delays - Information on the various kernel delay / sleep mechanisms
  -------------------------------------------------------------------
  
  This document seeks to answer the common question: "What is the
  RightWay (TM) to insert a delay?"
  
  This question is most often faced by driver writers who have to
  deal with hardware delays and who may not be the most intimately
  familiar with the inner workings of the Linux Kernel.
  
  
  Inserting Delays
  ----------------
  
  The first, and most important, question you need to ask is "Is my
  code in an atomic context?"  This should be followed closely by "Does
  it really need to delay in atomic context?" If so...
  
  ATOMIC CONTEXT:
  	You must use the *delay family of functions. These
  	functions use the jiffie estimation of clock speed
  	and will busy wait for enough loop cycles to achieve
  	the desired delay:
  
  	ndelay(unsigned long nsecs)
  	udelay(unsigned long usecs)

  	mdelay(unsigned long msecs)

  
  	udelay is the generally preferred API; ndelay-level
  	precision may not actually exist on many non-PC devices.
  
  	mdelay is macro wrapper around udelay, to account for
  	possible overflow when passing large arguments to udelay.
  	In general, use of mdelay is discouraged and code should
  	be refactored to allow for the use of msleep.
  
  NON-ATOMIC CONTEXT:
  	You should use the *sleep[_range] family of functions.
  	There are a few more options here, while any of them may
  	work correctly, using the "right" sleep function will
  	help the scheduler, power management, and just make your
  	driver better :)
  
  	-- Backed by busy-wait loop:
  		udelay(unsigned long usecs)
  	-- Backed by hrtimers:
  		usleep_range(unsigned long min, unsigned long max)
  	-- Backed by jiffies / legacy_timers
  		msleep(unsigned long msecs)
  		msleep_interruptible(unsigned long msecs)
  
  	Unlike the *delay family, the underlying mechanism
  	driving each of these calls varies, thus there are
  	quirks you should be aware of.
  
  
  	SLEEPING FOR "A FEW" USECS ( < ~10us? ):
  		* Use udelay
  
  		- Why not usleep?
  			On slower systems, (embedded, OR perhaps a speed-
  			stepped PC!) the overhead of setting up the hrtimers
  			for usleep *may* not be worth it. Such an evaluation
  			will obviously depend on your specific situation, but
  			it is something to be aware of.
  
  	SLEEPING FOR ~USECS OR SMALL MSECS ( 10us - 20ms):
  		* Use usleep_range
  
  		- Why not msleep for (1ms - 20ms)?
  			Explained originally here:
  				http://lkml.org/lkml/2007/8/3/250
  			msleep(1~20) may not do what the caller intends, and
  			will often sleep longer (~20 ms actual sleep for any
  			value given in the 1~20ms range). In many cases this
  			is not the desired behavior.
  
  		- Why is there no "usleep" / What is a good range?
  			Since usleep_range is built on top of hrtimers, the
  			wakeup will be very precise (ish), thus a simple
  			usleep function would likely introduce a large number
  			of undesired interrupts.
  
  			With the introduction of a range, the scheduler is
  			free to coalesce your wakeup with any other wakeup
  			that may have happened for other reasons, or at the
  			worst case, fire an interrupt for your upper bound.
  
  			The larger a range you supply, the greater a chance
  			that you will not trigger an interrupt; this should
  			be balanced with what is an acceptable upper bound on
  			delay / performance for your specific code path. Exact
  			tolerances here are very situation specific, thus it
  			is left to the caller to determine a reasonable range.
  
  	SLEEPING FOR LARGER MSECS ( 10ms+ )
  		* Use msleep or possibly msleep_interruptible
  
  		- What's the difference?
  			msleep sets the current task to TASK_UNINTERRUPTIBLE
  			whereas msleep_interruptible sets the current task to
  			TASK_INTERRUPTIBLE before scheduling the sleep. In
  			short, the difference is whether the sleep can be ended
  			early by a signal. In general, just use msleep unless
  			you know you have a need for the interruptible variant.