term: support kernel PPS on FreeBSD - #425
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The watch is much shorter than the Linux one: FreeBSD serial drivers implement the PPS source on the TTY itself, so there is no line discipline to attach, no sysfs scan, and no udev retry loop. The watch requires the sysctl that configures the device PPS capture (dev.uart.N.pps_mode for uart(4), hw.usb.ucom.pps_mode for ucom(4)) to select exactly the requested pin: the invert and narrow-pulse options change what the captured edges mean, so a mismatch is reported as unavailable, naming the value to set, and automatic method selection falls back to poll. With the options clear, kernel ASSERT means the pin became asserted in the TIOCM sense, so the sequence accounting is shared with the Linux watch, moved unchanged to kernelpps.go. Verified on FreeBSD 15.1 against a u-blox receiver on an onboard 16950 UART and a CASIC receiver on an FT232H USB adapter: 30 seconds each with no missed edges, a 6 us timestamp spread on the UART, and about 130 us of USB status-report latency on the FT232H.
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The watch is much shorter than the Linux one: FreeBSD serial drivers implement the PPS source on the TTY itself, so there is no line discipline to attach, no sysfs scan, and no udev retry loop. The watch requires the sysctl that configures the device PPS capture (dev.uart.N.pps_mode for uart(4), hw.usb.ucom.pps_mode for ucom(4)) to select exactly the requested pin: the invert and narrow-pulse options change what the captured edges mean, so a mismatch is reported as unavailable, naming the value to set, and automatic method selection falls back to poll. With the options clear, kernel ASSERT means the pin became asserted in the TIOCM sense, so the sequence accounting is shared with the Linux watch, moved unchanged to kernelpps.go.
Verified on FreeBSD 15.1 against a u-blox receiver on an onboard 16950 UART and a CASIC receiver on an FT232H USB adapter: 30 seconds each with no missed edges, a 6 us timestamp spread on the UART, and about 130 us of USB status-report latency on the FT232H.
Based on the freebsd-kpps branch, which it merges after.
Part of #426.