[SOLVED] UDEV rule for rtc: what am I doing wrong?

mstrimel · 10-30-2020, 01:59 PM

hi,
I need to durably change the permissions on /dev/rtc0 so that another user can write to it. Thus, I wrote a udev rule, ran "udevadm trigger" and rebooted. It did not work.
my custom udev file has one line, as follows:

Code:

ACTION=="ADD", ATTR{DEVNAME}=="/dev/rtc0", GROUP="mythtv", MODE="0666"

I saved it as /etc/udev/rules.d/40-rtc-permissions.rules

udevadm info returned the following name for my device:

Code:

/devices/pnp0/00:02/rtc/rtc

When I run

Code:

udevadm test /devices/pnp0/00:02/rtc/rtc

, I get a list of the rules being read, with my rule second on the list (that's good, right?), but then this:

Quote:

Invalid inotify descriptor.
DEVPATH=/devices/pnp0/00:02/rtc/rtc0
DEVNAME=/dev/rtc0
MAJOR=249
MINOR=0
ACTION=add
SUBSYSTEM=rtc
DEVLINKS=/dev/rtc
USEC_INITIALIZED=1681953
Unload module index
Unloaded link configuration context.

and the permissions still are incorrect. What am I doing wrong?

mstrimel · 10-31-2020, 07:15 AM

Well, I still don't know what was wrong, but after a bunch of fooling around here is the rule that works.

Code:

KERNEL=="rtc0", SUBSYSTEM=="rtc", KERNELS=="00:02", GROUP="mythtv", MODE="0666"

One trick was that apparently at least one of the match keys -- in my case, KERNELS -- must reference the parent device. To get the parent device attributes, you must run "udevadm info -a [yourdevice]". The section where the attributes are pluralized, like "KERNELS" vs. "KERNEL", relate to the parent.
Another thing is that you have to run udevadm control --reload-rules and udevadm trigger each time you change something, in order to test it properly with udevadm test.
I hope this helps someone!