Bug with clock nanoseconds

[Aitor Viana]

Just tried in tsim-leon and happens the same.
I didn't completely understand what's the problem Manuel could you explain
to me again? is early in the morning :D

Best regards,

Aitor


On Mon, Mar 30, 2009 at 6:49 PM, Joel Sherrill <joel.sherrill at oarcorp.com>wrote:

> Manuel Coutinho wrote:
> >
> > Hi
> >
> >
> >
> > We have discovered a bug of RTEMS relating to the clock. Even though
> > we only tested it for the ERC32 board (for now), we believe it is
> > common throughout all the targets.
> >
>
> No you have discovered a bug in the ERC32's get nanoseconds since last
> tick handler.
> Apparently it doesn't recognize that is has been longer than a tick
> since the last clock
> tick.  It should determine that is has overflowed and return 1 clock
> tick + partial clock
> tick.
>
> It is returning the wrong answer.
>
> >
> >
> > The test consists in checking if the time given by rtems_clock_get is
> > always increasing. The code is as follows:
> >
> >
> >
> > .....
>
>
> ....
> >
> > As you can see, the problem is raised when a clock tick occurs: the
> > time read in “new” is older than “old”. This cannot happen since “new”
> > is updated AFTER “old”! This problem occurs if a clock interrupt is
> > triggered after the number of clock ticks have been read but BEFORE
> > the nanosecond field has been read. The resulting number of clock
> > ticks is “small” (should be plus one).
> >
> >
> >
> > We can use the interrupt pending CPU registers info to determine if,
> > while we are reading the number of clock ticks that occurred and the
> > nanoseconds, a clock interrupt could be pending.
> >
> >
> >
> > We do not have enough knowledge to say that this solution can be used
> > for all boards (determining if an interrupt is pending). In the SPARC
> > architecture, at least, it is possible. If it is possible in all
> > architectures, then a solution to RTEMS would be to change the code of
> > the _TOD_Get_uptime function to:
> >
> >
> >
> >
> >
> > void _TOD_Get_uptime(struct timespec *uptime) {
> >
> >     ISR_Level level;
> >
> >     struct timespec offset;
> >
> >     volatile uint32_t pending;
> >
> >
> >
> >     /* assume uptime checked by caller */
> >
> >
> >
> >     offset.tv_sec = 0;
> >
> >     offset.tv_nsec = 0;
> >
> >
> >
> >     _ISR_Disable(level);
> >
> >     *uptime = _TOD_Uptime;
> >
> >     if (_Watchdog_Nanoseconds_since_tick_handler)
> >
> >         offset.tv_nsec = (*_Watchdog_Nanoseconds_since_tick_handler)();
> >
> >
> >
> >     /* code added: */
> >
> >     pending = isClockInterruptPending();
> >
> >
> >
> >     _ISR_Enable(level);
> >
> >
> >
> >     /* code added */
> >
> >     /* if an interrupt occurred while interrupts were disabled and the
> > nanoseconds is too little */
> >
> >     /* means that a clock interrupt occurred BEFORE the nanoseconds
> > were read */
> >
> >     if (pending && offset.tv_nsec <
> > rtems_configuration_get_microseconds_per_tick() / 2) {
> >
> >         struct timespec clockTick = {0,
> > rtems_configuration_get_microseconds_per_tick()*1000};
> >
> >         _Timespec_Add_to(&offset, &clockTick); /* so add one clock
> > tick to the offset */
> >
> >     }
> >
> >
> >
> >     /* else, the clock tick occurred AFTER the nanoseconds were read,
> > so no problem */
> >
> >
> >
> >     _Timespec_Add_to(uptime, &offset);
> >
> > }
> >
> >
> >
> >
> >
> > At least, with these modifications, the test passes :)!
> >
> >
> >
> > Kind regards
> >
> > Manuel Coutinho
> >
>
>
> --
> Joel Sherrill, Ph.D.             Director of Research & Development
> joel.sherrill at OARcorp.com        On-Line Applications Research
> Ask me about RTEMS: a free RTOS  Huntsville AL 35805
>   Support Available             (256) 722-9985
>
>
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