1 =================================================================== 2 delays - Information on the various kernel delay / sleep mechanisms 3 =================================================================== 4 5 This document seeks to answer the common question: "What is the 6 RightWay (TM) to insert a delay?" 7 8 This question is most often faced by driver writers who have to 9 deal with hardware delays and who may not be the most intimately 10 familiar with the inner workings of the Linux Kernel. 11 12 13 Inserting Delays 14 ---------------- 15 16 The first, and most important, question you need to ask is "Is my 17 code in an atomic context?" This should be followed closely by "Does 18 it really need to delay in atomic context?" If so... 19 20 ATOMIC CONTEXT: 21 You must use the `*delay` family of functions. These 22 functions use the jiffy 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 38 NON-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
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