Re: [gentoo-user] Re: Which USB device on which controller?

[Mark Knecht <markknecht@gmail.com>]

On Sat, Feb 28, 2009 at 6:54 AM, Grant <emailgrant@gmail.com> wrote:
>>>>> Sounds like it's a 1.1 device to me.
>>>>
>>>> Yep, that's what it sounds like to me too.
>>>>
>>>> Dale
>>>
>>> But that's OK isn't it?  I don't need 2.0 speeds between each webcam
>>> and the controller, I just need the increased overall bandwidth of a
>>> 2.0 controller so one of the 1.1 webcams doesn't use all of it.
>>
>> The 2.0 controller doesn't _have_ increased bandwith if it's
>> talking to 1.1 devices.  In that case, the 2.0 controller is
>> transferring data at the same speed as a 1.1 controller.
>
> I thought the total bandwidth available for a controller was different
> than the bandwidth at which it communicates with one device.  You're
> saying any 1.1 device that uses 12 mbit/s will 100% monopolize a 2.0
> controller so no other devices can function?
>

Yes. USB is not like 1394/Firewire. Firewire will increase and
decrease bandwidth as different devices take control of the bus. There
is negotiation between devices and the device that owns the bus
controls how the bus is used.

With USB a 1.1 device on the bus causes the 2.0 controller to operate
at 1.1 speed so there is only 12Mb/S on that bus once you plug the 1.1
web cam in.

> Taking a different approach, since I have 2 USB controllers (EHCIx1,
> OHCIx1) why can't I operate one webcam on one controller and one
> webcam on the other controller so they can both function?
>

If the hardware is working correctly, if the drivers are independent,
and IF IF IF the two physical ports you chose are actually going to
completely different controllers then you can. I do this. This sort of
thing becomes an issue with USB when you make a mistake an plug an old
USB mouse or keyboard into the wrong physical port and bring your 2.0
bus speed down to 1.1 bandwidth.

Over and out,
Mark

> - Grant
>
>>> I get the feeling I have a misconception somewhere along the
>>> line here. Could someone straighten me out?
>>
>> A Corvette going 3MPH will get to the finish line at exactly
>> the same time as a 4-year-old kid on a tricycle going 3MPH.  It
>> doesn't matter what the controller is capable of -- it matters
>> what speed it's actually talking.
>>
>> --
>> Grant
>
>