[Bug 1001603] New: LPC2XXX/17XX HW RTC driver can report wrong time/date data

[Bug 1001603] New: LPC2XXX/17XX HW RTC driver can report wrong time/date data

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           Summary: LPC2XXX/17XX HW RTC driver can report wrong time/date
                    data
           Product: eCos
           Version: CVS
          Platform: Other (please specify)
        OS/Version: Cortex-M
            Status: UNCONFIRMED
          Severity: normal
          Priority: low
         Component: Wallclock
        AssignedTo: unassigned@bugs.ecos.sourceware.org
        ReportedBy: bernard.fouche@kuantic.com
                CC: ecos-bugs@ecos.sourceware.org
             Class: Advice Request


I'm looking at the HW RTC driver for LPC2XXX/LP17XX and I see this:

struct time {
  volatile cyg_uint32 sec;
  volatile cyg_uint32 min;
  volatile cyg_uint32 hour;
  volatile cyg_uint32 dom;
  volatile cyg_uint32 dow;
  volatile cyg_uint32 doy;
  volatile cyg_uint32 month;
  volatile cyg_uint32 year;
};

struct rtcdev {
  volatile cyg_uint32 ilr;
  volatile cyg_uint32 ctc;
  volatile cyg_uint32 ccr;
  volatile cyg_uint32 ciir;
  volatile cyg_uint32 amr;
  volatile cyg_uint32 ctime[3];
  struct time time;
  cyg_uint32 dummy[8];
  struct time alarm;
#ifndef CYGHWR_HAL_LPC_RTC_32768HZ
  volatile cyg_uint32 preint;
  volatile cyg_uint32 prefrac;
#endif
};

static struct rtcdev * const rtc =
  (struct rtcdev *) CYGARC_HAL_LPC2XXX_REG_RTC_BASE;
...[snip]...

cyg_uint32
Cyg_WallClock::get_hw_seconds(void)
{
  return _simple_mktime(rtc->time.year,
                        rtc->time.month,
                        rtc->time.dom,
                        rtc->time.hour,
                        rtc->time.min,
                        rtc->time.sec);
}

The problem is:

- time data is spread in different HW registers that can't be read in a single
shot by the MCU.
- the code do not consider the case when unfortunately the RTC will change of
second exactly between different register reads.

For instance get_hw_seconds() is called at 23:59:59, a few ns before the RTC is
about to move to 00:00:00: the function can return 23:00:59, 00:59:00, 
anything is possible according to the way the compiler orders the processing of
the arguments before calling _simple_mktime() (the same for the date).

The solution is to read the whole set of registers twice: if they don't have
the same value, then read them again. If there are similar, then report the
result since it will be coherent.

  Bernard

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[Bug 1001603] LPC2XXX/17XX HW RTC driver can report wrong time/date data

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--- Comment #1 from Bernard Fouché <bernard.fouche@kuantic.com> 2012-06-01 08:53:23 BST ---
Created an attachment (id=1780)
 --> (http://bugs.ecos.sourceware.org/attachment.cgi?id=1780)
Fix race condition in get_hw_seconds()

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Subject: [Bug 1001603] LPC2XXX/17XX HW RTC driver can report wrong time/date
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--- Comment #1 from Bernard Fouché <bernard.fouche@kuantic.com> 2012-06-01 08:53:23 BST ---
Created an attachment (id=1780)
 --> (http://bugs.ecos.sourceware.org/attachment.cgi?id=1780)
Fix race condition in get_hw_seconds()

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Subject: [Bug 1001603] LPC2XXX/17XX HW RTC driver can report wrong time/date
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--- Comment #2 from Bernard Fouché <bernard.fouche@kuantic.com> 2012-06-01 08:55:50 BST ---
An improvement would be to store in the 'General Purpose Registers' (the
'dummy' item of struct RTC), a string like 'ECOS' (fits in a single 4 bytes
register): if this string isn't found at init, then clears the set of
registers. If the string is found, considers the registers give correct time. I
do this at application level, it could be moved here and this feature added as
a CDL option.

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--- Comment #2 from Bernard Fouché <bernard.fouche@kuantic.com> 2012-06-01 08:55:50 BST ---
An improvement would be to store in the 'General Purpose Registers' (the
'dummy' item of struct RTC), a string like 'ECOS' (fits in a single 4 bytes
register): if this string isn't found at init, then clears the set of
registers. If the string is found, considers the registers give correct time. I
do this at application level, it could be moved here and this feature added as
a CDL option.

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Subject: [Bug 1001605] New: much faster and simpler(!) replacement for
 _simple_mktime() (in wallclock.inl)
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           Summary: much faster and simpler(!) replacement for
                    _simple_mktime() (in wallclock.inl)
           Product: eCos
           Version: CVS
          Platform: All
        OS/Version: All
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: low
         Component: Wallclock
        AssignedTo: unassigned@bugs.ecos.sourceware.org
        ReportedBy: bernard.fouche@kuantic.com
                CC: ecos-bugs@ecos.sourceware.org
             Class: Advice Request


If _simple_mktime() is called for Dec 31st 2012, it will do:

  42 (2012-1970) additions of a value requiring 3 modulo's
+ 12 additions (of the month)
+ 1 substraction
+ 3 multiplications and 3 additions

That is a total of (42*3)+12+1+(3+3)\x145 operations.

The proposed change makes _simple_mktime() to use:

- 1 substraction, 1 multiplication, 1 addition and 1 substraction to calculate
a 'base' second value.
- 1 comparison that can lead to 1 substraction
- 1 addition, 1 substraction, 1 division
- 2 substractions, 1 division
- 1 addition, 1 substraction, 1 division
- 1 addition
- 1 substraction
- 1 multiplication
- 1 addition
- 1 multiplication
- 1 addition
- 1 multiplication
- 1 addition

That is a total of:

(1+1+1+1)+(1+1)+(1+1+1)+(2+1)+(1+1+1)+1+1+1+1+1+1+1+1# operations.

This count is nearly always the same whatever the time/date processed, which is
not the case for the original code, which takes 3 modulo's more per invocation
each year, since this function is called by all RTC HW drivers.

I don't think eCos runs on MCU/CPU without HW division, but in such a case this
would mean that 2/3rd of the modulo's in the current code are also done in
software (for 100 and 400 values).

I attach an example of the proposed replacement code that compiles under Linux
and uses brute force to check all possible values: it takes about 6 minutes to
run in a recent computer.

If someone can review and accept this small code change, I can make a patch vs
ecos/packages/io/wallclock/current/include/wallclock/wallclock.inl

  Bernard

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