Re: send data on "packet level"

On 8/7/07, Alan Stern <[email protected]> wrote:
> On Tue, 7 Aug 2007, Gabriel Maganis wrote:
>
> > On 8/7/07, Alan Stern <[email protected]> wrote:
> > > On Mon, 6 Aug 2007, Gabriel Maganis wrote:
> > >
> > > > Hello,
> > > >     How could one communicate with usb devices on the packet level
> > > > i.e. chapter 8 of the usb spec instead of the device requests in
> > > > chapter 9? I'd like to be able to send SETUP packets or DATA0 packets
> > > > and get the ACK coming from the devices versus a GET_DESCRIPTOR
> > > > request over the control pipe thru usb_control_msg.
> > >
> > > It depends on what you mean.  Individual packets in USB don't form a
> > > complete communication; not much below the level of a transaction is
> > > meaningful.  If that's good enough for you (a Setup transaction
> > > followed by an IN or OUT transaction for instance), you can do it by
> > > hacking the host controller drivers.
> > >
> > > If you really want control at the level of single packets, you will
> > > have to build your own hardware.  Normal host controllers don't provide
> > > that capability.
> > >
> > > But why would you want to do this?  What's wrong with the existing
> > > facilities?  I don't see any point in, say, just sending a SETUP
> > > packet.  You won't know whether the device has processed it without
> > > going through the remainder of the control transfer (data stage and
> > > status stage).  And if you're going to go through all that anyway, then
> > > why not let the existing drivers take care of the details for you?
> > >
> > > Alan Stern
> > >
> > >
> >
> > Hello Alan,
> >     My goal is to be able to examine the fields they talk about in
> > chapter 8 of the usb spec like the SYNC field and such. Is there a way
> > to look at those from a usb_control_msg? or, at leat usb_submit_urb?
>
>
> Not only is there no way to look at the SYNC and related fields from
> usb_control_msg() or usb_submit_urb() -- there is no way to look at
> them in software at all!  The hardware strips those fields out before
> storing the data in the computer's memory.
>
> If you have an oscilloscope or USB bus analyzer, you can monitor the
> USB data lines and see the fields directly.  Otherwise it's impossible.
>
> Alan Stern
>
>

I see. So the most I have access to from software are on the
transaction level like you described before i.e. a SETUP transaction
which is actually a SETUP packet followed by a DATA packet then an ACK
packet? And I can't even do so unless I hack the host controller
drivers?

Thanks,
Gabriel

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