[pvrusb2] Permission denied writing to cur_val

[Bjorn Danielsson]

Mike Isely <isely at isely.net> wrote:
>[...]
> As for why you need to be root, there isn't much the driver can do about 
> that.  From the viewpoint of inside the kernel there really isn't a way 
> to change the owner of those sysfs files.  The permissions can be 
> adjusted, but it's unwise for those fields to be world-writeable - since 
> then anything can take over control of the device.  With device control, 
> usually one changes the gid of the files to match a specific group (e.g. 
> "video") and then users with permission to operate the device need only 
> be associated with that group.  But even there the driver can't do 
> anything even if it wanted to, since the actual integer gid value for, 
> say, "video" is distro-specific.  One clean, theoretical, way to fix 
> this is for there to be a udev rule that executes a chgrp on all of 
> /sys/class/pvrusb2/sn-xxxxxx at the point when the device appears.  
> That has the additional advantage that udev can know the correct 
> distro-specific gid value to use.  But I haven't implemented this (this 
> is a task really outside of the driver).  But it would be a useful 
> exercrise for someone else to set up.

I posted such a script a while back:

http://www.isely.net/pipermail/pvrusb2/2008-April/001772.html

There was a discussion then about how to find out if the attached
video device was really a class/pvrusb2 device and not something
else, but my script does no harm in such a case. It just does a
bit of unnecessary work. What the script does is chgrp on all
of /sys/class/pvrusb2/*/ and then adds group write permission
for every file that has owner write permission.

The right way to invoke this script seems to be distro-dependent,
and this is the way to do it in Slackware 12.1:

1. Save the script as /lib/udev/rc.video and do chmod +x on it.
2. Edit the file /etc/udev/rules.d/40-video.rules and change the
   line:
KERNEL=="video[0-9]*",  NAME="v4l/video%n", SYMLINK+="%k", GROUP="video"
   into:
KERNEL=="video[0-9]*",  NAME="v4l/video%n", SYMLINK+="%k", GROUP="video", RUN+="rc.video"

Both the file paths and the contents of the rules-file will most
likely be different on other distros, and between different versions
of the same distro.

-- 
Bjorn Danielsson  <pvr-usb2 at dax.nu>