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diff --git a/Documentation/DocBook/kernel-api.tmpl b/Documentation/DocBook/kernel-api.tmpl index 488dd4a4945b..617c2d979975 100644 --- a/Documentation/DocBook/kernel-api.tmpl +++ b/Documentation/DocBook/kernel-api.tmpl @@ -645,4 +645,58 @@ X!Idrivers/video/console/fonts.c !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> |