Fixed inconsistent hardware clock timestamp.

[hsinkoyu]

Camera Model: D455
SDK Version: 2.54.2
Firmware Version: 5.15.1

Apply the attached patch to watch measurements of mean, minimum, maximum and
standard deviation of the interval between the hardware clock timestamp and
current time for every 2000 frames.
measure.patch

By the result, the hardware clock timestamp is inconsistent, and it may even
be a future time when the hardware clock (2^32) restarts from zero (on update_samples_base()).
error.log

 20/11 10:53:16,748 INFO [140162835150592] (global_timestamp_reader.cpp:321) HW Timestamp Interval (ms): mean=26.544191 min=21.134033 max=29.761719 stdev=1.751351
 20/11 10:54:23,444 INFO [140162835150592] (global_timestamp_reader.cpp:321) HW Timestamp Interval (ms): mean=25.889587 min=21.957764 max=30.401611 stdev=2.073406
 20/11 10:55:30,140 INFO [140162835150592] (global_timestamp_reader.cpp:321) HW Timestamp Interval (ms): mean=26.397918 min=22.964844 max=28.806885 stdev=1.125099
 20/11 10:56:36,836 INFO [140162835150592] (global_timestamp_reader.cpp:321) HW Timestamp Interval (ms): mean=25.553728 min=19.912354 max=30.630371 stdev=2.161987
 20/11 10:57:28,925 INFO [140162835150592] (global_timestamp_reader.cpp:139) update_samples_base(4294967.296000)
 20/11 10:57:29,025 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,059 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,092 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,125 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,159 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,192 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,225 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,259 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,292 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,326 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,359 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,392 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,426 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,459 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,492 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,526 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,559 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,592 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,626 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,659 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,692 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,726 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,759 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,792 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,826 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,859 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,892 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,926 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,959 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:29,992 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:30,026 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:30,059 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:30,092 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:30,126 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:30,159 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:30,193 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:30,226 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:30,259 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:30,293 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:30,326 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:30,359 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:30,393 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:30,426 ERROR [140162835150592] (global_timestamp_reader.cpp:298) Future Timestamp!
 20/11 10:57:43,532 INFO [140162835150592] (global_timestamp_reader.cpp:321) HW Timestamp Interval (ms): mean=22.965311 min=-207.109131 max=28.433105 stdev=20.851958
 20/11 10:58:50,228 INFO [140162835150592] (global_timestamp_reader.cpp:321) HW Timestamp Interval (ms): mean=25.265000 min=21.204590 max=29.512451 stdev=1.783758
 20/11 10:59:56,924 INFO [140162835150592] (global_timestamp_reader.cpp:321) HW Timestamp Interval (ms): mean=25.783524 min=20.166992 max=30.530518 stdev=2.243548
 20/11 11:01:03,653 INFO [140162835150592] (global_timestamp_reader.cpp:321) HW Timestamp Interval (ms): mean=26.359362 min=22.466309 max=30.238037 stdev=1.734098

double system_time(system_time_finish - _min_command_delay);

About this line of current code, the author thinks the hardware clock timer is retrieved and
is immediately sent back. That means the most duration (delay) of getting a hw clock
is spent before retrieving. Having this design is due to varying delay. I assume
the varying delay is caused by firmware design.

I tried the code that refers the delay happening after retrieving the hw clock.
This means the firmware receives the request and immediately processes it.

double system_time(system_time_start + _min_command_delay);

From the result, we have a consistent hardware clock timestamp.
fixed.log

 20/11 13:15:37,356 INFO [139883601434368] (global_timestamp_reader.cpp:321) HW Timestamp Interval (ms): mean=28.298203 min=28.126953 max=28.494873 stdev=0.063428
 20/11 13:16:44,052 INFO [139883601434368] (global_timestamp_reader.cpp:321) HW Timestamp Interval (ms): mean=28.250524 min=28.116455 max=28.454590 stdev=0.047277
 20/11 13:17:50,748 INFO [139883601434368] (global_timestamp_reader.cpp:321) HW Timestamp Interval (ms): mean=28.262912 min=28.094727 max=28.534424 stdev=0.070366
 20/11 13:18:57,445 INFO [139883601434368] (global_timestamp_reader.cpp:321) HW Timestamp Interval (ms): mean=28.264807 min=28.108887 max=28.530273 stdev=0.072174
 20/11 13:20:04,141 INFO [139883601434368] (global_timestamp_reader.cpp:321) HW Timestamp Interval (ms): mean=28.273861 min=28.120117 max=28.477783 stdev=0.053090
 20/11 13:20:38,256 INFO [139883601434368] (global_timestamp_reader.cpp:139) update_samples_base(4294967.296000)
 20/11 13:21:10,837 INFO [139883601434368] (global_timestamp_reader.cpp:321) HW Timestamp Interval (ms): mean=28.348879 min=28.144043 max=33.188965 stdev=0.461801
 20/11 13:22:17,533 INFO [139883601434368] (global_timestamp_reader.cpp:321) HW Timestamp Interval (ms): mean=28.267152 min=28.090820 max=28.541504 stdev=0.065626
 20/11 13:23:24,229 INFO [139883601434368] (global_timestamp_reader.cpp:321) HW Timestamp Interval (ms): mean=28.267967 min=28.081299 max=28.506592 stdev=0.078478
 20/11 13:24:30,925 INFO [139883601434368] (global_timestamp_reader.cpp:321) HW Timestamp Interval (ms): mean=28.262112 min=28.088135 max=28.674072 stdev=0.080410
 20/11 13:25:37,621 INFO [139883601434368] (global_timestamp_reader.cpp:321) HW Timestamp Interval (ms): mean=28.261170 min=28.087646 max=28.437500 stdev=0.058966

_last_request_time -= base_x;

This line of commit is because _last_request_time is also connected with the hw clock.
It should be modified like _prev_time.
The future timestamp could also be fixed.

Thanks.