Re: device enumeration

On 10/5/07, Joe Halpin <[email protected]> wrote:
> On 10/5/07, Alan Stern <[email protected]> wrote:
> > On Fri, 5 Oct 2007, jhalpin wrote:
> >
> > > Is there a way to specify the order in which devices are detected on bootup?
> > > It doesn't look like it to me, but I'm not sure I understand the code well
> > > enough to say.
> >
> > It is not possible.
> >
> > > We have a board running linux 2.6, which always seems to discover an
> > > external usb key first if one is there. This gets assigned to /dev/sda1, and
> > > the internal usb flash gets assigned to /dev/sdb1.
> > >
> > > However, if a key isn't plugged in, the internal flash gets assigned to
> > > /dev/sda1. Our software needs a way to tell which device file is associated
> > > with the internal flash.
> > >
> > > I can root around in /sys and get an answer, but I'm hoping there's a better
> > > way. Incidentally, this board does not run udev, so that's not an option
> > > unfortunately.
> >
> > Filesystem labels?
> >
> > If you want the answer without examining the contents of the storage
> > devices then looking through /sys is the only way to do it.  That's
> > what udev does.  It doesn't have to be hard.
> >
> > For example, just get the target of the /sys/block/sda/device symbolic
> > link.  Right now on my system I have:
> >
> > $ ls -l /sys/block/sda/device
> > lrwxrwxrwx 1 root root 0 Oct  5 11:13 /sys/block/sda/device ->
> > ../../devices/pci0000:00/0000:00:1d.7/usb8/8-4/8-4:1.0/host0/target0:0:0/0:0:0:0/
> >
> > Depending on your hardware arrangement, one of the path components at
> > or near the "usbN" part (usb8 above, or 8-4) should tell you which
> > device you're looking at.
>
> The problem is that the internal flash gets a different address
> depending on whether or not a usb key is plugged in. For example,
> here's what I get when the board is booted with no key in the external
> port:
>
> # ls -l /sys/block/sda/device
> lrwxrwxrwx    1 root     0               0 Oct 23 18:31
> /sys/block/sda/device ->
> ../../devices/platform/ehci_platform.4523/usb1/1-1/1-1.2/1-1.2:1.0/host0/target0:0:0/0:0:0:0
>
>
> This is what happens when the board is booted with a key in the external port:
> # ls -l /sys/block/sda/device
> lrwxrwxrwx    1 root     0               0 Oct 23 18:13
> /sys/block/sda/device ->
> ../../devices/platform/ehci_platform.4523/usb1/1-1/1-1.1/1-1.1:1.0/host0/target0:0:0/0:0:0:0
>
>
> So, we're looking to see if we can get udev installed. In the meantime
> I'm checking sys for the model of the internal flash in
> /sys/bus/scsi/drivers/sd/*/model. As you say, it's not hard, but I
> don't like it because we have to depend on the model name of the flash
> not changing.
>
> Thanks
>
> Joe
>

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