217 lines
7.2 KiB
Plaintext
217 lines
7.2 KiB
Plaintext
CPU frequency and voltage scaling code in the Linux(TM) kernel
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L i n u x C P U F r e q
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C P U D r i v e r s
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- information for developers -
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Dominik Brodowski <linux@brodo.de>
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Clock scaling allows you to change the clock speed of the CPUs on the
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fly. This is a nice method to save battery power, because the lower
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the clock speed, the less power the CPU consumes.
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Contents:
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---------
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1. What To Do?
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1.1 Initialization
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1.2 Per-CPU Initialization
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1.3 verify
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1.4 target or setpolicy?
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1.5 target
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1.6 setpolicy
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2. Frequency Table Helpers
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1. What To Do?
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==============
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So, you just got a brand-new CPU / chipset with datasheets and want to
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add cpufreq support for this CPU / chipset? Great. Here are some hints
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on what is necessary:
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1.1 Initialization
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------------------
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First of all, in an __initcall level 7 (module_init()) or later
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function check whether this kernel runs on the right CPU and the right
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chipset. If so, register a struct cpufreq_driver with the CPUfreq core
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using cpufreq_register_driver()
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What shall this struct cpufreq_driver contain?
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cpufreq_driver.name - The name of this driver.
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cpufreq_driver.owner - THIS_MODULE;
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cpufreq_driver.init - A pointer to the per-CPU initialization
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function.
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cpufreq_driver.verify - A pointer to a "verification" function.
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cpufreq_driver.setpolicy _or_
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cpufreq_driver.target - See below on the differences.
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And optionally
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cpufreq_driver.exit - A pointer to a per-CPU cleanup function.
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cpufreq_driver.resume - A pointer to a per-CPU resume function
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which is called with interrupts disabled
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and _before_ the pre-suspend frequency
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and/or policy is restored by a call to
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->target or ->setpolicy.
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cpufreq_driver.attr - A pointer to a NULL-terminated list of
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"struct freq_attr" which allow to
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export values to sysfs.
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1.2 Per-CPU Initialization
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--------------------------
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Whenever a new CPU is registered with the device model, or after the
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cpufreq driver registers itself, the per-CPU initialization function
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cpufreq_driver.init is called. It takes a struct cpufreq_policy
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*policy as argument. What to do now?
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If necessary, activate the CPUfreq support on your CPU.
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Then, the driver must fill in the following values:
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policy->cpuinfo.min_freq _and_
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policy->cpuinfo.max_freq - the minimum and maximum frequency
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(in kHz) which is supported by
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this CPU
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policy->cpuinfo.transition_latency the time it takes on this CPU to
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switch between two frequencies in
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nanoseconds (if appropriate, else
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specify CPUFREQ_ETERNAL)
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policy->cur The current operating frequency of
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this CPU (if appropriate)
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policy->min,
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policy->max,
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policy->policy and, if necessary,
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policy->governor must contain the "default policy" for
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this CPU. A few moments later,
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cpufreq_driver.verify and either
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cpufreq_driver.setpolicy or
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cpufreq_driver.target is called with
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these values.
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For setting some of these values, the frequency table helpers might be
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helpful. See the section 2 for more information on them.
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1.3 verify
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------------
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When the user decides a new policy (consisting of
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"policy,governor,min,max") shall be set, this policy must be validated
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so that incompatible values can be corrected. For verifying these
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values, a frequency table helper and/or the
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cpufreq_verify_within_limits(struct cpufreq_policy *policy, unsigned
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int min_freq, unsigned int max_freq) function might be helpful. See
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section 2 for details on frequency table helpers.
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You need to make sure that at least one valid frequency (or operating
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range) is within policy->min and policy->max. If necessary, increase
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policy->max first, and only if this is no solution, decrease policy->min.
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1.4 target or setpolicy?
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----------------------------
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Most cpufreq drivers or even most cpu frequency scaling algorithms
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only allow the CPU to be set to one frequency. For these, you use the
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->target call.
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Some cpufreq-capable processors switch the frequency between certain
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limits on their own. These shall use the ->setpolicy call
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1.4. target
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-------------
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The target call has three arguments: struct cpufreq_policy *policy,
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unsigned int target_frequency, unsigned int relation.
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The CPUfreq driver must set the new frequency when called here. The
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actual frequency must be determined using the following rules:
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- keep close to "target_freq"
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- policy->min <= new_freq <= policy->max (THIS MUST BE VALID!!!)
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- if relation==CPUFREQ_REL_L, try to select a new_freq higher than or equal
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target_freq. ("L for lowest, but no lower than")
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- if relation==CPUFREQ_REL_H, try to select a new_freq lower than or equal
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target_freq. ("H for highest, but no higher than")
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Here again the frequency table helper might assist you - see section 2
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for details.
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1.5 setpolicy
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---------------
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The setpolicy call only takes a struct cpufreq_policy *policy as
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argument. You need to set the lower limit of the in-processor or
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in-chipset dynamic frequency switching to policy->min, the upper limit
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to policy->max, and -if supported- select a performance-oriented
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setting when policy->policy is CPUFREQ_POLICY_PERFORMANCE, and a
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powersaving-oriented setting when CPUFREQ_POLICY_POWERSAVE. Also check
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the reference implementation in arch/i386/kernel/cpu/cpufreq/longrun.c
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2. Frequency Table Helpers
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==========================
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As most cpufreq processors only allow for being set to a few specific
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frequencies, a "frequency table" with some functions might assist in
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some work of the processor driver. Such a "frequency table" consists
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of an array of struct cpufreq_freq_table entries, with any value in
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"index" you want to use, and the corresponding frequency in
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"frequency". At the end of the table, you need to add a
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cpufreq_freq_table entry with frequency set to CPUFREQ_TABLE_END. And
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if you want to skip one entry in the table, set the frequency to
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CPUFREQ_ENTRY_INVALID. The entries don't need to be in ascending
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order.
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By calling cpufreq_frequency_table_cpuinfo(struct cpufreq_policy *policy,
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struct cpufreq_frequency_table *table);
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the cpuinfo.min_freq and cpuinfo.max_freq values are detected, and
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policy->min and policy->max are set to the same values. This is
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helpful for the per-CPU initialization stage.
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int cpufreq_frequency_table_verify(struct cpufreq_policy *policy,
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struct cpufreq_frequency_table *table);
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assures that at least one valid frequency is within policy->min and
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policy->max, and all other criteria are met. This is helpful for the
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->verify call.
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int cpufreq_frequency_table_target(struct cpufreq_policy *policy,
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struct cpufreq_frequency_table *table,
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unsigned int target_freq,
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unsigned int relation,
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unsigned int *index);
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is the corresponding frequency table helper for the ->target
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stage. Just pass the values to this function, and the unsigned int
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index returns the number of the frequency table entry which contains
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the frequency the CPU shall be set to. PLEASE NOTE: This is not the
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"index" which is in this cpufreq_table_entry.index, but instead
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cpufreq_table[index]. So, the new frequency is
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cpufreq_table[index].frequency, and the value you stored into the
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frequency table "index" field is
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cpufreq_table[index].index.
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