[PATCH v3 1/1] USB: core: let USB device know device node

[arnd@arndb.de (Arnd Bergmann)]

On Friday 15 January 2016 10:11:06 Alan Stern wrote:
> On Fri, 15 Jan 2016, Arnd Bergmann wrote:
> 
> > On Friday 15 January 2016 17:17:27 Peter Chen wrote:
> > > Although most of USB devices are hot-plug's, there are still some devices
> > > are hard wired on the board, eg, for HSIC and SSIC interface USB devices.
> > > If these kinds of USB devices are multiple functions, and they can supply
> > > other interfaces like i2c, gpios for other devices, we may need to
> > > describe these at device tree.
> > > 
> > > In this commit, it uses "reg" in dts as port number to match the port
> > > number decided by USB core, if they are the same, then the device node
> > > is for the device we are creating for USB core.
> > > 
> > > Signed-off-by: Peter Chen <peter.chen@freescale.com>
> > 
> > Acked-by: Arnd Bergmann <arnd@arndb.de>
> > 
> > 
> > Just one last question:
> > 
> > >  	dev->active_duration = -jiffies;
> > >  #endif
> > > -	if (root_hub)	/* Root hub always ok [and always wired] */
> > > +	if (root_hub) {	/* Root hub always ok [and always wired] */
> > >  		dev->authorized = 1;
> > > -	else {
> > > +		dev->of_node = bus->controller->of_node;
> > 
> > 
> > You are adding the of_node of the controller to the root hub, which I
> > guess means that we now have two 'struct device' instances with the
> > same of_node. They have different bus_types, so I think that is ok,
> > but I wonder if it would be better to leave out the of_node for the
> > root hub to avoid the confusion. Can you think of a case where we
> > actually want to add properties for the root hub that we can't do
> > more easily in the host controller?
> 
> There may not be any such cases, but there's still a good reason for
> setting the root hub's of_node pointer: to initialize the recursion
> along the USB device tree.
> 
> This leaves the question of whether OF will always use the same node to
> represent the host controller and the root hub.  In other words, if a
> motherboard has a fixed device plugged into a fixed root-hub port, will
> the DT description make that device a child of the host controller?  
> Or will there be a node in between (to represent the root hub)?

Good question. I'm sure the answer is somewhere in this document

http://www.firmware.org/1275/bindings/usb/usb-1_0.ps

but unfortunately I don't understand USB addressing well enough
to answer this. I think the key sentence is the definition of the
"reg" property:

| prop-encoded-array: one integer, encoded as with encode-int.
| The "reg" property for a device node shall consist of the number of the
| USB hub port or the USB host controller port to which this USB device
| is attached. As specified in [2] section 11.11.2.1, port numbers range
| from 1 to 255.

Does the root hub have a port number relative to the host controller,
or is it identical to the host controller from the addressing
perspective?

	Arnd