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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968" /><title>Chapter 17. Clock Framework</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><link rel="home" href="index.html" title="The Linux Kernel API" /><link rel="up" href="index.html" title="The Linux Kernel API" /><link rel="prev" href="re615.html" title="misc_deregister" /><link rel="next" href="re616.html" title="clk_get" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 17. Clock Framework</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="re615.html">Prev</a> </td><th width="60%" align="center"> </th><td width="20%" align="right"> <a accesskey="n" href="re616.html">Next</a></td></tr></table><hr /></div><div class="chapter" title="Chapter 17. Clock Framework"><div class="titlepage"><div><div><h2 class="title"><a id="clk"></a>Chapter 17. Clock Framework</h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="refentrytitle"><a href="re616.html"><span>clk_get</span></a></span><span class="refpurpose"> —
lookup and obtain a reference to a clock producer.
</span></dt><dt><span class="refentrytitle"><a href="re617.html"><span>clk_enable</span></a></span><span class="refpurpose"> —
inform the system when the clock source should be running.
</span></dt><dt><span class="refentrytitle"><a href="re618.html"><span>clk_disable</span></a></span><span class="refpurpose"> —
inform the system when the clock source is no longer required.
</span></dt><dt><span class="refentrytitle"><a href="re619.html"><span>clk_get_rate</span></a></span><span class="refpurpose"> —
obtain the current clock rate (in Hz) for a clock source. This is only valid once the clock source has been enabled.
</span></dt><dt><span class="refentrytitle"><a href="re620.html"><span>clk_put</span></a></span><span class="refpurpose"> —
"free" the clock source
</span></dt><dt><span class="refentrytitle"><a href="re621.html"><span>clk_round_rate</span></a></span><span class="refpurpose"> —
adjust a rate to the exact rate a clock can provide
</span></dt><dt><span class="refentrytitle"><a href="re622.html"><span>clk_set_rate</span></a></span><span class="refpurpose"> —
set the clock rate for a clock source
</span></dt><dt><span class="refentrytitle"><a href="re623.html"><span>clk_set_parent</span></a></span><span class="refpurpose"> —
set the parent clock source for this clock
</span></dt><dt><span class="refentrytitle"><a href="re624.html"><span>clk_get_parent</span></a></span><span class="refpurpose"> —
get the parent clock source for this clock
</span></dt><dt><span class="refentrytitle"><a href="re625.html"><span>clk_get_sys</span></a></span><span class="refpurpose"> —
get a clock based upon the device name
</span></dt><dt><span class="refentrytitle"><a href="re626.html"><span>clk_add_alias</span></a></span><span class="refpurpose"> —
add a new clock alias
</span></dt></dl></div><p>
The clock framework defines programming interfaces to support
software management of the system clock tree.
This framework is widely used with System-On-Chip (SOC) platforms
to support power management and various devices which may need
custom clock rates.
Note that these "clocks" don't relate to timekeeping or real
time clocks (RTCs), each of which have separate frameworks.
These <span class="structname">struct clk</span> instances may be used
to manage for example a 96 MHz signal that is used to shift bits
into and out of peripherals or busses, or otherwise trigger
synchronous state machine transitions in system hardware.
</p><p>
Power management is supported by explicit software clock gating:
unused clocks are disabled, so the system doesn't waste power
changing the state of transistors that aren't in active use.
On some systems this may be backed by hardware clock gating,
where clocks are gated without being disabled in software.
Sections of chips that are powered but not clocked may be able
to retain their last state.
This low power state is often called a <span class="emphasis"><em>retention
mode</em></span>.
This mode still incurs leakage currents, especially with finer
circuit geometries, but for CMOS circuits power is mostly used
by clocked state changes.
</p><p>
Power-aware drivers only enable their clocks when the device
they manage is in active use. Also, system sleep states often
differ according to which clock domains are active: while a
"standby" state may allow wakeup from several active domains, a
"mem" (suspend-to-RAM) state may require a more wholesale shutdown
of clocks derived from higher speed PLLs and oscillators, limiting
the number of possible wakeup event sources. A driver's suspend
method may need to be aware of system-specific clock constraints
on the target sleep state.
</p><p>
Some platforms support programmable clock generators. These
can be used by external chips of various kinds, such as other
CPUs, multimedia codecs, and devices with strict requirements
for interface clocking.
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