1===================================================================
2delays - Information on the various kernel delay / sleep mechanisms
3===================================================================
4
5This document seeks to answer the common question: "What is the
6RightWay (TM) to insert a delay?"
7
8This question is most often faced by driver writers who have to
9deal with hardware delays and who may not be the most intimately
10familiar with the inner workings of the Linux Kernel.
11
12
13Inserting Delays
14----------------
15
16The first, and most important, question you need to ask is "Is my
17code in an atomic context?"  This should be followed closely by "Does
18it really need to delay in atomic context?" If so...
19
20ATOMIC CONTEXT:
21	You must use the `*delay` family of functions. These
22	functions use the jiffie estimation of clock speed
23	and will busy wait for enough loop cycles to achieve
24	the desired delay:
25
26	ndelay(unsigned long nsecs)
27	udelay(unsigned long usecs)
28	mdelay(unsigned long msecs)
29
30	udelay is the generally preferred API; ndelay-level
31	precision may not actually exist on many non-PC devices.
32
33	mdelay is macro wrapper around udelay, to account for
34	possible overflow when passing large arguments to udelay.
35	In general, use of mdelay is discouraged and code should
36	be refactored to allow for the use of msleep.
37
38NON-ATOMIC CONTEXT:
39	You should use the `*sleep[_range]` family of functions.
40	There are a few more options here, while any of them may
41	work correctly, using the "right" sleep function will
42	help the scheduler, power management, and just make your
43	driver better :)
44
45	-- Backed by busy-wait loop:
46
47		udelay(unsigned long usecs)
48
49	-- Backed by hrtimers:
50
51		usleep_range(unsigned long min, unsigned long max)
52
53	-- Backed by jiffies / legacy_timers
54
55		msleep(unsigned long msecs)
56		msleep_interruptible(unsigned long msecs)
57
58	Unlike the `*delay` family, the underlying mechanism
59	driving each of these calls varies, thus there are
60	quirks you should be aware of.
61
62
63	SLEEPING FOR "A FEW" USECS ( < ~10us? ):
64		* Use udelay
65
66		- Why not usleep?
67			On slower systems, (embedded, OR perhaps a speed-
68			stepped PC!) the overhead of setting up the hrtimers
69			for usleep *may* not be worth it. Such an evaluation
70			will obviously depend on your specific situation, but
71			it is something to be aware of.
72
73	SLEEPING FOR ~USECS OR SMALL MSECS ( 10us - 20ms):
74		* Use usleep_range
75
76		- Why not msleep for (1ms - 20ms)?
77			Explained originally here:
78				https://lore.kernel.org/r/15327.1186166232@lwn.net
79
80			msleep(1~20) may not do what the caller intends, and
81			will often sleep longer (~20 ms actual sleep for any
82			value given in the 1~20ms range). In many cases this
83			is not the desired behavior.
84
85		- Why is there no "usleep" / What is a good range?
86			Since usleep_range is built on top of hrtimers, the
87			wakeup will be very precise (ish), thus a simple
88			usleep function would likely introduce a large number
89			of undesired interrupts.
90
91			With the introduction of a range, the scheduler is
92			free to coalesce your wakeup with any other wakeup
93			that may have happened for other reasons, or at the
94			worst case, fire an interrupt for your upper bound.
95
96			The larger a range you supply, the greater a chance
97			that you will not trigger an interrupt; this should
98			be balanced with what is an acceptable upper bound on
99			delay / performance for your specific code path. Exact
100			tolerances here are very situation specific, thus it
101			is left to the caller to determine a reasonable range.
102
103	SLEEPING FOR LARGER MSECS ( 10ms+ )
104		* Use msleep or possibly msleep_interruptible
105
106		- What's the difference?
107			msleep sets the current task to TASK_UNINTERRUPTIBLE
108			whereas msleep_interruptible sets the current task to
109			TASK_INTERRUPTIBLE before scheduling the sleep. In
110			short, the difference is whether the sleep can be ended
111			early by a signal. In general, just use msleep unless
112			you know you have a need for the interruptible variant.
113
114	FLEXIBLE SLEEPING (any delay, uninterruptible)
115		* Use fsleep
116