.  Depending on what sort of hardware you have, the 
bus numbers will change.


On Mon, 23 Apr 2012, Anil Nair wrote:

>   You meant to say port numbers are fixed but bus address changes
> dynamically, also does the host controller
>   fixes the port number?

Yes.

> If USB Bus bandwidth is not available, how does it
> assign port numbers?

The port numbers are determined by the hardware -- each set of wires
has its own port number.  The numbers have nothing to do with
bandwidth.

Alan Stern

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