[ubuntu-x] chmod 666 /dev/dri/card0

[Tormod Volden]

Hi,
I have seen many forum posts etc where people have to chmod 666
/dev/dri/card0 in order to get HW acceleration, but though it was some
kind of snake oil. Today I experienced this issue myself, and got
interested.

It turns out my /dev/dri/card0 has root:video 660, and this seemed to
be fine since I expected I was in the video group. However, since
Intrepid, the user created by the desktop installer does not get added
to the video group. The same goes for a number of groups listed in the
users-admin GUI. This is usually explained with the reasoning that
these groups are obsolete and not in use (like LP #188759). Of course,
users-admin should not list those groups in this case. Furthermore,
user-admin adds video to the "Desktop" users it creates, so there
certainly inconsistencies here.

On the other hand, debian libdrm has been keeping a patch for ages,
setting the default permissions to 666 instead of 660. Is this because
we don't want to use the "video" group for this?

Now checking the code, it seems libdrm will use this patched setting
in the case it has to create the device itself. Which it will not
always do, I guess that's why my device now only has 660. Is udev
responsible for creating this device? Are there default settings in
the kernel code?

Thanks for any clarifications,
Tormod