[PATCH 2/4] Documentation: Add timers/timers-howto.txt

From: Patrick Pannuto
Date: Mon Aug 02 2010 - 18:01:30 EST

This file seeks to explain the nuances in various delays;
many driver writers are not necessarily familiar with the
various kernel timers, their shortfalls, and quirks. When
faced with

ndelay, udelay, mdelay, usleep_range, msleep, and msleep_interrubtible

the question "How do I just wait 1 ms for my hardware to
latch?" has the non-intuitive "best" answer:

This patch is followed by a series of checkpatch additions
that seek to help kernel hackers pick the best delay.

Signed-off-by: Patrick Pannuto <ppannuto@xxxxxxxxxxxxxx>
Documentation/timers/timers-howto.txt | 105 +++++++++++++++++++++++++++++++++
1 files changed, 105 insertions(+), 0 deletions(-)
create mode 100644 Documentation/timers/timers-howto.txt

diff --git a/Documentation/timers/timers-howto.txt b/Documentation/timers/timers-howto.txt
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+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...
+ 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(unsgined 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.
+ 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.
+ * 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.
+ * 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.

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