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

[Peter Chen <hzpeterchen-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On Sat, Jan 16, 2016 at 11:40:32AM -0500, Alan Stern wrote:
> On Sat, 16 Jan 2016, Arnd Bergmann wrote:
> 
> > > 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?
> 
> The root hub does not have a port number relative to the host
> controller.  In a sense, the root hub is _part_ of the host controller;
> it represents the controller's connection to the USB bus (whereas the
> logical device we commonly associate with the host controller
> represents the controller's connection to the upstream PCI/whatever
> bus).
> 
> You can also ask whether port numbers relative to the root hub are
> the same as the port numbers relative to the controller.  Under normal
> circumstances they are the same.  However, it is possible that they
> will be different for USB-3 host controllers (!).
> 
> This is because USB-3 uses two buses per controller: the legacy
> Low-Speed/Full-Speed/High-Speed bus compatible with USB-1.1 and USB-2
> devices, and the new SuperSpeed bus compatible with USB-3 devices.  
> Even though there's only the one controller, each bus has its own root
> hub and the ports for each root hub are numbered starting from 1.  
> Thus, for example, an xHCI controller with 6 LS/FS/HS ports and 3 SS
> ports would have a USB-2 root hub with ports 1-6 and a USB-3 root hub
> with ports 1-3.  However, xHCI treats the controller as a single entity
> and numbers all the ports sequentially; thus the controller would have
> ports 1-9.  (Even though the three SS ports would be the same physical
> objects as three of the LS/FS/HS ports -- USB-3 ports literally contain
> two independent sets of signal wires.)

Sorry, I have not USB-3 platform now.
I have several questions:

- For USB-3, how many times hub_configure() will be called for root hub?
One or two?

- Which the value of hub->descriptor->bNbrPorts for root hub?

> 
> This isn't merely a convention in the specifications; the hardware
> actually uses both numbering schemes (although I don't know about USB-3
> controllers other than xHCI; they might be different).  Untangling this
> mess might be difficult.  I don't know how existing DT files handle it.
> 

-- 

Best Regards,
Peter Chen
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