1da177e4c3
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
216 lines
7.2 KiB
Text
216 lines
7.2 KiB
Text
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 (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 3
|
|
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 arch/i386/kernel/cpu/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.
|
|
|