116 lines
4.0 KiB
ReStructuredText
116 lines
4.0 KiB
ReStructuredText
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===================================================================
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delays - Information on the various kernel delay / sleep mechanisms
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===================================================================
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This document seeks to answer the common question: "What is the
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RightWay (TM) to insert a delay?"
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This question is most often faced by driver writers who have to
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deal with hardware delays and who may not be the most intimately
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familiar with the inner workings of the Linux Kernel.
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Inserting Delays
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----------------
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The first, and most important, question you need to ask is "Is my
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code in an atomic context?" This should be followed closely by "Does
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it really need to delay in atomic context?" If so...
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ATOMIC CONTEXT:
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You must use the `*delay` family of functions. These
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functions use the jiffie estimation of clock speed
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and will busy wait for enough loop cycles to achieve
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the desired delay:
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ndelay(unsigned long nsecs)
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udelay(unsigned long usecs)
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mdelay(unsigned long msecs)
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udelay is the generally preferred API; ndelay-level
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precision may not actually exist on many non-PC devices.
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mdelay is macro wrapper around udelay, to account for
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possible overflow when passing large arguments to udelay.
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In general, use of mdelay is discouraged and code should
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be refactored to allow for the use of msleep.
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NON-ATOMIC CONTEXT:
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You should use the `*sleep[_range]` family of functions.
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There are a few more options here, while any of them may
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work correctly, using the "right" sleep function will
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help the scheduler, power management, and just make your
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driver better :)
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-- Backed by busy-wait loop:
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udelay(unsigned long usecs)
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-- Backed by hrtimers:
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usleep_range(unsigned long min, unsigned long max)
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-- Backed by jiffies / legacy_timers
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msleep(unsigned long msecs)
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msleep_interruptible(unsigned long msecs)
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Unlike the `*delay` family, the underlying mechanism
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driving each of these calls varies, thus there are
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quirks you should be aware of.
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SLEEPING FOR "A FEW" USECS ( < ~10us? ):
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* Use udelay
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- Why not usleep?
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On slower systems, (embedded, OR perhaps a speed-
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stepped PC!) the overhead of setting up the hrtimers
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for usleep *may* not be worth it. Such an evaluation
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will obviously depend on your specific situation, but
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it is something to be aware of.
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SLEEPING FOR ~USECS OR SMALL MSECS ( 10us - 20ms):
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* Use usleep_range
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- Why not msleep for (1ms - 20ms)?
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Explained originally here:
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https://lore.kernel.org/r/15327.1186166232@lwn.net
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msleep(1~20) may not do what the caller intends, and
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will often sleep longer (~20 ms actual sleep for any
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value given in the 1~20ms range). In many cases this
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is not the desired behavior.
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- Why is there no "usleep" / What is a good range?
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Since usleep_range is built on top of hrtimers, the
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wakeup will be very precise (ish), thus a simple
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usleep function would likely introduce a large number
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of undesired interrupts.
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With the introduction of a range, the scheduler is
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free to coalesce your wakeup with any other wakeup
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that may have happened for other reasons, or at the
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worst case, fire an interrupt for your upper bound.
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The larger a range you supply, the greater a chance
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that you will not trigger an interrupt; this should
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be balanced with what is an acceptable upper bound on
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delay / performance for your specific code path. Exact
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tolerances here are very situation specific, thus it
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is left to the caller to determine a reasonable range.
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SLEEPING FOR LARGER MSECS ( 10ms+ )
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* Use msleep or possibly msleep_interruptible
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- What's the difference?
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msleep sets the current task to TASK_UNINTERRUPTIBLE
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whereas msleep_interruptible sets the current task to
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TASK_INTERRUPTIBLE before scheduling the sleep. In
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short, the difference is whether the sleep can be ended
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early by a signal. In general, just use msleep unless
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you know you have a need for the interruptible variant.
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FLEXIBLE SLEEPING (any delay, uninterruptible)
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* Use fsleep
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