- CPU frequency and voltage scaling code in the Linux(TM) kernel
+.. SPDX-License-Identifier: GPL-2.0
+===============================================
+How to Implement a new CPUFreq Processor Driver
+===============================================
- L i n u x C P U F r e q
+Authors:
- C P U D r i v e r s
- - information for developers -
+ - Dominik Brodowski <linux@brodo.de>
+ - Rafael J. Wysocki <rafael.j.wysocki@intel.com>
+ - Viresh Kumar <viresh.kumar@linaro.org>
+.. Contents
- Dominik Brodowski <linux@brodo.de>
- Rafael J. Wysocki <rafael.j.wysocki@intel.com>
- Viresh Kumar <viresh.kumar@linaro.org>
-
-
-
- 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/target_index or setpolicy?
-1.5 target/target_index
-1.6 setpolicy
-1.7 get_intermediate and target_intermediate
-2. Frequency Table Helpers
+ 1. What To Do?
+ 1.1 Initialization
+ 1.2 Per-CPU Initialization
+ 1.3 verify
+ 1.4 target/target_index or setpolicy?
+ 1.5 target/target_index
+ 1.6 setpolicy
+ 1.7 get_intermediate and target_intermediate
+ 2. Frequency Table Helpers
chipset. If so, register a struct cpufreq_driver with the CPUfreq core
using cpufreq_register_driver()
-What shall this struct cpufreq_driver contain?
+What shall this struct cpufreq_driver contain?
.name - The name of this driver.
cpufreq driver registers itself, the per-policy initialization function
cpufreq_driver.init is called if no cpufreq policy existed for the CPU.
Note that the .init() and .exit() routines are called only once for the
-policy and not for each CPU managed by the policy. It takes a struct
-cpufreq_policy *policy as argument. What to do now?
+policy and not for each CPU managed by the policy. 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/target_index is called
- with these values.
-policy->cpus Update this with the masks of the
- (online + offline) CPUs that do DVFS
- along with this CPU (i.e. that share
- clock/voltage rails with it).
++-----------------------------------+--------------------------------------+
+|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/target_index is|
+| | called with these values. |
++-----------------------------------+--------------------------------------+
+|policy->cpus | Update this with the masks of the |
+| | (online + offline) CPUs that do DVFS |
+| | along with this CPU (i.e. that share|
+| | clock/voltage rails with it). |
++-----------------------------------+--------------------------------------+
For setting some of these values (cpuinfo.min[max]_freq, policy->min[max]), the
frequency table helpers might be helpful. See the section 2 for more information
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 cpufreq_verify_within_limits(struct cpufreq_policy *policy,
-unsigned int min_freq, unsigned int max_freq) function might be helpful.
+values 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
1.4 target or target_index or setpolicy or fast_switch?
-------------------------------------------------------
-Most cpufreq drivers or even most cpu frequency scaling algorithms
+Most cpufreq drivers or even most cpu frequency scaling algorithms
only allow the CPU frequency to be set to predefined fixed values. For
these, you use the ->target(), ->target_index() or ->fast_switch()
callbacks.
1.5. target/target_index
------------------------
-The target_index call has two arguments: struct cpufreq_policy *policy,
-and unsigned int index (into the exposed frequency table).
+The target_index call has two arguments: ``struct cpufreq_policy *policy``,
+and ``unsigned int`` index (into the exposed frequency table).
The CPUfreq driver must set the new frequency when called here. The
actual frequency must be determined by freq_table[index].frequency.
It should always restore to earlier frequency (i.e. policy->restore_freq) in
case of errors, even if we switched to intermediate frequency earlier.
-Deprecated:
+Deprecated
----------
-The target call has three arguments: struct cpufreq_policy *policy,
+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
this callback isn't allowed. This callback must be highly optimized to
do switching as fast as possible.
-This function has two arguments: struct cpufreq_policy *policy and
-unsigned int target_frequency.
+This function has two arguments: ``struct cpufreq_policy *policy`` and
+``unsigned int target_frequency``.
1.7 setpolicy
-------------
-The setpolicy call only takes a struct cpufreq_policy *policy as
+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
cpufreq_for_each_valid_entry(pos, table) - iterates over all entries,
excluding CPUFREQ_ENTRY_INVALID frequencies.
-Use arguments "pos" - a cpufreq_frequency_table * as a loop cursor and
-"table" - the cpufreq_frequency_table * you want to iterate over.
+Use arguments "pos" - a ``cpufreq_frequency_table *`` as a loop cursor and
+"table" - the ``cpufreq_frequency_table *`` you want to iterate over.
-For example:
+For example::
struct cpufreq_frequency_table *pos, *driver_freq_table;
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