CPUfreq.txt
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S3C24XX CPUfreq support
=======================
Introduction
------------
The S3C24XX series support a number of power saving systems, such as
the ability to change the core, memory and peripheral operating
frequencies. The core control is exported via the CPUFreq driver
which has a number of different manual or automatic controls over the
rate the core is running at.
There are two forms of the driver depending on the specific CPU and
how the clocks are arranged. The first implementation used as single
PLL to feed the ARM, memory and peripherals via a series of dividers
and muxes and this is the implementation that is documented here. A
newer version where there is a separate PLL and clock divider for the
ARM core is available as a separate driver.
Layout
------
The code core manages the CPU specific drivers, any data that they
need to register and the interface to the generic drivers/cpufreq
system. Each CPU registers a driver to control the PLL, clock dividers
and anything else associated with it. Any board that wants to use this
framework needs to supply at least basic details of what is required.
The core registers with drivers/cpufreq at init time if all the data
necessary has been supplied.
CPU support
-----------
The support for each CPU depends on the facilities provided by the
SoC and the driver as each device has different PLL and clock chains
associated with it.
Slow Mode
---------
The SLOW mode where the PLL is turned off altogether and the
system is fed by the external crystal input is currently not
supported.
sysfs
-----
The core code exports extra information via sysfs in the directory
devices/system/cpu/cpu0/arch-freq.
Board Support
-------------
Each board that wants to use the cpufreq code must register some basic
information with the core driver to provide information about what the
board requires and any restrictions being placed on it.
The board needs to supply information about whether it needs the IO bank
timings changing, any maximum frequency limits and information about the
SDRAM refresh rate.
Document Author
---------------
Ben Dooks, Copyright 2009 Simtec Electronics
Licensed under GPLv2