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Sebastian Huber
d9f5345df6
Let A be the current calculation of the frequency accumulator (pps_fcount)
update in pps_event()
scale = (uint64_t)1 << 63;
scale /= captc->tc_frequency;
scale *= 2;
bt.sec = 0;
bt.frac = 0;
bintime_addx(&bt, scale * tcount);
bintime2timespec(&bt, &ts);
hardpps(tsp, ts.tv_nsec + 1000000000 * ts.tv_sec);
and hardpps(..., delta_nsec):
u_nsec = delta_nsec;
if (u_nsec > (NANOSECOND >> 1))
u_nsec -= NANOSECOND;
else if (u_nsec < -(NANOSECOND >> 1))
u_nsec += NANOSECOND;
pps_fcount += u_nsec;
This change introduces a new calculation which is slightly simpler and more
straight forward. Name it B.
Consider the following sample values with a tcount of 2000000100 and a
tc_frequency of 2000000000 (2GHz).
For A, the scale is 9223372036. Then scale * tcount is 18446744994337203600
which is larger than UINT64_MAX (= 18446744073709551615). The result is
920627651984 == 18446744994337203600 % UINT64_MAX. Since all operands are
unsigned the result is well defined through modulo arithmetic. The result of
bintime2timespec(&bt, &ts) is 49. This is equal to the correct result
1000000049 % NANOSECOND.
In hardpps(), both conditional statements are not executed and pps_fcount is
incremented by 49.
For the new calculation B, we have 1000000000 * tcount is 2000000100000000000
which is less than UINT64_MAX. This yields after the division with tc_frequency
the correct result of 1000000050 for delta_nsec.
In hardpps(), the first conditional statement is executed and pps_fcount is
incremented by 50.
This shows that both methods yield roughly the same results. However, method B
is easier to understand and requires fewer conditional statements.
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/604
Real-Time Executive for Multiprocessing Systems (RTEMS)
-------------------------------------------------------
RTEMS, Real-Time Executive for Multiprocessor Systems, is a real-time executive
(kernel) which provides a high performance environment for embedded
applications with the following features:
- standards based user interfaces
- multitasking capabilities
- homogeneous and heterogeneous multiprocessor systems
- event-driven, priority-based, preemptive scheduling
- optional rate monotonic scheduling
- intertask communication and synchronization
- priority inheritance
- responsive interrupt management
- dynamic memory allocation
- high level of user configurability
- open source with a friendly user license
Project git repositories are located at https://git.rtems.org/
RTEMS Kernel: : https://git.rtems.org/rtems/
RTEMS Source Builder : https://git.rtems.org/rtems-source-builder/
RTEMS Tools : https://git.rtems.org/rtems-tools/
RTEMS Documentation : https://git.rtems.org/rtems-docs/
RTEMS FreeBSD : https://git.rtems.org/rtems-libbsd/
Online documentation is available at https://docs.rtems.org/
RTEMS User Manual : https://docs.rtems.org/branches/master/user/index.html
RTEMS RSB Manual : https://docs.rtems.org/branches/master/rsb/index.html
RTEMS Classic API : https://docs.rtems.org/branches/master/c-user/index.html
RTEMS POSIX API : https://docs.rtems.org/branches/master/posix-users/index.html
RTEMS Doxygen for CPUKit : https://docs.rtems.org/doxygen/branches/master/
RTEMS POSIX 1003.1 Compliance Guide :
https://docs.rtems.org/branches/master/posix-compliance/index.html
- Details the standards base functionality and profiles RTEMS supportsXo
RTEMS Developers Wiki : http://devel.rtems.org
- Bug reporting, community knowledge and tutorials.
RTEMS Mailing Lists : https://lists.rtems.org/mailman/listinfo
- The RTEMS Project maintains mailing lists which are used for most
discussions:
* For general-purpose questions related to using RTEMS, use the rtems-users
ml: https://lists.rtems.org/mailman/listinfo/users
* For questions and discussion related to development of RTEMS, use the
rtems-devel ml: https://lists.rtems.org/mailman/listinfo/devel
The version number for this software is indicated in the VERSION file.