1 ==============================================
2 delays - Information on the various kernel del
3 ==============================================
4
5 This document seeks to answer the common quest
6 RightWay (TM) to insert a delay?"
7
8 This question is most often faced by driver wr
9 deal with hardware delays and who may not be t
10 familiar with the inner workings of the Linux
11
12
13 Inserting Delays
14 ----------------
15
16 The first, and most important, question you ne
17 code in an atomic context?" This should be fo
18 it really need to delay in atomic context?" If
19
20 ATOMIC CONTEXT:
21 You must use the `*delay` family of fu
22 functions use the jiffy estimation of
23 and will busy wait for enough loop cyc
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;
31 precision may not actually exist on ma
32
33 mdelay is macro wrapper around udelay,
34 possible overflow when passing large a
35 In general, use of mdelay is discourag
36 be refactored to allow for the use of
37
38 NON-ATOMIC CONTEXT:
39 You should use the `*sleep[_range]` fa
40 There are a few more options here, whi
41 work correctly, using the "right" slee
42 help the scheduler, power management,
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
52
53 -- Backed by jiffies / legacy_timers
54
55 msleep(unsigned long msecs)
56 msleep_interruptible(unsigned
57
58 Unlike the `*delay` family, the underl
59 driving each of these calls varies, th
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, (em
68 stepped PC!) the overh
69 for usleep *may* not b
70 will obviously depend
71 it is something to be
72
73 SLEEPING FOR ~USECS OR SMALL MSECS ( 1
74 * Use usleep_range
75
76 - Why not msleep for (1ms - 20
77 Explained originally h
78 https://lore.k
79
80 msleep(1~20) may not d
81 will often sleep longe
82 value given in the 1~2
83 is not the desired beh
84
85 - Why is there no "usleep" / W
86 Since usleep_range is
87 wakeup will be very pr
88 usleep function would
89 of undesired interrupt
90
91 With the introduction
92 free to coalesce your
93 that may have happened
94 worst case, fire an in
95
96 The larger a range you
97 that you will not trig
98 be balanced with what
99 delay / performance fo
100 tolerances here are ve
101 is left to the caller
102
103 SLEEPING FOR LARGER MSECS ( 10ms+ )
104 * Use msleep or possibly mslee
105
106 - What's the difference?
107 msleep sets the curren
108 whereas msleep_interru
109 TASK_INTERRUPTIBLE bef
110 short, the difference
111 early by a signal. In
112 you know you have a ne
113
114 FLEXIBLE SLEEPING (any delay, uninterr
115 * Use fsleep
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