--- /dev/null
+.. SPDX-License-Identifier: GPL-2.0
+
+===============================================
+How to Implement a new CPUFreq Processor Driver
+===============================================
+
+Authors:
+
+
+ - Dominik Brodowski <linux@brodo.de>
+ - Rafael J. Wysocki <rafael.j.wysocki@intel.com>
+ - Viresh Kumar <viresh.kumar@linaro.org>
+
+.. 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?
+==============
+
+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?
+
+ .name - The name of this driver.
+
+ .init - A pointer to the per-policy initialization function.
+
+ .verify - A pointer to a "verification" function.
+
+ .setpolicy _or_ .fast_switch _or_ .target _or_ .target_index - See
+ below on the differences.
+
+And optionally
+
+ .flags - Hints for the cpufreq core.
+
+ .driver_data - cpufreq driver specific data.
+
+ .resolve_freq - Returns the most appropriate frequency for a target
+ frequency. Doesn't change the frequency though.
+
+ .get_intermediate and target_intermediate - Used to switch to stable
+ frequency while changing CPU frequency.
+
+ .get - Returns current frequency of the CPU.
+
+ .bios_limit - Returns HW/BIOS max frequency limitations for the CPU.
+
+ .exit - A pointer to a per-policy cleanup function called during
+ CPU_POST_DEAD phase of cpu hotplug process.
+
+ .stop_cpu - A pointer to a per-policy stop function called during
+ CPU_DOWN_PREPARE phase of cpu hotplug process.
+
+ .suspend - A pointer to a per-policy suspend function which is called
+ with interrupts disabled and _after_ the governor is stopped for the
+ policy.
+
+ .resume - A pointer to a per-policy resume function which is called
+ with interrupts disabled and _before_ the governor is started again.
+
+ .ready - A pointer to a per-policy ready function which is called after
+ the policy is fully initialized.
+
+ .attr - A pointer to a NULL-terminated list of "struct freq_attr" which
+ allow to export values to sysfs.
+
+ .boost_enabled - If set, boost frequencies are enabled.
+
+ .set_boost - A pointer to a per-policy function to enable/disable boost
+ frequencies.
+
+
+1.2 Per-CPU Initialization
+--------------------------
+
+Whenever a new CPU is registered with the device model, or after the
+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?
+
+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). |
++-----------------------------------+--------------------------------------+
+
+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
+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 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 target_index or setpolicy or fast_switch?
+-------------------------------------------------------
+
+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.
+
+Some cpufreq capable processors switch the frequency between certain
+limits on their own. These shall use the ->setpolicy() callback.
+
+
+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 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
+----------
+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 2
+for details.
+
+1.6. fast_switch
+----------------
+
+This function is used for frequency switching from scheduler's context.
+Not all drivers are expected to implement it, as sleeping from within
+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``.
+
+
+1.7 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 drivers/cpufreq/longrun.c
+
+1.8 get_intermediate and target_intermediate
+--------------------------------------------
+
+Only for drivers with target_index() and CPUFREQ_ASYNC_NOTIFICATION unset.
+
+get_intermediate should return a stable intermediate frequency platform wants to
+switch to, and target_intermediate() should set CPU to that frequency, before
+jumping to the frequency corresponding to 'index'. Core will take care of
+sending notifications and driver doesn't have to handle them in
+target_intermediate() or target_index().
+
+Drivers can return '0' from get_intermediate() in case they don't wish to switch
+to intermediate frequency for some target frequency. In that case core will
+directly call ->target_index().
+
+NOTE: ->target_index() should restore to policy->restore_freq in case of
+failures as core would send notifications for that.
+
+
+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_frequency_table entries, with driver specific
+values in "driver_data", the corresponding frequency in "frequency" and
+flags set. At the end of the table, you need to add a
+cpufreq_frequency_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 sorted in any
+particular order, but if they are cpufreq core will do DVFS a bit
+quickly for them as search for best match is faster.
+
+The cpufreq table is verified automatically by the core if the policy contains a
+valid pointer in its policy->freq_table field.
+
+cpufreq_frequency_table_verify() 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.
+
+cpufreq_frequency_table_target() is the corresponding frequency table
+helper for the ->target stage. Just pass the values to this function,
+and this function returns the of the frequency table entry which
+contains the frequency the CPU shall be set to.
+
+The following macros can be used as iterators over cpufreq_frequency_table:
+
+cpufreq_for_each_entry(pos, table) - iterates over all entries of frequency
+table.
+
+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.
+
+For example::
+
+ struct cpufreq_frequency_table *pos, *driver_freq_table;
+
+ cpufreq_for_each_entry(pos, driver_freq_table) {
+ /* Do something with pos */
+ pos->frequency = ...
+ }
+
+If you need to work with the position of pos within driver_freq_table,
+do not subtract the pointers, as it is quite costly. Instead, use the
+macros cpufreq_for_each_entry_idx() and cpufreq_for_each_valid_entry_idx().
projects / linux-2.6-block.git / blobdiff