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author | Linus Torvalds <torvalds@linux-foundation.org> | 2019-07-09 22:34:26 +0300 |
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committer | Linus Torvalds <torvalds@linux-foundation.org> | 2019-07-09 22:34:26 +0300 |
commit | e9a83bd2322035ed9d7dcf35753d3f984d76c6a5 (patch) | |
tree | 66dc466ff9aec0f9bb7f39cba50a47eab6585559 /Documentation/timers/timers-howto.rst | |
parent | 7011b7e1b702cc76f9e969b41d9a95969f2aecaa (diff) | |
parent | 454f96f2b738374da4b0a703b1e2e7aed82c4486 (diff) | |
download | linux-e9a83bd2322035ed9d7dcf35753d3f984d76c6a5.tar.xz |
Merge tag 'docs-5.3' of git://git.lwn.net/linux
Pull Documentation updates from Jonathan Corbet:
"It's been a relatively busy cycle for docs:
- A fair pile of RST conversions, many from Mauro. These create more
than the usual number of simple but annoying merge conflicts with
other trees, unfortunately. He has a lot more of these waiting on
the wings that, I think, will go to you directly later on.
- A new document on how to use merges and rebases in kernel repos,
and one on Spectre vulnerabilities.
- Various improvements to the build system, including automatic
markup of function() references because some people, for reasons I
will never understand, were of the opinion that
:c:func:``function()`` is unattractive and not fun to type.
- We now recommend using sphinx 1.7, but still support back to 1.4.
- Lots of smaller improvements, warning fixes, typo fixes, etc"
* tag 'docs-5.3' of git://git.lwn.net/linux: (129 commits)
docs: automarkup.py: ignore exceptions when seeking for xrefs
docs: Move binderfs to admin-guide
Disable Sphinx SmartyPants in HTML output
doc: RCU callback locks need only _bh, not necessarily _irq
docs: format kernel-parameters -- as code
Doc : doc-guide : Fix a typo
platform: x86: get rid of a non-existent document
Add the RCU docs to the core-api manual
Documentation: RCU: Add TOC tree hooks
Documentation: RCU: Rename txt files to rst
Documentation: RCU: Convert RCU UP systems to reST
Documentation: RCU: Convert RCU linked list to reST
Documentation: RCU: Convert RCU basic concepts to reST
docs: filesystems: Remove uneeded .rst extension on toctables
scripts/sphinx-pre-install: fix out-of-tree build
docs: zh_CN: submitting-drivers.rst: Remove a duplicated Documentation/
Documentation: PGP: update for newer HW devices
Documentation: Add section about CPU vulnerabilities for Spectre
Documentation: platform: Delete x86-laptop-drivers.txt
docs: Note that :c:func: should no longer be used
...
Diffstat (limited to 'Documentation/timers/timers-howto.rst')
-rw-r--r-- | Documentation/timers/timers-howto.rst | 112 |
1 files changed, 112 insertions, 0 deletions
diff --git a/Documentation/timers/timers-howto.rst b/Documentation/timers/timers-howto.rst new file mode 100644 index 000000000000..7e3167bec2b1 --- /dev/null +++ b/Documentation/timers/timers-howto.rst @@ -0,0 +1,112 @@ +=================================================================== +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. |