[Triumf-linux-managers] FYI - adventures in udev, and PCI slot numbering

[Andrew Daviel]

FYI

I was recently looking at how devices in Linux get numbered - I have a PC 
with two identical video capture cards and wanted to make sure they are 
always numbered the same way round after a power outage.

I'm not sure whether I really needed to do this, but in any case I have a 
recipe to ensure that the card in the lower PCI slot is always 
/dev/video1 regardless of whether there are 1 or 2 cards fitted 
(normally, if there's only one card it would be video0).
Similarly one can pin a particular NIC to eth0 by MAC address.

One can build udev rules and put them in /etc/udev/rules.d, e.g. I have
   KERNEL=="video*", ID=="0000:08:01.0",  BUS=="pci", NAME="video1"
to pin PCI slot 08:01 to /dev/video1

This is documented in e.g.
http://www.reactivated.net/writing_udev_rules.html

One can find suitable parameters using e.g.
   $ udevinfo -a -p  `udevinfo -q path -n /dev/video0`
In SL6, this is now
   $ udevadm info -a -p `udevadm info -q path -n /dev/video0`

The same technique should work to assign invariant device names to USB 
devices such as audio adapters, printers and webcams, if a suitable 
identifier can be found (serial number etc.)


The PCI slots found with "lspci" seem to be invariant to power-cycling 
and whether the BIOS is set to PnP OS or not. This may be obvious but I 
had wondered whether the slot numbers were physical ones or logical ones 
assigned by the BIOS. E.g. the second PCI slot on my motherboard is 
08:01 regardless of whether it has a video card or an audio card in it, 
and whether the first slot (08:00) is populated.

PCI slots seen in Windows device manager match those seen in Linux with 
lspci (again ought to be obvious), e.g.
   02:09.3 System peripheral: Ricoh Co Ltd xD-Picture Card Controller (rev 12)
   PCI bus 2 device 9 function 3 - Ricoh xD-Picture Card Controller

Incidentally, lspci version 3 (standard in SL6) has a "-q" option to 
query the PCI database online, while pcitools 3 builds easily on SL5. So 
you can easily identify previously "unknown" devices.



-- 
Andrew Daviel, TRIUMF, Canada
Tel. +1 (604) 222-7376  (Pacific Time)
Network Security Manager