On Tue, Jun 20, 2023 at 10:07:14PM -0700, Jacob Keller wrote: > On 6/11/2023 11:53 PM, Miroslav Lichvar wrote: > > If you need to synchronize multiple PHCs to each other, it's better to > > use one phc2sys instance to synchronize the system clock to the source > > PHC and then another phc2sys instance to synchronize the rest of PHCs > > to the system clock. > > > > In theory the kernel could be extended with an interface to perform the > comparison between two clocks in-kernel without a switch. That would be > a bit better than doing it in user space.
The drivers could provide a callback for other drivers to read the lowest part of the timestamp and another callback to complete the timestamp. One issue with this approach would be increased delay due to the measurement doing 3 PCIe reads instead of 1 PCIe read and 2 system clock reads. Another issue might with hardware which cannot read the lowest part of the timestamp (latching) twice without reading the rest, if such a thing exists. A better approach would be to use the system clock as a reference, similarly to what you described for PTM, except doing it multiple times in the style of PTP_SYS_OFFSET_EXTENDED with interleaved readings SYS, PHC1, SYS, PHC2, SYS, PHC1, SYS ... and combining the measurements in the kernel or user space. -- Miroslav Lichvar _______________________________________________ Linuxptp-users mailing list Linuxptp-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/linuxptp-users
