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d218ed7739
This routine can't fail unless the frequency table is invalid and doesn't contain any valid entries. Make it return the index and WARN() in case it is used for an invalid table. Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
273 lines
9.5 KiB
Plaintext
273 lines
9.5 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/target_index or setpolicy?
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1.5 target/target_index
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1.6 setpolicy
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1.7 get_intermediate and target_intermediate
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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.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/
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target_index - 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
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function called during CPU_POST_DEAD
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phase of cpu hotplug process.
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cpufreq_driver.stop_cpu - A pointer to a per-CPU stop function
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called during CPU_DOWN_PREPARE phase of
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cpu hotplug process.
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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/target_index 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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cpufreq_driver.get_intermediate
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and target_intermediate Used to switch to stable frequency while
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changing CPU frequency.
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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/target_index is called
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with these values.
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For setting some of these values (cpuinfo.min[max]_freq, policy->min[max]), the
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frequency table helpers might be helpful. See the section 2 for more information
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on them.
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SMP systems normally have same clock source for a group of cpus. For these the
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.init() would be called only once for the first online cpu. Here the .init()
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routine must initialize policy->cpus with mask of all possible cpus (Online +
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Offline) that share the clock. Then the core would copy this mask onto
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policy->related_cpus and will reset policy->cpus to carry only online cpus.
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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/target_index 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/target_index 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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1.5. target/target_index
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-------------
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The target_index call has two arguments: struct cpufreq_policy *policy,
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and unsigned int index (into the exposed frequency table).
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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 by freq_table[index].frequency.
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It should always restore to earlier frequency (i.e. policy->restore_freq) in
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case of errors, even if we switched to intermediate frequency earlier.
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Deprecated:
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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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1.6 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 drivers/cpufreq/longrun.c
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1.7 get_intermediate and target_intermediate
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--------------------------------------------
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Only for drivers with target_index() and CPUFREQ_ASYNC_NOTIFICATION unset.
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get_intermediate should return a stable intermediate frequency platform wants to
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switch to, and target_intermediate() should set CPU to to that frequency, before
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jumping to the frequency corresponding to 'index'. Core will take care of
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sending notifications and driver doesn't have to handle them in
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target_intermediate() or target_index().
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Drivers can return '0' from get_intermediate() in case they don't wish to switch
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to intermediate frequency for some target frequency. In that case core will
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directly call ->target_index().
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NOTE: ->target_index() should restore to policy->restore_freq in case of
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failures as core would send notifications for that.
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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_frequency_table entries, with any value in
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"driver_data" 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_frequency_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_table_validate_and_show(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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unsigned int target_freq,
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unsigned int relation);
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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 this function
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returns the number of the frequency table entry which contains
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the frequency the CPU shall be set to.
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The following macros can be used as iterators over cpufreq_frequency_table:
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cpufreq_for_each_entry(pos, table) - iterates over all entries of frequency
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table.
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cpufreq-for_each_valid_entry(pos, table) - iterates over all entries,
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excluding CPUFREQ_ENTRY_INVALID frequencies.
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Use arguments "pos" - a cpufreq_frequency_table * as a loop cursor and
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"table" - the cpufreq_frequency_table * you want to iterate over.
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For example:
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struct cpufreq_frequency_table *pos, *driver_freq_table;
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cpufreq_for_each_entry(pos, driver_freq_table) {
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/* Do something with pos */
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pos->frequency = ...
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}
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