Difference between 2 jiffies

What is jiffies:

• The original kernel timer system (called the "timer wheel) was based on incrementing a kernel-internal value (jiffies) every timer interrupt.
• The timer interrupt rate (and jiffy increment rate) is defined by a compile-time constant called HZ.
• Different platforms use different values for HZ.
• The kernel used 100 as the value for HZ, yielding a jiffy interval of 10 ms.
• With 2.4, the HZ value for i386 was changed to 1000, yeilding a jiffy interval of 1 ms.
• Process:
• When jiffy count is incremented then a process gets time to run.
• The jiffies are incremented by the timer interrupt that tells the scheduler to reschedule
•  If the process calls yield() or sleep(), for example, then rescheduling takes place immediately.
• Thus the context switch to a next available running process not necessarily occurs at the jiffy boundary.
• It lists /proc/timer_list and /proc/timer_stats.
• Tick generation:
• It is updated every tick 
• tick is triggered by timer/clock interrupt.
• The interrupt is generated by Intel 8253 or HPET hardware
• So it won't update until the hardware was property initialized.
• Difference:
• There is a better solution to ensure correct jiffy wrap handling in *ALL* kernel code: make jiffy wrap in first five minutes of uptime.
• #define INITIAL_JIFFIES (0xffffffffUL & (unsigned long)(-300*HZ))
• Hence jiffies is using the full 32 bit, after that overflow happen.
• So jiffies.h file comment as maximum jiffie value is (MAX_INT >> 1) is a false comment.
• Don't use MAX_JIFFY_OFFSET as max value since this is wrong.
• https://lwn.net/Articles/22874/

Program:

Wrong program:

• This below code is wrong, because jiffies is using the full 32 bit confirmed by after bootup jiffies overflow in 5 min, 
• So after bootup just checked the jiffies value, it is using full 32 bit before overflow.
• Hence below code is wrong, this is wrongly used in some of kernel code.

inline unsigned long jiffies_time_diff(unsigned long start, unsigned long end)
{
    if (end >= start)
        return end - start;

    return end + (MAX_JIFFY_OFFSET - start);
}

Correct program:

inline unsigned long jiffies_time_diff(unsigned long start, unsigned long end)
{  

       unsigned long diff = 0;

       /* This code is handle the overflow also, by converting to long.

            if jiffies start time is 13 and after overflowed end time is 5 then, coverting to sign

            difference value is -8, the 1st complement +1 value of -8 is stored as big value in unsiged diff.

            This is the correct difference */

       diff = (long)end - (long) start;

       return diff;
}