linux/Documentation/timers/timers-howto.txt
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   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
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