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Old 10-30-2020, 02:59 PM   #1
mstrimel
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Registered: Apr 2004
Location: USA
Distribution: Mint 19 Cinnamon
Posts: 53

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UDEV rule for rtc: what am I doing wrong?


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?
 
Old 10-31-2020, 08:15 AM   #2
mstrimel
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Registered: Apr 2004
Location: USA
Distribution: Mint 19 Cinnamon
Posts: 53

Original Poster
Rep: Reputation: 0
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!

Last edited by mstrimel; 10-31-2020 at 08:17 AM.
 
  


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