why do we have wall_jiffies?

[Paul Mackerras]

In kernel/timer.c we currently have jiffies_64, of which jiffies is
the least-significant long-sized piece, and wall_jiffies.  The code
that updates them looks like this (from kernel/timer.c):

static inline void update_times(void)
{
	unsigned long ticks;

	ticks = jiffies - wall_jiffies;
	if (ticks) {
		wall_jiffies += ticks;
		update_wall_time(ticks);
	}
	calc_load(ticks);
}
  
/*
 * The 64-bit jiffies value is not atomic - you MUST NOT read it
 * without sampling the sequence number in xtime_lock.
 * jiffies is defined in the linker script...
 */

void do_timer(struct pt_regs *regs)
{
	jiffies_64++;
	update_times();
	softlockup_tick(regs);
}

In other places there is code that uses (jiffies - wall_jiffies).
However I can't see any way that jiffies and wall_jiffies could ever
be different (except for a few nanoseconds while executing the code
above).  I also can't see any way that `ticks' could ever be anything
other than 1.

Is the wall_jiffies stuff just a leftover from days when we used to do
timekeeping from a softirq?  Or am I missing something fundamental?

Thanks,
Paul.
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