Re: USB device nodes in device tree

[Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>]

On Saturday 04 April 2015 12:08:36 Florian Fainelli wrote:
> Le 30/03/2015 12:35, Alan Stern a écrit :
> > On Mon, 30 Mar 2015, Valentin Longchamp wrote:
> > 
> >> Hello,
> >>
> >> We are currently developing a board with an USB MFD device (I2C and GPIOs are to
> >> be supported). The device is soldered on the board and is the only one on the
> >> bus, so the bus is not really "dynamic".
> >>
> >> Since it's an USB device, it should be dynamically detected by the kernel and it
> >> would not require a node in the board's DTS. However, I need to have the devices
> >> which are "behind" the MFD USB device to be in the DTS (I2C bus topology, and
> >> some of the GPIOs are to be used directly by some other DTS nodes as well).
> >>
> >> Is there a way to add a node for USB device in a DTS ? Is there an available
> >> example for this ?
> > 
> > No, there is no way to do it as far as I know.  The main problem is 
> > that Device Tree is static whereas USB devices are dynamic.
> 
> The PCI(e) bus has the same problem, yet you can specify a PCI device
> child node, and have a compatible string which will match the vendor
> id/device id tuple, device class etc... such that you can use Device
> Tree to add additional information not necessarily available in other
> ways such as MAC addresses and similar.
> 
> Once the PCI bus is scanned, pci_device present in Device Tree get a
> device_node pointer assigned.
> 
> I don't think there is anything doing this yet for USB devices, but
> maybe that's something that should be there?
> 

It's come up a number of times. Basically we should support this, and I
believe it's relatively straightforward to do, someone just has to 
implement it, but setting the of_node pointer for a USB device that
gets added and if that has a matching node.

There is a binding for USB that we never implemented in Linux. It
used to be at http://playground.sun.com/1275/bindings/usb/usb-1_0.ps
and I can send a copy to anyone that needs one. Most of the contents
are irrelevant unless you want to implement a full Open Firmware
rather than a flattened device tree. 

The important parts are:

#address-cells: needs to be 1
#size-cells: needs to be 0

reg: '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.'

	Arnd
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