From: Alan Stern <[EMAIL PROTECTED]>
To: Steve Calfee <[EMAIL PROTECTED]>
CC: linux-usb-devel@lists.sourceforge.net
Subject: Re: [linux-usb-devel] Fwd: Re: linux as a hub?
Date: Mon, 11 Dec 2006 15:27:23 -0500 (EST)
On Mon, 11 Dec 2006, Steve Calfee wrote:
> >>Among other problems, a full-speed hub has to be able to handle both
> >>low-speed and full-speed devices attached to its downstream ports, in
> >>spite of the fact that the upstream port is full-speed. If a
low-speed
> >>device was attached to a downstream port then the upstream port would
> >>receive data at low speed, and would not be able to understand that
data
> >>or forward it to the downstream port.
> >>
>
> This is not true. Alan, you are confusing High speed with Full speed.
All
> full and low speed transactions appear on the downstream ports of all
full
> speed hubs. Upstream only goes up through the chain directly to the root
-
> what I mean is hubs don't echo downstream data that is going upstream.
But
> the upstream data is still sent at low speed, if that is what the device
> sends.
You misunderstood me. I was talking about downstream data, not upstream
data. Data destined for a device behind a hub is received at the hub's
upstream-facing port at the speed used by the device, not the speed used
by the hub.
In more detail: Suppose you have a Linux box with a full-speed USB device
interface as the upstream port and you want to use it as though it were a
hub, connecting to a low-speed device on the downstream side. Now when
the actual host (i.e., not the Linux box mentioned above but the host
connected to its upstream port) wants to talk to the low-speed device, it
will send a packet using low-speed bit timings. This packet will not
be received by the Linux box's device interface, which can handle only
full-speed timings. Hence the Linux box won't be aware of the packet and
won't forward it to the low-speed device.
Alan Stern
Oh, I see I did misunderstand you, sorry.
What you are saying is that whatever gadget hardware interface you have on
your PC cannot recognize a "PRE" pid and downshift to low speed. That is
probably true - I have never seen one in any gadget type USB interface. But
a HW interface could do that (for example, all fs hubs) - but it still would
not be possilble to emulate a hub without real hub repeater electronics in
the hw interface.
Sorry about the noise,
Steve
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