OS: Android 7.1
Board: Firefly-RK3399
Kernel: v4.4.55

devfreq介紹：

rk3288平臺上, gpu和ddr有自己的一套dvfs機制，而在rk3399平臺，使用了系統的devfreq框架。

devfreq 是核心開發者定義的一套支援動態調整裝置頻率和電壓的的框架模型。它能有效的降
低該裝置的功耗,同時兼顧其效能。
devfreq 通過不同的變頻策略,選擇一個合適的頻率供裝置使用,目前的核心版本提供了以下
幾種策略:

• Simple Ondemand :根據負載動態調頻調壓;
• Userspace:使用者自己設定電壓和頻率,系統不會自動調整;
• Powersave:功耗優先,始終將頻率設定在最低值;
• Performance:效能優先,始終將頻率設定為最高值。

devfreq和cpufreq的功能類似，只是前者用於控制device的clock,後者用來控制cpu.

dts配置：
dmc, dynamic memory controller

    dmc: dmc {
        compatible = "rockchip,rk3399-dmc";
        devfreq-events = <&dfi>;
        interrupts = <GIC_SPI 1 IRQ_TYPE_LEVEL_HIGH 0
 
>;
        clocks = <&cru SCLK_DDRCLK>;
        clock-names = "dmc_clk";
        ddr_timing = <&ddr_timing>;
        status = "disabled";
    };

dfi,全稱不曉得，用於監控dmc的loading模組

    dfi: dfi@ff630000 {
        reg = <0x00 0xff630000 0x00 0x4000>;
        compatible = "rockchip,rk3399-dfi";
        rockchip,pmu = <&pmugrf>;
        clocks = <&cru PCLK_DDR_MON
 
>;
        clock-names = "pclk_ddr_mon";
        status = "disabled";
    };

ddr timing:

ddr_timing: ddr_timing {
        compatible = "rockchip,ddr-timing";
        ddr3_speed_bin = <21>;
        pd_idle = <0>;
        sr_idle = <0>;
        sr_mc_gate_idle = <0>;
        srpd_lite_idle  = <0>;
        standby_idle = <0>;
        dram_dll_dis_freq = <300>;
        phy_dll_dis_freq = <125>;

        ddr3_odt_dis_freq = <333>;
        ddr3_drv = <DDR3_DS_40ohm>;
        ddr3_odt = <DDR3_ODT_120ohm>;
        phy_ddr3_ca_drv = <PHY_DRV_ODT_40>;
        phy_ddr3_dq_drv = <PHY_DRV_ODT_40>;
        phy_ddr3_odt = <PHY_DRV_ODT_240>;

        lpddr3_odt_dis_freq = <333>;
        lpddr3_drv = <LP3_DS_34ohm>;
        lpddr3_odt = <LP3_ODT_240ohm>;
        phy_lpddr3_ca_drv = <PHY_DRV_ODT_40>;
        phy_lpddr3_dq_drv = <PHY_DRV_ODT_40>;
        phy_lpddr3_odt = <PHY_DRV_ODT_240>;

        lpddr4_odt_dis_freq = <333>;
        lpddr4_drv = <LP4_PDDS_60ohm>;
        lpddr4_dq_odt = <LP4_DQ_ODT_40ohm>;
        lpddr4_ca_odt = <LP4_CA_ODT_40ohm>;
        phy_lpddr4_ca_drv = <PHY_DRV_ODT_40>;
        phy_lpddr4_ck_cs_drv = <PHY_DRV_ODT_80>;
        phy_lpddr4_dq_drv = <PHY_DRV_ODT_80>;
        phy_lpddr4_odt = <PHY_DRV_ODT_60>;
    };

freq table:

    dmc_opp_table: opp-table3 {
        compatible = "operating-points-v2";

        [email protected]200000000 {
            opp-hz = /bits/ 64 <200000000>;
            opp-microvolt = <825000>;
        };
        [email protected]297000000 {
            opp-hz = /bits/ 64 <297000000>;
            opp-microvolt = <850000>;
        };
        [email protected]400000000 {
            opp-hz = /bits/ 64 <400000000>;
            opp-microvolt = <850000>;
        };
        [email protected]594000000 {
            opp-hz = /bits/ 64 <594000000>;
            opp-microvolt = <900000>;
        };
        [email protected]800000000 {
            opp-hz = /bits/ 64 <800000000>;
            opp-microvolt = <900000>;
        };
    };

驅動檔案：
目錄：
kernel/drivers/devfreq
檔案：
governor_simpleondemand.c：　策略控制
devfreq.c: devfreq framework
devfreq-event.c: 　event驅動，結合dfi
rockchip_dmc.c: rk dmc驅動
rockchip-dfi.c: rk dfi驅動

驅動流程：

• dmc註冊：

rockchip_dmcfreq_probe ->  rockchip_dmc.c
  devm_regulator_get(dev, "center"); 　//需要在dts中配置center-supply 屬性
  devfreq_event_get_edev_by_phandle //獲取dfi device
  devfreq_event_enable_edev //使能dfi監測
  of_get_ddr_timings　//獲取ddr timing
  rockchip_dmcfreq_init_freq_table //讀取scaling table表
  clk_get_rate(data->dmc_clk);　//獲取要設定的預設ddr頻率
  devm_devfreq_add_device　//新增到devfreq框架中，預設策略是　"simple_ondemand"。

核心在於profile結構：

static struct devfreq_dev_profile rockchip_devfreq_dmc_profile = {
    .polling_ms = 200, //loading查詢週期
    .target     = rockchip_dmcfreq_target,//設定電壓和頻率
    .get_dev_status = rockchip_dmcfreq_get_dev_status, //獲取loading
    .get_cur_freq   = rockchip_dmcfreq_get_cur_freq, //獲取當前頻率
};

• dfi註冊：

rockchip_dfi_probe -> rockchip-dfi.c
  devm_devfreq_event_add_edev //作為一個event device新增到devfreq框架中

核心在於event ops：

static const struct devfreq_event_ops rockchip_dfi_ops = {
    .disable = rockchip_dfi_disable,
    .enable = rockchip_dfi_enable,
    .get_event = rockchip_dfi_get_event,
    .set_event = rockchip_dfi_set_event,
};

• loading獲取及頻率設定：

系統會在polling_ms個週期也就是200ms讀取一次loading，然後決定是否需要調整電壓和頻率。

devfreq_simple_ondemand_handler -> governor_simpleondemand.c
  devfreq_monitor_start　-> devfreq.c
    queue_delayed_work -> 
      devfreq_monitor ->    
        update_devfreq -> 
          devfreq->governor->get_target_freq -> 
            devfreq_simple_ondemand_func -> governor_simpleondemand.c
              devfreq_update_stats ->
                df->profile->get_dev_status ->
                  rockchip_dmcfreq_get_dev_status -> rockchip_dmc.c
                    devfreq_event_get_event -> devfreq-event.c
                      edev->desc->ops->get_event ->
                        rockchip_dfi_get_event ->
                          rockchip_dfi_get_busier_ch //獲取dmc每個channel的busy情況，也就是loading
           devfreq->profile->get_cur_freq -> //獲取當前頻率
             rockchip_dmcfreq_get_cur_freq rockchip_dmc.c
           devfreq->profile->target ->　　//設定頻率和電壓
           　　rockchip_dmcfreq_target -> rockchip_dmc.c