3

On my Beaglebone Black I added a I2C real-time-clock to not being reliant on ntpd to maintain accurate timing. The outcome is that there are two special device files in /dev. These are /dev/rtc0 and /dev/rtc1 but there is also /dev/rtc which is a symlink to /dev/rtc0.

enter image description here

/dev/rtc0 is the real-time-clock within the ARM SOC on the board, /dev/rtc1 is the I2C device. At the moment I'm using scripts that read and write the time manually to the I2C clock but I'd rather like the symlink /dev/rtc to point to /dev/rtc1.

Hence the question, how can this be done? The Linux distro on my beaglebone black is Arch Linux which uses systemd for all the house keeping.

When I delete the symlink and create a new one pointing to /dev/rtc1 not surprisingly it is reset after the next reboot and I didn't find any config files or systemd-units so far.

Help is much appreciated.

2

That udev rule hint pointed me in the right direction. After a quick review of writing udev rules I did the following.

udevadm info -a -p /sys/class/rtc/rtc1

The output (shortened) revealed some useful properties to define a udev rule.

looking at device '/devices/platform/ocp/4802a000.i2c/i2c-1/1-0068/rtc/rtc1':
KERNEL=="rtc1"
SUBSYSTEM=="rtc"
DRIVER==""
ATTR{date}=="2015-12-04"
ATTR{hctosys}=="0"
ATTR{max_user_freq}=="64"
ATTR{name}=="ds1307"
ATTR{since_epoch}=="1449230817"
ATTR{time}=="12:06:57"
...

So the rules file needs to reside in /etc/udev/rules.d/ with a naming scheme like 99-rtc1.rules.

The files content is

KERNEL=="rtc1", SUBSYSTEM=="rtc", DRIVER=="", ATTR{name}=="ds1307", SYMLINK="rtc", MODE="0666"

To test the rule you can run

udevadm test /sys/class/rtc/rtc1

and the important lines in the output are

...
creating link '/dev/rtc' to '/dev/rtc1'
atomically replace '/dev/rtc'
...

The result in /dev is the desired configuration.

enter image description here

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