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Diffstat (limited to 'Documentation/cpu-freq/cpu-drivers.txt')
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diff --git a/Documentation/cpu-freq/cpu-drivers.txt b/Documentation/cpu-freq/cpu-drivers.txt new file mode 100644 index 000000000000..43c743903dd7 --- /dev/null +++ b/Documentation/cpu-freq/cpu-drivers.txt @@ -0,0 +1,216 @@ + 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. + |