Re: [Xen-devel] [PATCH V4 3/7] libxl: add pvusb API [and 1 more messages]

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

George Dunlap writes ("Re: [Xen-devel] [PATCH V4 3/7] libxl: add pvusb API"):
> Remember that the path you gave in your previous e-mail isn't the path
> for the *usb device*, it's the path for the *block device*.  It
> contains a PCI address, but it looks like it also contains part of the
> USB topology.  Are you sure that's actually a stable interface, or
> does it just happen that on your hardware the discovery always happens
> in the same order?

The block device is (in path terms) underneath the usb device,
obviously.  Not all of that path is relevant to identifying the
USB device.

> On my system /sys/bus/usb/devices/2-3.3 is a link to
> /sys/devices/pci0000:00/0000:00:1d.7/usb2/2-3/2-3.3/.  This contains
> the pci bus address, but it also contains the bus number, which we've
> just said may be unstable across reboots.

You mean the 2 in `usb2' ?  I think that bus number is the bus number
within the controller, not globally.

> I suppose it might be possible to specify <buspci,port> -- the pci
> address of the root bus, and the topology from there.  In theory I
> guess that should be stable?

Yes.  The whole point of paths like this is that they are stable if
the physical topology doesn't change.  So on my netbook

  /dev/disk/by-path/pci-0000:00:1d.7-usb-0:1:1.0-scsi-0:0:0:0-part1

always refers to the 1st MBR partition on logical device 0 on the USB
storage device plugged into the USB port physically on the front left
of the computer.

> In any case, at the moment you're essentially inventing from whole
> cloth a new way of specifying USB devices that (as far as I know)
> isn't supported by any other program that uses USB.

If you can't specify the device by hardware path, you can't specify it
deterministically.

And as you can see it _is_ supported by other programs that use USB.
"mount" can use it!

I think the hardware path to the controller, at least, should be
treated as an opaque OS-specific string.  It might have a different
format on BSD.

Ian.

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