CPU frequency and voltage scaling code in the Linux(TM) kernel
  
  
  		         L i n u x    C P U F r e q
  
  			   C P U   D r i v e r s 
  
  		       - information for developers -
  
  
  		    Dominik Brodowski  <linux@brodo.de>
  
  
  
     Clock scaling allows you to change the clock speed of the CPUs on the
      fly. This is a nice method to save battery power, because the lower
              the clock speed, the less power the CPU consumes.
  
  
  Contents:
  ---------
  1.   What To Do?
  1.1  Initialization
  1.2  Per-CPU Initialization
  1.3  verify
  1.4  target or setpolicy?
  1.5  target
  1.6  setpolicy
  2.   Frequency Table Helpers
  
  
  
  1. What To Do?
  ==============
  
  So, you just got a brand-new CPU / chipset with datasheets and want to
  add cpufreq support for this CPU / chipset? Great. Here are some hints
  on what is necessary:
  
  
  1.1 Initialization
  ------------------
  
  First of all, in an __initcall level 7 (module_init()) or later
  function check whether this kernel runs on the right CPU and the right
  chipset. If so, register a struct cpufreq_driver with the CPUfreq core
  using cpufreq_register_driver()
  
  What shall this struct cpufreq_driver contain? 
  
  cpufreq_driver.name -		The name of this driver.
  
  cpufreq_driver.owner -		THIS_MODULE;
  
  cpufreq_driver.init -		A pointer to the per-CPU initialization 
  				function.
  
  cpufreq_driver.verify -		A pointer to a "verification" function.
  
  cpufreq_driver.setpolicy _or_ 
  cpufreq_driver.target -		See below on the differences.
  
  And optionally
  
  cpufreq_driver.exit -		A pointer to a per-CPU cleanup function.
  
  cpufreq_driver.resume -		A pointer to a per-CPU resume function
  				which is called with interrupts disabled
  				and _before_ the pre-suspend frequency
  				and/or policy is restored by a call to
  				->target or ->setpolicy.
  
  cpufreq_driver.attr -		A pointer to a NULL-terminated list of
  				"struct freq_attr" which allow to
  				export values to sysfs.
  
  
  1.2 Per-CPU Initialization
  --------------------------
  
  Whenever a new CPU is registered with the device model, or after the
  cpufreq driver registers itself, the per-CPU initialization function 
  cpufreq_driver.init is called. It takes a struct cpufreq_policy
  *policy as argument. What to do now?
  
  If necessary, activate the CPUfreq support on your CPU.
  
  Then, the driver must fill in the following values:
  
  policy->cpuinfo.min_freq _and_
  policy->cpuinfo.max_freq -	the minimum and maximum frequency 
  				(in kHz) which is supported by 
  				this CPU
  policy->cpuinfo.transition_latency   the time it takes on this CPU to

  				switch between two frequencies in
  				nanoseconds (if appropriate, else
  				specify CPUFREQ_ETERNAL)

  
  policy->cur			The current operating frequency of
  				this CPU (if appropriate)
  policy->min, 
  policy->max, 
  policy->policy and, if necessary,
  policy->governor		must contain the "default policy" for
  				this CPU. A few moments later,
  				cpufreq_driver.verify and either
  				cpufreq_driver.setpolicy or
  				cpufreq_driver.target is called with
  				these values.
  
  For setting some of these values, the frequency table helpers might be
  helpful. See the section 2 for more information on them.
  
  
  1.3 verify
  ------------
  
  When the user decides a new policy (consisting of
  "policy,governor,min,max") shall be set, this policy must be validated
  so that incompatible values can be corrected. For verifying these
  values, a frequency table helper and/or the
  cpufreq_verify_within_limits(struct cpufreq_policy *policy, unsigned
  int min_freq, unsigned int max_freq) function might be helpful. See
  section 2 for details on frequency table helpers.
  
  You need to make sure that at least one valid frequency (or operating
  range) is within policy->min and policy->max. If necessary, increase
  policy->max first, and only if this is no solution, decrease policy->min.
  
  
  1.4 target or setpolicy?
  ----------------------------
  
  Most cpufreq drivers or even most cpu frequency scaling algorithms 
  only allow the CPU to be set to one frequency. For these, you use the
  ->target call.
  
  Some cpufreq-capable processors switch the frequency between certain
  limits on their own. These shall use the ->setpolicy call
  
  
  1.4. target
  -------------
  
  The target call has three arguments: struct cpufreq_policy *policy,
  unsigned int target_frequency, unsigned int relation.
  
  The CPUfreq driver must set the new frequency when called here. The
  actual frequency must be determined using the following rules:
  
  - keep close to "target_freq"
  - policy->min <= new_freq <= policy->max (THIS MUST BE VALID!!!)
  - if relation==CPUFREQ_REL_L, try to select a new_freq higher than or equal
    target_freq. ("L for lowest, but no lower than")
  - if relation==CPUFREQ_REL_H, try to select a new_freq lower than or equal
    target_freq. ("H for highest, but no higher than")

  Here again the frequency table helper might assist you - see section 2

  for details.
  
  
  1.5 setpolicy
  ---------------
  
  The setpolicy call only takes a struct cpufreq_policy *policy as
  argument. You need to set the lower limit of the in-processor or
  in-chipset dynamic frequency switching to policy->min, the upper limit
  to policy->max, and -if supported- select a performance-oriented
  setting when policy->policy is CPUFREQ_POLICY_PERFORMANCE, and a
  powersaving-oriented setting when CPUFREQ_POLICY_POWERSAVE. Also check

  the reference implementation in drivers/cpufreq/longrun.c

  
  
  
  2. Frequency Table Helpers
  ==========================
  
  As most cpufreq processors only allow for being set to a few specific
  frequencies, a "frequency table" with some functions might assist in
  some work of the processor driver. Such a "frequency table" consists
  of an array of struct cpufreq_freq_table entries, with any value in
  "index" you want to use, and the corresponding frequency in
  "frequency". At the end of the table, you need to add a
  cpufreq_freq_table entry with frequency set to CPUFREQ_TABLE_END. And
  if you want to skip one entry in the table, set the frequency to 
  CPUFREQ_ENTRY_INVALID. The entries don't need to be in ascending
  order.
  
  By calling cpufreq_frequency_table_cpuinfo(struct cpufreq_policy *policy,
  					struct cpufreq_frequency_table *table);
  the cpuinfo.min_freq and cpuinfo.max_freq values are detected, and
  policy->min and policy->max are set to the same values. This is
  helpful for the per-CPU initialization stage.
  
  int cpufreq_frequency_table_verify(struct cpufreq_policy *policy,
                                     struct cpufreq_frequency_table *table);
  assures that at least one valid frequency is within policy->min and
  policy->max, and all other criteria are met. This is helpful for the
  ->verify call.
  
  int cpufreq_frequency_table_target(struct cpufreq_policy *policy,
                                     struct cpufreq_frequency_table *table,
                                     unsigned int target_freq,
                                     unsigned int relation,
                                     unsigned int *index);
  
  is the corresponding frequency table helper for the ->target
  stage. Just pass the values to this function, and the unsigned int
  index returns the number of the frequency table entry which contains
  the frequency the CPU shall be set to. PLEASE NOTE: This is not the
  "index" which is in this cpufreq_table_entry.index, but instead
  cpufreq_table[index]. So, the new frequency is
  cpufreq_table[index].frequency, and the value you stored into the
  frequency table "index" field is
  cpufreq_table[index].index.