[argyllcms] More on instrument access
- From: Leonard Evens <len@xxxxxxxxxxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Sun, 06 Jan 2008 16:04:31 -0600
I've been spending an enormous amount of time trying to figure out how
to get access to an instrument as a user other than root. In the
process I've learned quite a lot about udev rules, how to create them
and how to test them, which I suppose will eventually be of use. In the
process, I believe I've found a bug in the way udev works with my latest
Fedora kernel
kernel-2.6.23.12-52.fc7
But, after trying all sorts of complicated things, I did finally find
something which works. The argyll web page suggests that for the latest
versions of udev, something like the following should work.
# i1Display
SYSFS{idVendor}=="0971", SYSFS{idProduct}=="2003", GROUP="plugdev"
That certainly won't work for me since I don't have a group "plugdev".
But I tried it with a group I do have and belong to, and it did change
the group ownership of the device but left the permissions at 0644 which
didn't do me any good.
So I tried
# i1Display
SYSFS{idVendor}=="0971", SYSFS{idProduct}=="2003", MODE:="0666"
instead, and much to my amazement, this did work. In my case the device
is /dev/bus/usb/004/xxx. Here 'xxx' is a positive integer given as three
digits possibly starting with leading zero(s). It is incremented by one
each time I plug in the device and a new instance is created, the
previous instance having disappeared. The ':=' is an instruction to
preserve the assignment even if later rules try to change it. I think
just using '=' for the assignment won't work because I've tried similar
things in the past, but I haven't actually tested that. "004" refers to
the particular usb port I always use, and I presume it would work
similarly if I chose another port. So wherever the device is plugged
in, every user should have access to it with the argyll programs.
Perhaps the web page should be altered to reflect what I've outlined
here. Of course, given apparent problems with the current kernel and
udev, I can't be sure this will survive the next kernal upgrade, but I
have hopes.
One of the strange things I found is the following. The argylll web
page suggests as an earlier approach the rule
SYSFS{idVendor}=="0971", SYSFS{idProduct}=="2003", PROGRAM="/bin/sh -c
'K=%k; K=$${K#usbdev}; printf bus/usb/%%03i/%%03i $${K%%%%.*}
$${K#*.}'", NAME="%c", MODE="666"
This actually sets the permissions properly on the device, although the
PROGRAM statement returns with error status 1, in which case the NAME
assignment is not made. That is because %k has the value 4-1 and the
shell syntax results in something printf can't parse. udevtest seems to
tell me in this case that the device is created with mode 0644. The net
result of using this rule is that the permissions are set properly but
how the fragment of the rule MODE="666" remaining after PROGRAM and
NAME are ignored, is implemented is entirely unclear.
--
Leonard Evens <len@xxxxxxxxxxxxxxxxxxxxx>
Mathematics Department, Northwestern University
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