Hi,

I have a couple of questions I was hoping somebody could help me with. This needs some background context and is a little lengthy, so I broke it into two headings below--"Background" and "My Questions".


Background
---------

I support RHEL-based customized linux system installers (and runtime) at my work. I have a requirement where, given a device name such as "/dev/sdb", I need to determine where or not it is a USB device (attached to a USB bus). I need to be able to do this without the use of udev, since I need to support this during installation time (where udev is not running).

I need to do this in RHEL5, but also need to make sure it works for RHEL6 for when it becomes available in the future. I have been told by Redhat that RHEL6 will be based on Fedora 10/11. Note that I'm currently testing this on RHEL5 kernel 2.6.18-128.1.6.el5 and the Fedora 10 kernel 2.6.27.5-117.fc10.i686.

I have a method which seems to work for RHEL5, and is as follows, using /dev/sdb to illustrate:

a) I grab the target of the following symlink:

>ls -l /sys/block/sdb/device
lrwxrwxrwx 1 root root 0 Aug 15 10:32 /sys/block/sdb/device -> ../../devices/pci0000:00/0000:00:11.0/0000:02:04.0/usb1/1-1/1-1:1.0/host1/target1:0:0/1:0:0:0

and resolve it to it's absolute path:

/sys/devices/pci0000:00/0000:00:11.0/0000:02:04.0/usb1/1-1/1-1:1.0/host1/target1:0:0/1:0:0:0

Upon inspection of the target, I see the path element "usb1" and so can infer that it is on a usb bus. But I prefer to be more determinstic (strings can change I suppose), so I continue with step (b).

b) I list all "usb bus" type symlinks on the system:

>ls -l /sys/bus/usb/devices
total 0
lrwxrwxrwx 1 root root 0 Aug 15 10:18 1-0:1.0 -> ../../../devices/pci0000:00/0000:00:11.0/0000:02:04.0/usb1/1-0:1.0
lrwxrwxrwx 1 root root 0 Aug 15 10:35 1-1 -> ../../../devices/pci0000:00/0000:00:11.0/0000:02:04.0/usb1/1-1
lrwxrwxrwx 1 root root 0 Aug 15 10:35 1-1:1.0 -> ../../../devices/pci0000:00/0000:00:11.0/0000:02:04.0/usb1/1-1/1-1:1.0
lrwxrwxrwx 1 root root 0 Aug 15 10:18 2-0:1.0 -> ../../../devices/pci0000:00/0000:00:11.0/0000:02:00.0/usb2/2-0:1.0
lrwxrwxrwx 1 root root 0 Aug 15 10:18 usb1 -> ../../../devices/pci0000:00/0000:00:11.0/0000:02:04.0/usb1
lrwxrwxrwx 1 root root 0 Aug 15 10:18 usb2 -> ../../../devices/pci0000:00/0000:00:11.0/0000:02:00.0/usb2

and resolve them to their absolute paths:

/sys/devices/pci0000:00/0000:00:11.0/0000:02:04.0/usb1/1-0:1.0
/sys/devices/pci0000:00/0000:00:11.0/0000:02:04.0/usb1/1-1
/sys/devices/pci0000:00/0000:00:11.0/0000:02:04.0/usb1/1-1/1-1:1.0
/sys/devices/pci0000:00/0000:00:11.0/0000:02:00.0/usb2/2-0:1.0
/sys/devices/pci0000:00/0000:00:11.0/0000:02:04.0/usb1
/sys/devices/pci0000:00/0000:00:11.0/0000:02:00.0/usb2

If at least one of the absolute symlink paths just above matches the first part of the absolute path from (a), the I conclude that the device (/dev/sdb) must be on a usb bus.


When I try to apply the same algorithm in Fedora 10 (RHEL6), things are a little different in step (a).

>ls -l /sys/block/sdb/device
lrwxrwxrwx 1 root root 0 2009-08-10 20:49 /sys/block/sdb/device -> ../../../4:0:0:0

Note the considerably shorter target of the symlink. I can't infer anything about USB from this path, and cannot compare it against the symlinks found in /sys/bus/usb.

I then tried the following:

>cd /sys/block/sdb
>ls -l device
lrwxrwxrwx 1 root root 0 2009-08-10 21:23 device -> ../../../4:0:0:0
>cd ../../../4:0:0:0
>pwd
/sys/devices/pci0000:00/0000:00:11.0/0000:02:03.0/usb1/1-1/1-1:1.0/host4/target4:0:0/4:0:0:0

The above path is the result I want and will make the algorithm as I described for RHEL5 work in Fedora 10 (RHEL6) as well.

Making it a bit more "productized" for inclusion into a script:

>ls -l /sys/block/sdb/device
lrwxrwxrwx 1 root root 0 2009-08-10 21:23 /sys/block/sdb/device -> ../../../4:0:0:0

>pushd . > /dev/null; cd /sys/block/sdb/../../../4:0:0:0; pwd; popd > /dev/null
/sys/devices/pci0000:00/0000:00:11.0/0000:02:03.0/usb1/1-1/1-1:1.0/host4/target4:0:0/4:0:0:0

So, it would seem that there is something magical about the target of the /sys/block/sdb/device symlink. If you do straight "ls -l" on it, you get an abbreviated relative path; if you follow the actual link and do a "pwd" you get the full device path. The way I see it, I have to use the method describe just above (following the link instead of just using "ls -l").


My Questions
-----------

- What is this piece of magic that is happening in RHEL6 with the symlink?

- Is my method of following the symlink (i.e., "cd" then "pwd") a valid one? Am I missing something? I would like one algorithm that I would like to implement right now and have it work for both RHEL5 and RHEL6 (I don't want to revisit this in the future). The approach described above for Fedora 10 does work in RHEL5 as well and so it would be the basis of the algorithm.

- Is my overall algorithm for determining if a device on the usb bus valid? (ie., comparing links from /sys/block/<dev>/device and /sys/bus/usb)?


Thanks,
Mark

Have you tried the `lsscsi` command, provided of course that you (can) have it available at the point at which you need it?
At very least, I believe lsscsi does not depend on udev. It queries sysfs as you have.

I haven't yet fully analyzed what you've posted above, but if `lsscsi` does not (or isn't available) give you what you need, please post again.

Sample:
Thanks,
Sasha

Thanks for the reply...I just checked both of my test machines and neither seem to have the lsscsi command. From looking at the output in your post, I'm not quite sure what to depend on. I know that these commands often read data from the device and include it in the output. I tend to not want to rely on that data, but more on what the kernel itself provides. Do you know if this data is kernel-provided or read from the device?

Thanks,
Mark

The information is kernel provided, and is derived from sysfs.

From the man page:

You could do something like:

dupe