Re: [chrony-users] Newbie Help Needed

[Stuart Maclean <stuart@xxxxxxxxxxxxxxxxxx>]

Hi Miroslav, all,

I'm including my code below to answer your question as why my CSAC data
source is apparently so good.  To the best of my knowledge I am NOT
combining the CSAC data and PPS timestamps, but the code below should
answer that.

The CSAC data SOCK sample is composed thus:

char bufData[] = secs since epoch as a decimal string, read from the
CSAC over serial.

I should point out that the CSAC's time string ends with a \n, and that
I use the serial device tty in canonical mode, so my read is not
satisfied until ALL of the data is in the tty kernel layer. I normally
use canonical mode for line-oriented inputs as it simplifies user space
code, you just need one read, not many.  Then:

int t = atoi( bufData );
struct sock_sample ss;
gettimeofday( &ss.tv, NULL );
ss.offset = -(ss.tv.tv_sec - t); // or negative value of that?
ss.pulse = 0;
ss.leap = 0;
ss._pad = 0;
ss.magic = SOCK_MAGIC;
send(fd1, &ss, sizeof ss, 0);



The 'Linux system time at CSAC PPS fire event' data SOCK sample is
composed thus:

char bufPPS[] = seconds and useconds of system time when PPS fired, as a
string in the form SSSS.UUUUUU, read from a char device.

Again, the string is ended by a \n, and again I use canonical mode and
hence need only a single read to get this string from kernel space back
to user space.  Then:

int sysSecs, sysUsecs;
sscanf( bufPPS, "%u.%u", &sysSecs, &sysUsecs );
struct sock_sample ss;
ss.tv.tv_sec = sysSecs;
ss.tv.tv_usec = sysUsecs;
ss.offset = sysUsecs  / 1e6;
ss.pulse = 1;
ss.leap = 0;
ss._pad = 0;
ss.magic = SOCK_MAGIC;
send(fd2, &ss, sizeof ss, 0);	

Should I have relayed that time back to user space with nanosecond
resolution?  I was just using microsecond resolution, and hence calling
do_gettimeofday() in the ISR in my device driver.

Given your comment about my CSAC data reflock being 'good enough on its
own', I may play with eliminating the PPS driver entirely.

Your comments welcomed.

Stuart

On 01/11/2018 04:34 AM, Miroslav Lichvar wrote:
> On Wed, Jan 10, 2018 at 12:06:00PM -0800, Stuart Maclean wrote:
>> My results, after about 12 hours of chronyd running, are
> 
>> $ ./chronyc  sourcestats
>> 210 Number of sources = 2
>>
>> Name/IP Address            NP  NR  Span  Frequency  Freq Skew  Offset
>> Std Dev
>> ==============================================================================
>>
>> CSAC                        6   3    80     -0.009      0.009   -519ns
>>  67ns
>> PPS                        64  42  1009     +0.000      0.005     +2ns
>> 3481ns
> 
> That looks good, but I'm wondering how is it possible that the CSAC
> source is so stable and has such a small offset relative to the PPS.
> Is your code combining the CSAC timestamp with the PPS timestamp? If
> it was really that good, you could probably just the CSAC source
> alone.
> 
> If it was a GPS NMEA source, you would see offset and jitter in
> milliseconds.
> 

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