hrtimer:high-resolution kernel timers:

  hrtimers的誕生是由於核心開發者在使用過程中發現，原始的定時器kernel/timers.c,已經可以滿足所有場景的，但是在實際的大量測試中發現還是無法滿足所有場景，所以hrtimers就誕生了。這裡簡單介紹下關鍵結構體和一個應用場景，具體遠離下篇部落格再來分析。

1.結構體簡單介紹

1.struct hrtimer

這個是高精度物件的成員變數，核心註釋已經很詳細了，具體細節下篇來說，這裡重點關注的是function,這個是我們需要實現的方法。

/**
 * struct hrtimer - the
 
 basic hrtimer structure
 * @node:   timerqueue node, which also manages node.expires,
 *      the absolute expiry time in the hrtimers internal
 *      representation. The time is related to the clock on
 *      which the timer is based. Is setup by adding
 *      slack to the _softexpires value. For non range timers
 *      identical to
 
 _softexpires.
 * @_softexpires: the absolute earliest expiry time of the hrtimer.
 *      The time which was given as expiry time when the timer
 *      was armed.
 * @function:   timer expiry callback function
 * @base:   pointer to the timer base (per cpu and per clock)
 * @state:  state information (See bit values above
 
)
 * @start_pid: timer statistics field to store the pid of the task which
 *      started the timer
 * @start_site: timer statistics field to store the site where the timer
 *      was started
 * @start_comm: timer statistics field to store the name of the process which
 *      started the timer
 *
 * The hrtimer structure must be initialized by hrtimer_init()
 */
struct hrtimer {
    struct timerqueue_node      node;
    ktime_t             _softexpires;
    enum hrtimer_restart        (*function)(struct hrtimer *);
    struct hrtimer_clock_base   *base;
    unsigned long           state;
#ifdef CONFIG_TIMER_STATS
    int             start_pid;
    void                *start_site;
    char                start_comm[16];
#endif
};

2.ktime_t

這個用來表示時間，單位為納米。在啟動定時器之前，需要設定一個時間來決定何時呼叫回撥函式。

typedef union ktime ktime_t;

/*
 * ktime_t:
 *
 * A single 64-bit variable is used to store the hrtimers
 * internal representation of time values in scalar nanoseconds. The
 * design plays out best on 64-bit CPUs, where most conversions are
 * NOPs and most arithmetic ktime_t operations are plain arithmetic
 * operations.
 *
 */
union ktime {
    s64 tv64;
};

2.hrtimers應用場景

1).建立定時器物件&設定首次觸發時間

最近在做一個Camera的flash驅動，該camera需要pwm來控制亮度，而硬體上該GPIO口，沒有pwm功能，所以只能來模擬pwm了。

本地flash控制物件中巢狀的有hrtimer物件：

struct flash_gpio_ctrl {
    struct hrtimer *flash_timer;
    ktime_t kt;
    int flash_level;
    int IsFlash_stop;
    int loop_count;
    uint32_t gpio_enf;
    uint32_t gpio_enm; //pwm
};

建立定時器以及啟動定時器：

static void flash_pwm_start(void)
{
    hrtimer_init(flash_gpio_ctrl.flash_timer,CLOCK_MONOTONIC,HRTIMER_MODE_REL);
    flash_gpio_ctrl.flash_timer->function = hrtimer_handler;                                                                                                                    
    flash_gpio_ctrl.kt = ktime_set(0, 10000);
    hrtimer_start(flash_gpio_ctrl.flash_timer,flash_gpio_ctrl.kt,HRTIMER_MODE_REL);
    pr_err("timer_start");
}

• 1.hrtimer_init：這個是用來建立timer定時器物件，由核心封裝好了，具體細節先不用關係，只需要把定時器物件巢狀進我們定義的結構體中就行了。

• 2.function:這個就是回撥函式指標，這裡註冊回撥函式。

• 3.ktime_set(0, 10000)：設定首次觸發時間，這裡我設定了10ms。

• 4.hrtimer_start();這裡傳入定時器物件，和觸發時間，就開啟定時器了。

2).回撥函式預覽

static enum hrtimer_restart  hrtimer_handler(struct hrtimer *timer)
{
    flash_flag = !flash_flag;

    gpio_direction_output(flash_gpio_ctrl.gpio_enm, 0);
    if(flash_gpio_ctrl.loop_count != 1){
        if (flash_flag) {
            gpio_direction_output(flash_gpio_ctrl.gpio_enm, 1);
            flash_gpio_ctrl.kt = ktime_set(0, BASE_TIME * (flash_gpio_ctrl.flash_level) * 1000);
            hrtimer_forward_now(flash_gpio_ctrl.flash_timer, flash_gpio_ctrl.kt);
            //pr_err("pulse low level:%d",flash_gpio_ctrl.flash_level);
        } else {
            gpio_direction_output(flash_gpio_ctrl.gpio_enm, 0);
            flash_gpio_ctrl.kt = ktime_set(0, 50000 - (BASE_TIME * (flash_gpio_ctrl.flash_level) * 1000));
            hrtimer_forward_now(flash_gpio_ctrl.flash_timer, flash_gpio_ctrl.kt);
            //pr_err("pulse high levle");
        }
        flash_gpio_ctrl.loop_count--;
        pr_err("flash_timer out\n");
        return HRTIMER_RESTART;
    }else{
        flash_gpio_ctrl.loop_count--;
        return HRTIMER_NORESTART;
    }
}

• 1.loop_count:這裡我設定的是一個迴圈觸發的定時器，所以為了不讓定時器無休止的進行下去，設定一個觸發次數。次數減為0，則推出定時器。

• 2.HRTIMER_RESTART：這個返回值告訴定時器核心，這裡是定時器會重新觸發，而且上面又重新設定了觸發的時間。

• 3.hrtimer_forward_now：重新設定定時器觸發的時間。

至此定時器就已經執行起來了，每割一定時間都會觸發回撥函式。而且產生的pwm能使flash亮起來，但是由於佔空比不是很準確，所以flash亮度有些不穩定。(此外還有一種方法是啟動一個執行緒，來同步產生控制訊號)