我们来看看conservative governer。这个governer会在cpu loadding 超过向上阈值是一点一点的增加cpu freq.
如果低于向下的阈值，则会减小频率.
最重要的common_dbs_data如下。
358 static struct common_dbs_data cs_dbs_cdata = {
359 .governor = GOV_CONSERVATIVE,
360 .attr_group_gov_sys = &cs_attr_group_gov_sys,
361 .attr_group_gov_pol = &cs_attr_group_gov_pol,
362 .get_cpu_cdbs = get_cpu_cdbs,
363 .get_cpu_dbs_info_s = get_cpu_dbs_info_s,
364 .gov_dbs_timer = cs_dbs_timer,
365 .gov_check_cpu = cs_check_cpu,
366 .gov_ops = &cs_ops,
367 .init = cs_init,
368 .exit = cs_exit,
369 };
这个governer 和ondemand governer一样也是通过cs_dbs_timer中调用cs_check_cup 来切换频率.
105 static void cs_dbs_timer(struct work_struct *work)
106 {
107 struct cs_cpu_dbs_info_s *dbs_info = container_of(work,
108 struct cs_cpu_dbs_info_s, cdbs.work.work);
109 unsigned int cpu = dbs_info->cdbs.cur_policy->cpu;
110 struct cs_cpu_dbs_info_s *core_dbs_info = &per_cpu(cs_cpu_dbs_info,
111 cpu);
112 struct dbs_data *dbs_data = dbs_info->cdbs.cur_policy->governor_data;
113 struct cs_dbs_tuners *cs_tuners = dbs_data->tuners;
114 int delay = delay_for_sampling_rate(cs_tuners->sampling_rate);
115 bool modify_all = true;
116
117 mutex_lock(&core_dbs_info->cdbs.timer_mutex);
118 if (!need_load_eval(&core_dbs_info->cdbs, cs_tuners->sampling_rate))
119 modify_all = false;
120 else
121 dbs_check_cpu(dbs_data, cpu);
122
123 gov_queue_work(dbs_data, dbs_info->cdbs.cur_policy, delay, modify_all);
124 mutex_unlock(&core_dbs_info->cdbs.timer_mutex);
125 }
这个函数123行每过delay 时间运行cs_dbs_timer，121行调用dbs_check_cpu 来check cpu freq.
dbs_check_cpu最后会调用cs_check_cpu
47 static void cs_check_cpu(int cpu, unsigned int load)
48 {
49 struct cs_cpu_dbs_info_s *dbs_info = &per_cpu(cs_cpu_dbs_info, cpu);
50 struct cpufreq_policy *policy = dbs_info->cdbs.cur_policy;
51 struct dbs_data *dbs_data = policy->governor_data;
52 struct cs_dbs_tuners *cs_tuners = dbs_data->tuners;
53
54 /*
55 * break out if we 'cannot' reduce the speed as the user might
56 * want freq_step to be zero
57 */
58 if (cs_tuners->freq_step == 0)
59 return;
60
61 /* Check for frequency increase */
62 if (load > cs_tuners->up_threshold) {
63 dbs_info->down_skip = 0;
64
65 /* if we are already at full speed then break out early */
66 if (dbs_info->requested_freq == policy->max)
67 return;
68
69 dbs_info->requested_freq += get_freq_target(cs_tuners, policy);
70
71 if (dbs_info->requested_freq > policy->max)
72 dbs_info->requested_freq = policy->max;
73
74 __cpufreq_driver_target(policy, dbs_info->requested_freq,
75 CPUFREQ_RELATION_H);
76 return;
77 }
78
79 /* if sampling_down_factor is active break out early */
80 if (++dbs_info->down_skip < cs_tuners->sampling_down_factor)
81 return;
82 dbs_info->down_skip = 0;
83
84 /* Check for frequency decrease */
85 if (load < cs_tuners->down_threshold) {
86 unsigned int freq_target;
87 /*
88 * if we cannot reduce the frequency anymore, break out early
89 */
90 if (policy->cur == policy->min)
91 return;
92
93 freq_target = get_freq_target(cs_tuners, policy);
94 if (dbs_info->requested_freq > freq_target)
95 dbs_info->requested_freq -= freq_target;
96 else
97 dbs_info->requested_freq = policy->min;
98
99 __cpufreq_driver_target(policy, dbs_info->requested_freq,
100 CPUFREQ_RELATION_L);
101 return;
102 }
103 }
这个函数的63 行发现如果cpu loadding大于阈值的话，就69行增加freq，在74行将频率设定到cpu的寄存器中。
85行，如果loadding小于向下的阈值，95行减小频率，99行将减小后的频率设定的cpu寄存器中.
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