287 lines
11 KiB
ReStructuredText
287 lines
11 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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===============================================
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How to Implement a new CPUFreq Processor Driver
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===============================================
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Authors:
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- Dominik Brodowski <linux@brodo.de>
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- Rafael J. Wysocki <rafael.j.wysocki@intel.com>
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- Viresh Kumar <viresh.kumar@linaro.org>
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.. Contents
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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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.name - The name of this driver.
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.init - A pointer to the per-policy initialization function.
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.verify - A pointer to a "verification" function.
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.setpolicy _or_ .fast_switch _or_ .target _or_ .target_index - See
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below on the differences.
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And optionally
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.flags - Hints for the cpufreq core.
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.driver_data - cpufreq driver specific data.
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.get_intermediate and target_intermediate - Used to switch to stable
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frequency while changing CPU frequency.
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.get - Returns current frequency of the CPU.
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.bios_limit - Returns HW/BIOS max frequency limitations for the CPU.
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.exit - A pointer to a per-policy cleanup function called during
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CPU_POST_DEAD phase of cpu hotplug process.
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.suspend - A pointer to a per-policy suspend function which is called
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with interrupts disabled and _after_ the governor is stopped for the
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policy.
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.resume - A pointer to a per-policy resume function which is called
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with interrupts disabled and _before_ the governor is started again.
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.ready - A pointer to a per-policy ready function which is called after
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the policy is fully initialized.
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.attr - A pointer to a NULL-terminated list of "struct freq_attr" which
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allow to export values to sysfs.
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.boost_enabled - If set, boost frequencies are enabled.
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.set_boost - A pointer to a per-policy function to enable/disable boost
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frequencies.
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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-policy initialization function
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cpufreq_driver.init is called if no cpufreq policy existed for the CPU.
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Note that the .init() and .exit() routines are called only once for the
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policy and not for each CPU managed by the policy. It takes a ``struct
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cpufreq_policy *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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+-----------------------------------+--------------------------------------+
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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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+-----------------------------------+--------------------------------------+
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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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+-----------------------------------+--------------------------------------+
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|policy->cur | The current operating frequency of |
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| | this CPU (if appropriate) |
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+-----------------------------------+--------------------------------------+
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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|
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| | called with these values. |
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+-----------------------------------+--------------------------------------+
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|policy->cpus | Update this with the masks of the |
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| | (online + offline) CPUs that do DVFS |
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| | along with this CPU (i.e. that share|
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| | clock/voltage rails with it). |
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+-----------------------------------+--------------------------------------+
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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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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 cpufreq_verify_within_limits(``struct cpufreq_policy *policy``,
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``unsigned int min_freq``, ``unsigned int max_freq``) function might be helpful.
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See 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 target_index or setpolicy or fast_switch?
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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 frequency to be set to predefined fixed values. For
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these, you use the ->target(), ->target_index() or ->fast_switch()
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callbacks.
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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() callback.
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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. fast_switch
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----------------
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This function is used for frequency switching from scheduler's context.
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Not all drivers are expected to implement it, as sleeping from within
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this callback isn't allowed. This callback must be highly optimized to
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do switching as fast as possible.
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This function has two arguments: ``struct cpufreq_policy *policy`` and
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``unsigned int target_frequency``.
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1.7 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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1.8 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 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 of
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an array of struct cpufreq_frequency_table entries, with driver specific
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values in "driver_data", the corresponding frequency in "frequency" and
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flags set. 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.
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And 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 sorted in any
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particular order, but if they are cpufreq core will do DVFS a bit
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quickly for them as search for best match is faster.
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The cpufreq table is verified automatically by the core if the policy contains a
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valid pointer in its policy->freq_table field.
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cpufreq_frequency_table_verify() assures that at least one valid
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frequency is within policy->min and policy->max, and all other criteria
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are met. This is helpful for the ->verify call.
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cpufreq_frequency_table_target() is the corresponding frequency table
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helper for the ->target stage. Just pass the values to this function,
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and this function returns the of the frequency table entry which
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contains 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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If you need to work with the position of pos within driver_freq_table,
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do not subtract the pointers, as it is quite costly. Instead, use the
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macros cpufreq_for_each_entry_idx() and cpufreq_for_each_valid_entry_idx().
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