firewire: core: add sysfs attribute for easier udev rules
This adds the attribute /sys/bus/firewire/devices/fw[0-9]+/units. It
can be used in udev rules like the following ones:
# IIDC devices: industrial cameras and some webcams
SUBSYSTEM=="firewire", ATTR{units}=="*0x00a02d:0x00010?*", GROUP="video"
# AV/C devices: camcorders, set-top boxes, TV sets, audio devices, ...
SUBSYSTEM=="firewire", ATTR{units}=="*0x00a02d:0x010001*", GROUP="video"
Background:
firewire-core manages two device types:
- fw_device is a FireWire node. A character device file is associated
with it.
- fw_unit is a unit directory on a node. Each fw_device may have 0..n
children of type fw_unit. The units tell us what kinds of protocols
a node implements.
We want to set ownership or ACLs or permissions of the character device
file of an fw_device, or/and create symlinks to it, based on available
protocols. Until now udev rules had to look at the fw_unit devices and
then modify their parent's character device file accordingly. This is
problematic for two reasons: 1) It happens sometime after the creation
of the fw_device, 2) an access policy may require that information from
all children is evaluated before a decision about the parent is made.
Problem 1) can ultimately not be avoided since this is the nature of
FireWire nodes: They may add or remove unit directories at any point in
time.
However, we can still help userland a lot by providing the protocol type
information of all units in a summary sysfs attribute directly at the
fw_device. This way,
- the information is immediately available at the affected device
when userspace goes about to handle an ADD or CHANGE event of the
fw_device,
- with most policies, it won't be necessary anymore to dig through
child attributes.
The new attribute is called "units". It contains space-separated tuples
of specifier_id and version of each present unit. The delimiter within
tuples is a colon. Specifier_id and version are printed as 0x%06x.
Here is an example of a node which implements an IPv4 unit and an IPv6
unit: $ cat /sys/bus/firewire/devices/fw2/units
0x00005e:0x000001 0x00005e:0x000002
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
2 files changed