CPU频率调节模式以及降频方法简介

在kernel中实现了两个driver，一个负责控制开关CPU的核数，叫做hot-plug驱动，另一个负责调整CPU的频率，叫做DVFS驱动。kernel中的driver会根据系统的负载，自动调整使用几个CPU和调整CPU频率。如果负载高了，提高频率，或者多开几个核，或者开大核。如果负载降下去了，就可以关大核，关核，降频。

以下是两个知名手机厂商开源的config配置文档：

60 CONFIG_CPU_FREQ=y                                                | 446 CONFIG_CPU_FREQ=y61 CONFIG_CPU_FREQ_STAT_DETAILS=y                                   | 447 CONFIG_CPU_FREQ_STAT=y62 CONFIG_CPU_FREQ_DEFAULT_GOV_INTERACTIVE=y                        | 448 CONFIG_CPU_FREQ_STAT_DETAILS=y63 CONFIG_CPU_FREQ_GOV_PERFORMANCE=y                                | 449 # CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE is not set64 CONFIG_CPU_FREQ_GOV_POWERSAVE=y                                  | 450 # CONFIG_CPU_FREQ_DEFAULT_GOV_POWERSAVE is not set                                        65 CONFIG_CPU_FREQ_GOV_USERSPACE=y                                  | 451 # CONFIG_CPU_FREQ_DEFAULT_GOV_USERSPACE is not set66 CONFIG_CPU_FREQ_GOV_ONDEMAND=y  # CONFIG_CPU_FREQ_DEFAULT_GOV_ONDEMAND is not set67 CONFIG_NET=y                                                     | 453 # CONFIG_CPU_FREQ_DEFAULT_GOV_CONSERVATIVE is not set68 CONFIG_PACKET=y                                                  | 454 CONFIG_CPU_FREQ_DEFAULT_GOV_INTERACTIVE=y69 CONFIG_UNIX=y                                                    | 455 # CONFIG_CPU_FREQ_GOV_PERFORMANCE is not set70 CONFIG_XFRM_MIGRATE=y                                            | 456 # CONFIG_CPU_FREQ_GOV_POWERSAVE is not set71 CONFIG_NET_KEY=y                                                 | 457 # CONFIG_CPU_FREQ_GOV_USERSPACE is not set72 CONFIG_INET=y                                                    | 458 # CONFIG_CPU_FREQ_GOV_ONDEMAND is not set73 CONFIG_IP_MULTICAST=y                                            | 459 CONFIG_CPU_FREQ_GOV_INTERACTIVE=y74 CONFIG_IP_ADVANCED_ROUTER=y                                      | 460 # CONFIG_CPU_FREQ_GOV_CONSERVATIVE is not set75 CONFIG_IP_MULTIPLE_TABLES=y                                      | 461 # CONFIG_CPUFREQ_DT is not set

在adb shell命令下，和cpu频率相关的目录：
/sys/devices/system/cpu/cpuX， X表示cpu number.
:/sys/devices/system/cpu/cpu0/cpufreq # ls
affected_cpus related_cpus scaling_governor
cpuinfo_cur_freq scaling_available_frequencies scaling_max_freq
cpuinfo_max_freq scaling_available_governors scaling_min_freq
cpuinfo_min_freq scaling_cur_freq scaling_setspeed
cpuinfo_transition_latency scaling_driver stats

这些文件节点的读写属性：

cpufreq_freq_attr_ro_perm(cpuinfo_cur_freq, 0400);936 cpufreq_freq_attr_ro(cpuinfo_min_freq);937 cpufreq_freq_attr_ro(cpuinfo_max_freq);938 cpufreq_freq_attr_ro(cpuinfo_transition_latency);939 cpufreq_freq_attr_ro(scaling_available_governors);940 cpufreq_freq_attr_ro(scaling_driver);941 cpufreq_freq_attr_ro(scaling_cur_freq);942 cpufreq_freq_attr_ro(bios_limit);943 cpufreq_freq_attr_ro(related_cpus);944 cpufreq_freq_attr_ro(affected_cpus);945 cpufreq_freq_attr_rw(scaling_min_freq);946 cpufreq_freq_attr_rw(scaling_max_freq);                                                                                                                                                   947 cpufreq_freq_attr_rw(scaling_governor);948 cpufreq_freq_attr_rw(scaling_setspeed);

cpuinfo_cur_freq：当前cpu正在运行的工作频率
cpuinfo_max_freq：该文件指定了处理器能够运行的最高工作频率（单位: 千赫兹）
cpuinfo_min_freq ：该文件指定了处理器能够运行的最低工作频率（单位: 千赫兹）
cpuinfo_transition_latency：该文件定义了处理器在两个不同频率之间切换时所需要的时间（单位：纳秒）
scaling_available_frequencies：所有支持的主频率列表（单位: 千赫兹）
scaling_available_governors：该文件显示当前内核中支持的所有cpufreq governor类型
scaling_cur_freq：被governor和cpufreq核决定的当前CPU工作频率。该频率是内核认为该CPU当前运行的主频率
scaling_driver：该文件显示该CPU正在使用何种cpufreq driver
scaling_governor：通过echo命令，能够改变当前处理器的governor类型
scaling_max_freq：显示当前policy的上下限（单位: 千赫兹）需要注意的是，当改变cpu policy时，需要首先设置scaling_max_freq, 然后才是scaling_min_freq

scaling_setspeed：如果用户选择了“userspace” governor, 那么可以设置cpu工作主频率到某一个指定值。只需要这个值在scaling_min_freq 和 scaling_max_freq之间即可。

工作模式：cat scaling_available_governors
:/sys/devices/system/cpu/cpu0/cpufreq # cat scaling_available_governors
ondemand userspace powersave interactive performance
CPU的频率调节模式：
1. Performance. 不考虑耗电，只用最高频率。
2. Interactive. 直接上最高频率，然后看CPU负荷慢慢降低。
3. Powersave. 通常以最低频率运行，流畅度会受影响，一般不会用这个吧！
4. Userspace. 可以在用户空间手动调节频率。
5. Ondemand. 定期检查负载，根据负载来调节频率。

对于这项飞思卡尔的实现：
默认使用了performance,不过freescale在boot完成后改成了interactive.
device/fsl/tek_mx6/init.rc:
on property:sys.boot_completed=1
# Set default CPU frequency governor
# Set timer 40ms, min sample 60ms,hispeed at cpufreq MAX freq in freq_table at load 40%
write /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor interactive
最终可通过scaling_governor文件查看。

工作频率：
当前支持的cpu调节模式可通过 scaling_available_frequencies 查看。
:/sys/devices/system/cpu/cpu0/cpufreq # cat scaling_available_frequencies
1092000 988000 858000 793000 637000 494000 364000 221000

相关的数据结构：
kernel-4.4$ vi ./include/linux/cpufreq.h +59

struct cpufreq_policy {/* CPUs sharing clock, require sw coordination */cpumask_var_t        cpus;   /* Online CPUs only */cpumask_var_t     related_cpus; /* Online + Offline CPUs */cpumask_var_t     real_cpus; /* Related and present */unsigned int        shared_type; /* ACPI: ANY or ALL affected CPUsshould set cpufreq */unsigned int     cpu;    /* cpu managing this policy, must be online */struct clk        *clk;struct cpufreq_cpuinfo cpuinfo;/* see above */unsigned int     min;    /* in kHz */unsigned int        max;    /* in kHz */unsigned int        cur;    /* in kHz, only needed if cpufreq* governors are used */unsigned int        restore_freq; /* = policy->cur before transition */unsigned int     suspend_freq; /* freq to set during suspend */unsigned int      policy; /* see above */unsigned int     last_policy; /* policy before unplug */struct cpufreq_governor  *governor; /* see below */void          *governor_data;bool         governor_enabled; /* governor start/stop flag */char            last_governor[CPUFREQ_NAME_LEN]; /* last governor used */struct work_struct update; /* if update_policy() needs to be* called, but you're in IRQ context */struct cpufreq_user_policy user_policy;struct cpufreq_frequency_table   *freq_table;struct list_head        policy_list;struct kobject      kobj;struct completion  kobj_unregister;/** The rules for this semaphore:* - Any routine that wants to read from the policy structure will*   do a down_read on this semaphore.* - Any routine that will write to the policy structure and/or may take away*   the policy altogether (eg. CPU hotplug), will hold this lock in write*   mode before doing so.** Additional rules:* - Lock should not be held across*     __cpufreq_governor(data, CPUFREQ_GOV_POLICY_EXIT);*/struct rw_semaphore    rwsem;/** Fast switch flags:* - fast_switch_possible should be set by the driver if it can*   guarantee that frequency can be changed on any CPU sharing the*   policy and that the change will affect all of the policy CPUs then.* - fast_switch_enabled is to be set by governors that support fast*   freqnency switching with the help of cpufreq_enable_fast_switch().*/bool                    fast_switch_possible;bool                    fast_switch_enabled;/* Cached frequency lookup from cpufreq_driver_resolve_freq. */unsigned int cached_target_freq;int cached_resolved_idx;/* Synchronization for frequency transitions */bool           transition_ongoing; /* Tracks transition status */spinlock_t        transition_lock;wait_queue_head_t   transition_wait;struct task_struct  *transition_task; /* Task which is doing the transition *//* cpufreq-stats */struct cpufreq_stats   *stats;/* For cpufreq driver's internal use */void         *driver_data;
};

根据平台以及默认的最大频率来选择对应的频率列表。
所以降频有两种方法：
1. 直接编译静态修改频率列表。
2. 通过scaling_max_freq文件动态写入。

/ # cat /proc/cpufreq/cpufreq_ptpod_freq_volt
cat /proc/cpufreq/cpufreq_ptpod_freq_volt
[0] = { .cpufreq_khz = 1092000, .cpufreq_volt = 119375, .cpufreq_volt_org = 125000, },
[1] = { .cpufreq_khz = 988000, .cpufreq_volt = 114375, .cpufreq_volt_org = 121875, },
[2] = { .cpufreq_khz = 858000, .cpufreq_volt = 108125, .cpufreq_volt_org = 118750, },
[3] = { .cpufreq_khz = 793000, .cpufreq_volt = 105000, .cpufreq_volt_org = 115000, },
[4] = { .cpufreq_khz = 637000, .cpufreq_volt = 101250, .cpufreq_volt_org = 110000, },
[5] = { .cpufreq_khz = 494000, .cpufreq_volt = 101250, .cpufreq_volt_org = 105000, },
[6] = { .cpufreq_khz = 364000, .cpufreq_volt = 101250, .cpufreq_volt_org = 100000, },
[7] = { .cpufreq_khz = 221000, .cpufreq_volt = 101250, .cpufreq_volt_org = 95000, },
/ # cat proc/cpufreq/cpufreq_oppidx
cat proc/cpufreq/cpufreq_oppidx
[MT_CPU_DVFS_LITTLE/0]
cpufreq_oppidx = 1
OP(1092000, 119375),
OP(988000, 114375),
OP(858000, 108125),
OP(793000, 105000),
OP(637000, 101250),
OP(494000, 101250),
OP(364000, 101250),
OP(221000, 101250),

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