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

[Juergen Gross <jgross@xxxxxxxx>]

On 06/16/2015 06:34 PM, George Dunlap wrote:
> On Tue, Jun 16, 2015 at 4:59 PM, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx> wrote:
> > George Dunlap writes ("Re: [Xen-devel] [PATCH V4 3/7] libxl: add pvusb API [and 1 
> > more messages]"):
> > > > 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.
> > > That would be great if it were true, but I'm not convinced that the
> > > above path doesn't include a globally-enumerated bus number, which might
> > > change across reboots if it's enumerated in a different order.
> > >
> > > It may be that the format above is *not* based on the sysfs path of the
> > > device; that the '0' immediately after the USB means "the first (and
> > > perhaps only) controller at this PCI address".  In which case, yes,
> > > having a string like this might be a unique identifier for a particular
> > > port that would be stable across reboots.  That needs some investigation.
> > That does seem to be the case:
> What seems to be the case -- that it contains the global bus, or not?
>
> > Looking at the output of udevadm monitor --property for sdc1 (on
> > another plug):
> >
> > DEVPATH=/devices/pci0000:00/0000:00:1d.7/usb1/1-1/1-1:1.0/host22/target22:0:0/22:0:0:0/block/sdc/sdc1
> > ID_PATH=pci-0000:00:1d.7-usb-0:1:1.0-scsi-0:0:0:0
> > ID_PATH_TAG=pci-0000_00_1d_7-usb-0_1_1_0-scsi-0_0_0_0
> >
> > I don't know where that ID_PATH comes from.
> It looks like that's constructed in udev:
>
> http://cgit.freedesktop.org/systemd/systemd/tree/src/udev/udev-builtin-path_id.c
>
> See handle_usb() (line 542) in particular.
>
> If I'm reading it right, what it basically does it take the
> [bus]-[port], and replace it with usb-0:[port].  (IOW, the '0' is
> hard-coded.)
>
> Also, if I'm reading it right, this won't work properly for Juergen's
> system that has two USB devices at the same pci address -- they'll
> both end up resolving to [pciaddr]-usb-0:[whatever].
>
> Juergen -- can you give this a try?  Run "udevadm info" on the two USB
> busses that share the same PCI slot, and see if the ID_PATH is the
> same for both?
This was the correct question. :-)

The ID_PATH is the same, but:

It turns out that the two busses are for one physical hcd. One is the
bus for USB 3.0, the other one for USB 2.0. I guess the bus is just
selected by the capability of the plugged in device.

So the [port] in "usb-0:[port]" is unique across the two logical
busses.


Juergen


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