Fedora kernels missing hardpps
hmurray at megapathdsl.net
Mon Jun 20 09:34:32 UTC 2016
Mumble. Long story.
There are two parts of PPS processing in the kernel. One is RFC 2783 which
describes an API for capturing the time when a pulse happens. The other is
RFC 1589 which describes a PLL which basically moves all the timekeeping work
into the kernel. If you turn on flag3 with the PPS driver, it tries to
activate the kernel PLL.
I hadn't tried the kernel PLL for a while, maybe because I turned it off ages
ago when the Linux PPS area was being rewritten. So I tried it. It gives a
not-implemented error. But I thought the code was there.
Poking around, that chunk of code in the kernel depends upon NTP_PPS which
bool "PPS kernel consumer support"
depends on !NO_HZ
This option adds support for direct in-kernel time
synchronization using an external PPS signal.
It doesn't work on tickless systems at the moment.
The kernels shipped with Fedora, Debian, and Ubuntu all have NO_HZ turned on.
I haven't found the config file for any of the ARM kernels. One try got the
I guess I'll try building a kernel and/or running on FreeBSD or NetBSD.
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