kerneldoc for <linux/clk.h>

!Edrivers/i2c/i2c-core.c

  </chapter>

  <chapter id="clk">

     <title>Clock Framework</title>

     <para>

	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 <structname>struct clk</structname> 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.

     </para>

     <para>

	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 <emphasis>retention

	mode</emphasis>.

	This mode still incurs leakage currents, especially with finer

	circuit geometries, but for CMOS circuits power is mostly used

	by clocked state changes.

     </para>

     <para>

	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.

     </para>

     <para>

        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.

     </para>

!Iinclude/linux/clk.h

  </chapter>

</book>