Re: pulse-per-second API status

[Dennis Ferguson <dennis.c.ferguson%gmail.com@localhost>]

On 2 Nov, 2013, at 03:33 , Alan Barrett <apb%cequrux.com@localhost> wrote:
> On Fri, 01 Nov 2013, Greg Troxel wrote:
>>> But if NetBSD enables PPS on ucom, there's going to be an expectation that 
>>> it is good enough for stratum-1 timekeeping, like PPS on real serial ports.
>> 
>> I don't think there's any such expectation created.
>> [...]
>> People who expect the same as serial PPS are confused, and we are not 
>> responsible for that.
> 
> I think that PPS on a device with very high "interrupt" latency is 
> sufficiently similar to PPS on a device with low interrupt latency that it 
> deserves to have the same API.  I don't think it even needs a sysctl to 
> enable it.
> 
> I think that it just needs careful documentation, in ucom(4) and wherever we 
> document the PPS API.  Maybe the documentation for applications like ntpd 
> should also warn against using PPS on USB interfaces.

I think this is correct.  How accurate is "accurate enough" is a value judgement
which the operating system probably shouldn't be making for you.  I was going to
point out that the TI processor on my Beaglebone board has a hardware timestamp
peripheral which eliminates interrupt latency entirely, and hence can be 
accurate
to single digit nanoseconds rather than just single digit microseconds (or
milliseconds), but I wouldn't expect this fact to make anyone want to add sysctl
warnings about how comparatively sloppy even the low interrupt latency devices 
are.

I think everyone should be informed about what is better and what is worse,
but in the end I don't think you should be denied, or even inconvenienced by, 
the
use of the hardware you do have just because of the existence of much better
hardware that you don't have.

Dennis Ferguson