PTP hardware support allows precise synchronization of a GPS with a NIC clock, and allows precise synchronization over the network between the clock of one NIC and the clock of another NIC. But almost all computer software is written to make use of the computer's system clock rather than a NIC clock. This means that the system clock needs to synchronized with the NIC clock, but PTP deals only with NIC clocks and does not help with this.
PTM is a PCI Express (PCIe) feature that enables devices on the PCIe bus to synchronize their clocks. In particular, it provides hardware support for precise synchronization of the NIC clock and the system clock. At a high-level, it's like PTP, but for the PCIe bus rather than the network. PTM requires support from both the network card and the computer (specifically the motherboards chipset's PCIe subsystem).
The Linux kernel enables applications to take advantage
of PTM using a system call that performs cross timestamping, which
means that a single system call fetches both the system time and the
time of a NIC clock simultaneously.
(The system call is the PTP_SYS_OFFSET_PRECISE ioctl.)
You can tell whether a particular NIC supports cross timestamping
by using the phc_ctl command from the linuxptp package with just the interface name as the argument. The last line of the output will say
has cross timestamping support if cross timestamping support is working for that NIC.
The e1000e driver supports cross timestamping for some on-motherboard NICs,
such as the Intel I219-V, in a device-specific way that does not depend on PTM.
Apart from that, support for cross timestamping is currently based on PTM.
You can check whether the PCIe subsystem has PTM support by searching for Precision Time Measurement in the output of sudo lspci -vv.
For PTM to work, it needs to be supported by the NIC device and the root complex (host bridge) and any PCI bridges
between them. You can use lspci -t to identify the relevant bridges.
The CPU also needs support for the Always Running Timer feature;
this corresponds to the art flag in /proc/cpuinfo.
The driver for the NIC also needs to have cross timestamping support: searching the Linux kernel for getcrosststamp will find drivers which might have support for cross timestamping
The only widely available NICs with support for PTM are the Intel i225/i226, which use the igc driver. However, the igc driver disables
support for PTM on the i225-V, which means that PTM works on the i225-LM, i226-V and i226-LM controllers. The i225-V is much more common than the more expensive i225-LM. The i225-T1 PCIe card uses the i225-LM controller.
It is not straightforward to predict whether a motherboard has the necessary support. I have tried a variety of systems, and the rule that covers the systems I've tried is that if the motherboard uses an 11th gen or newer chipset, it is likely to work. For motherboards with separate CPUs, I found the B660 and Z690 worked, but the Z490 didn't. (The Z490 had some PTM support but the bridge for the x16 slot didn't: I was using that slot since it was a ITX system.) For mini PCs with soldered on CPUs, I found that Jasper Lake (N5095/N6000) and Alder Lake-N (N100) work, but Gemini Lake (J4125) doesn't.