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Based on kernel version 3.16. Page generated on 2014-08-06 21:41 EST.

1	delays - Information on the various kernel delay / sleep mechanisms
2	-------------------------------------------------------------------
3	
4	This document seeks to answer the common question: "What is the
5	RightWay (TM) to insert a delay?"
6	
7	This question is most often faced by driver writers who have to
8	deal with hardware delays and who may not be the most intimately
9	familiar with the inner workings of the Linux Kernel.
10	
11	
12	Inserting Delays
13	----------------
14	
15	The first, and most important, question you need to ask is "Is my
16	code in an atomic context?"  This should be followed closely by "Does
17	it really need to delay in atomic context?" If so...
18	
19	ATOMIC CONTEXT:
20		You must use the *delay family of functions. These
21		functions use the jiffie estimation of clock speed
22		and will busy wait for enough loop cycles to achieve
23		the desired delay:
24	
25		ndelay(unsigned long nsecs)
26		udelay(unsigned long usecs)
27		mdelay(unsigned long msecs)
28	
29		udelay is the generally preferred API; ndelay-level
30		precision may not actually exist on many non-PC devices.
31	
32		mdelay is macro wrapper around udelay, to account for
33		possible overflow when passing large arguments to udelay.
34		In general, use of mdelay is discouraged and code should
35		be refactored to allow for the use of msleep.
36	
37	NON-ATOMIC CONTEXT:
38		You should use the *sleep[_range] family of functions.
39		There are a few more options here, while any of them may
40		work correctly, using the "right" sleep function will
41		help the scheduler, power management, and just make your
42		driver better :)
43	
44		-- Backed by busy-wait loop:
45			udelay(unsigned long usecs)
46		-- Backed by hrtimers:
47			usleep_range(unsigned long min, unsigned long max)
48		-- Backed by jiffies / legacy_timers
49			msleep(unsigned long msecs)
50			msleep_interruptible(unsigned long msecs)
51	
52		Unlike the *delay family, the underlying mechanism
53		driving each of these calls varies, thus there are
54		quirks you should be aware of.
55	
56	
57		SLEEPING FOR "A FEW" USECS ( < ~10us? ):
58			* Use udelay
59	
60			- Why not usleep?
61				On slower systems, (embedded, OR perhaps a speed-
62				stepped PC!) the overhead of setting up the hrtimers
63				for usleep *may* not be worth it. Such an evaluation
64				will obviously depend on your specific situation, but
65				it is something to be aware of.
66	
67		SLEEPING FOR ~USECS OR SMALL MSECS ( 10us - 20ms):
68			* Use usleep_range
69	
70			- Why not msleep for (1ms - 20ms)?
71				Explained originally here:
72					http://lkml.org/lkml/2007/8/3/250
73				msleep(1~20) may not do what the caller intends, and
74				will often sleep longer (~20 ms actual sleep for any
75				value given in the 1~20ms range). In many cases this
76				is not the desired behavior.
77	
78			- Why is there no "usleep" / What is a good range?
79				Since usleep_range is built on top of hrtimers, the
80				wakeup will be very precise (ish), thus a simple
81				usleep function would likely introduce a large number
82				of undesired interrupts.
83	
84				With the introduction of a range, the scheduler is
85				free to coalesce your wakeup with any other wakeup
86				that may have happened for other reasons, or at the
87				worst case, fire an interrupt for your upper bound.
88	
89				The larger a range you supply, the greater a chance
90				that you will not trigger an interrupt; this should
91				be balanced with what is an acceptable upper bound on
92				delay / performance for your specific code path. Exact
93				tolerances here are very situation specific, thus it
94				is left to the caller to determine a reasonable range.
95	
96		SLEEPING FOR LARGER MSECS ( 10ms+ )
97			* Use msleep or possibly msleep_interruptible
98	
99			- What's the difference?
100				msleep sets the current task to TASK_UNINTERRUPTIBLE
101				whereas msleep_interruptible sets the current task to
102				TASK_INTERRUPTIBLE before scheduling the sleep. In
103				short, the difference is whether the sleep can be ended
104				early by a signal. In general, just use msleep unless
105				you know you have a need for the interruptible variant.
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