Documentation: Add timers/timers-howto.txt

This file seeks to explain the nuances in various delays;
many driver writers are not necessarily familiar with the
various kernel timers, their shortfalls, and quirks. When
faced with

ndelay, udelay, mdelay, usleep_range, msleep, and msleep_interrubtible

the question "How do I just wait 1 ms for my hardware to
latch?" has the non-intuitive "best" answer:
	usleep_range(1000,1500)

This patch is followed by a series of checkpatch additions
that seek to help kernel hackers pick the best delay.
Signed-off-by: NPatrick Pannuto <ppannuto@codeaurora.org>
Cc: apw@canonical.com
Cc: corbet@lwn.net
Cc: arjan@linux.intel.com
Cc: Randy Dunlap <rdunlap@xenotime.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
LKML-Reference: <1280786467-26999-3-git-send-email-ppannuto@codeaurora.org>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>

Documentation/timers/timers-howto.txt

+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(unsgined 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.