UM0851 datasheet(235/245 Pages) STMICROELECTRONICS

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UM0851

Power management section

Doc ID 16604 Rev 2

235/245

Ethernet and USB are asynchronous to data transfers, so once the network has been

initialized or the USB enumeration is complete, the clocks of the respective modules should

be on, for the modules to function properly.

The clock framework is mainly used by the drivers in the kernel space. This is done to allow

the drivers to fetch the clock from the clock dependency tree through the set of kernel

interfaces such as clk_get, clk_put, clk_enable etc. To illustrate this, a sample from the I2C

driver is given below:

1) During the initialization probe time, the calls are made to the clock framework to

get the clock. The return value is a handler to manage the clocks related

functionality in future.

dev->iclk = clk_get(dev->dev, "i2c_ick");

dev->fclk = clk_get(dev->dev, "i2c_fck");

2) /* Prior to the data transfers enable the clock, do the transfers and then disable

the clock.*/

clk_enable(dev->iclk);

clk_enable(dev->fclk);

/*DO DATA TRANSFERS*/

clk_disable(dev->fclk);

clk_disable(dev->iclk);

Some other kernel interfaces such as clk_get_rate() have been defined to get the clock rate,

and clk_set_rate() to set the clock rate.

10.4.3

Usage: CPUfreq user interface

The user interface to CPUfreq is through sysfs. CPUfreq provides the flexibility to manage

CPUs at a per-processor level (as long as hardware agrees to manage CPUs at that level).

The interface for each CPU will be under sysfs, typically at /sys/devices/system/cpu/

cpuX/cpufreq, where X ranges from 0 through N-1, with N being total number of logical

CPUs in the system.

The basic interfaces provided by CPUfreq are:

$: cd /sys/devices/system/cpu/cpu0/cpufreq

$: ls -la

affected_cpus

cpuinfo_cur_freq

: Current CPU freq

cpuinfo_max_freq

: Max CPU Freq

cpuinfo_min_freq

: Min CPU freq

scaling_available_frequencies

: All available CPU freq

scaling_available_governors

: lists out all the governors supported by the

kernel.

scaling_cur_freq

: cached value of current frequency from cpufreq

subsystem.

scaling_driver

: CPU Freq driver used to change the freq

scaling_governor

: User assigned governor

scaling_max_freq

: User Controlled Upper Limit

scaling_min_freq

: User Controlled Lower Limit

To show the available governors you can use

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