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

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Message-ID: <20120420161955.GA7090@kroah.com>
Date:	Fri, 20 Apr 2012 09:19:55 -0700
From:	Greg KH <greg@...ah.com>
To:	Anil Nair <anilcoll90@...il.com>
Cc:	Wang YanQing <Udknight@...il.com>,
	lkml <linux-kernel@...r.kernel.org>, linux-usb@...r.kernel.org
Subject: Re: [RFC]Is usb port number fixed?

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On Fri, Apr 20, 2012 at 09:39:37PM +0530, Anil Nair wrote:
> 
> 
>     >USB ports on a hub are all enumerated the same, but the root hubs
>     >aren't, and sometimes hubs on the same bus are not as well.
> 
>  
>   Does every device assigned a port number serially, that is in increasing
> order?

Yes.

>   If a USB port number is assigned to a USB device then are other USB devices
> able to use it,
>   if connected USB device is Disconnected?

It can, why?  USB devices don't care about their device number.

>     >But usually, things do come up in the same way all the time, so
>     >depending on your use case, you can rely on this, but it is recommended
>     >that you look for something else a bit more unique, like a serial number
>     >on a device (if it is present), if you wish to uniquely identify a
>     >device all the time.
> 
> 
>   Does the Linux Kernel keeps the history of connected devices? So that next
> time,   the same USB device get assigned the same USB port number.

No.  I think you are confusing port number with the USB device number,
both of which mean nothing to a USB device at all, and they shouldn't to
anyone else.

greg k-h
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