It's a good idea to make sure that everything is working at the kernel level before trying to set up NTP or PTP. Part of this is determining the device names that the Linux kernel is using for our hardware.
In this section, we will check that the PPS signal into the NIC is working properly.
If the NIC is enp1s0, then first do:
ethtool -T enp1s0 | grep PTP
This will give us the number of the PTP Hardware Clock, which will often be 0.
Make sure the interface is in an UP state.
ip link show enp1s0
If it's not up, then bring it up with
sudo ip link set enp1s0 up
Then we can do
echo 1 0 | sudo tee /sys/class/ptp/ptp0/pins/SDP0
Replace the 0 in ptp0 with whatever ethtool said was the number.
SDP0 here is the name of the pin to which the PPS is connected. In the echo 1 0, 1 means to use the pin for input and 0 means the pin should use input channel 0.
Now do:
echo 0 1 | sudo tee /sys/class/ptp/ptp0/extts_enable
This means to enable timestamping of pulses on channel 0. In the echo 0 1, 0 means channel 0 and 1 means to enable timestamping.
Now see if we're getting timestamps:
sudo cat /sys/class/ptp/ptp0/fifo
The cat command should output a line, which represents a timestamp of an input pulse and consists of 3 numbers: channel number, which is zero in this case, seconds count, nanoseconds count. Repeating the last command will give lines for successive input timestamps.
If cat outputs nothing, then it's not working.
The i210 and i225 NICs have the quirk that they timestamp both the rising and falling edges of the every pulse. This means that a PPS signal will result
in two timestamps per second. Most GPS receivers default to a pulse width of 0.1s, which means that you should see consecutive timestamps separated alternately
by 0.1s and 0.9s (i.e. 100,000,000 and 900,000,000 nanoseconds).
By looking at the output of cat you can determine what the pulse width is.
Both chrony or ts2phc need to be configured with the pulse width for NICs that timestamp both edges.
First determine the serial device name:
- a USB-RS232 or USB-TTL converter will usually show up as
/dev/ttyUSB0, but occasionally may show up as/dev/ttyACM0 - an RS232 connection using a DB9 port on the PC will usually show up as
/dev/ttyS0 - a GPS in M.2 slot will usually be
/dev/ttyACM0 - a GPS with a USB connection will be either
/dev/ttyACM0or/dev/ttyUSB0
Obviously you may need to change 0 in the device name to a larger number, if you have multiple such devices.
It's convenient if you are in the dialout group so you can access the serial devices
sudo usermod -G dialout -a jjc
Here jjc is your username. You'll need to logout and then login again for this to take effect.
Then do:
(stty 9600 -echo -icrnl; cat) </dev/ttyUSB0
Here 9600 is the speed. The most common default speed is 9600, but some receivers default to 38400.
You should see lines starting with $.
In particular look for a line starting with $GPRMC or $GNRMC. The number following that should be the current UTC time;
for example, 025713.00 means 02:57:13.00 UTC.
After another 8 commas, there will be a field that should have the current UTC date;
for example, 140923 means 14th Septemember 2023.
TODO: NetworkManager causes problems; use systemd-networkd instead. Need to enable systemd-networkd-wait-online@enp1s0 and disable NetworkManager-wait-online.service.
On Fedora:
sudo dnf install kernel-tools
On Debian:
sudo apt install linux-cpupower
Discover available frequencies with
cpupower frequency-info -l -m
Also
cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_frequencies
will show specific frequencies available.
Set them all to a constant specific frequency:
sudo cpupower frequency-set --min 1.6Ghz --max 1.6GHz
TODO
- Test whether this makes a measureable difference.
- Make it start automatically with a systemd service.
- Should we also try to control CPU idle states?
- Should we try to tweak scheduling (e.g. isolated cores, CPU affinity etc)?