[time-nuts] Re: PPS latency? User vs kernel mode

[Poul-Henning Kamp]

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Javier Herrero writes:

> - To implement a counter in the FPGA for use as the Linux clock source, 
> instead of the ARM timer
> - Implement harware timestamping on the PPS, and generate the interrupt 
> (and since I was there, I use an external clock source for the counter 
> like the GPSO that gives also the PPS signal, instead of the usually 
> crappy XO that drives the Zynq clocks)
> - And then have a lot of fun convincing the kernel to use the FPGA 
> counter as clock source, and converting raw PPS timestamp times to wall 
> clock in the kernel, to be able to give a good timestamp value to ntp/chrony

In FreeBSD you get all that for free:

	https://papers.freebsd.org/2002/phk-timecounters.files/timecounter.pdf

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