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diff --git a/Documentation/timers/timers-howto.txt b/Documentation/timers/timers-howto.txt deleted file mode 100644 index 038f8c77a076..000000000000 --- a/Documentation/timers/timers-howto.txt +++ /dev/null @@ -1,105 +0,0 @@ -delays - Information on the various kernel delay / sleep mechanisms -------------------------------------------------------------------- - -This document seeks to answer the common question: "What is the -RightWay (TM) to insert a delay?" - -This question is most often faced by driver writers who have to -deal with hardware delays and who may not be the most intimately -familiar with the inner workings of the Linux Kernel. - - -Inserting Delays ----------------- - -The first, and most important, question you need to ask is "Is my -code in an atomic context?" This should be followed closely by "Does -it really need to delay in atomic context?" If so... - -ATOMIC CONTEXT: - You must use the *delay family of functions. These - functions use the jiffie estimation of clock speed - and will busy wait for enough loop cycles to achieve - the desired delay: - - ndelay(unsigned long nsecs) - udelay(unsigned long usecs) - mdelay(unsigned long msecs) - - udelay is the generally preferred API; ndelay-level - precision may not actually exist on many non-PC devices. - - mdelay is macro wrapper around udelay, to account for - possible overflow when passing large arguments to udelay. - In general, use of mdelay is discouraged and code should - be refactored to allow for the use of msleep. - -NON-ATOMIC CONTEXT: - You should use the *sleep[_range] family of functions. - There are a few more options here, while any of them may - work correctly, using the "right" sleep function will - help the scheduler, power management, and just make your - driver better :) - - -- Backed by busy-wait loop: - udelay(unsigned long usecs) - -- Backed by hrtimers: - usleep_range(unsigned long min, unsigned long max) - -- Backed by jiffies / legacy_timers - msleep(unsigned long msecs) - msleep_interruptible(unsigned long msecs) - - Unlike the *delay family, the underlying mechanism - driving each of these calls varies, thus there are - quirks you should be aware of. - - - SLEEPING FOR "A FEW" USECS ( < ~10us? ): - * Use udelay - - - Why not usleep? - On slower systems, (embedded, OR perhaps a speed- - stepped PC!) the overhead of setting up the hrtimers - for usleep *may* not be worth it. Such an evaluation - will obviously depend on your specific situation, but - it is something to be aware of. - - SLEEPING FOR ~USECS OR SMALL MSECS ( 10us - 20ms): - * Use usleep_range - - - Why not msleep for (1ms - 20ms)? - Explained originally here: - http://lkml.org/lkml/2007/8/3/250 - msleep(1~20) may not do what the caller intends, and - will often sleep longer (~20 ms actual sleep for any - value given in the 1~20ms range). In many cases this - is not the desired behavior. - - - Why is there no "usleep" / What is a good range? - Since usleep_range is built on top of hrtimers, the - wakeup will be very precise (ish), thus a simple - usleep function would likely introduce a large number - of undesired interrupts. - - With the introduction of a range, the scheduler is - free to coalesce your wakeup with any other wakeup - that may have happened for other reasons, or at the - worst case, fire an interrupt for your upper bound. - - The larger a range you supply, the greater a chance - that you will not trigger an interrupt; this should - be balanced with what is an acceptable upper bound on - delay / performance for your specific code path. Exact - tolerances here are very situation specific, thus it - is left to the caller to determine a reasonable range. - - SLEEPING FOR LARGER MSECS ( 10ms+ ) - * Use msleep or possibly msleep_interruptible - - - What's the difference? - msleep sets the current task to TASK_UNINTERRUPTIBLE - whereas msleep_interruptible sets the current task to - TASK_INTERRUPTIBLE before scheduling the sleep. In - short, the difference is whether the sleep can be ended - early by a signal. In general, just use msleep unless - you know you have a need for the interruptible variant. |