Re: device enumeration

[Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>]

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.

Alan Stern


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