1delays - Information on the various kernel delay / sleep mechanisms 2------------------------------------------------------------------- 3 4This document seeks to answer the common question: "What is the 5RightWay (TM) to insert a delay?" 6 7This question is most often faced by driver writers who have to 8deal with hardware delays and who may not be the most intimately 9familiar with the inner workings of the Linux Kernel. 10 11 12Inserting Delays 13---------------- 14 15The first, and most important, question you need to ask is "Is my 16code in an atomic context?" This should be followed closely by "Does 17it really need to delay in atomic context?" If so... 18 19ATOMIC 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 37NON-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. 106