通過clk驅動框架可以看出，clk主要分core和hardware兩層，而core層的主要函式是clk_register，它是把clk註冊到系統中，而hardware層的註冊函式因種類而異，不過最終都會統一呼叫clk_register來註冊，本文只拿gate來舉例；另外本文介紹下操作clk的主要API的過程

1.clk_register

clk_register首先分配一個結構體clk，然後呼叫_clk_register來進行實際的註冊

1.1_clk_register

static int _clk_register(struct device *dev, struct
 
 clk_hw *hw, struct clk *clk)

這個函式主要根據hardware結構體clk_hw來對clk進行初始化：

clk->ops = hw->init->ops;
clk->hw = hw;
clk->flags = hw->init->flags;
clk->num_parents = hw->init->num_parents;

for (i = 0; i < clk->num_parents; i++) {
    clk->parent_names[i] = kstrdup(hw->init->parent_names[i],
                    GFP_KERNEL
 
);
}

接著呼叫__clk_init來進行初始化

1.2__clk_init

int __clk_init(struct device *dev, struct clk *clk)

下面根據程式碼分步驟來介紹下初始化過程：

• 檢查各個clk欄位是否符合要求
• 如果clk有多個parent，那麼分配一段記憶體儲存多個parent，並尋找現存系統中適配的parent
• 呼叫__clk_init_parent來找到parent，此步驟主要是用clk自己的ops->get_parent來尋找parent(上一步已經適配了parent，為什麼還要多此一舉，首先上一步有了條件是多個parent，另外上一步在適配parent的時候是利用的parent_names來做的，不能保證一定會尋找到)
• 把clk加入系統連結串列：clk有parent，那麼要加入clk->parent->children；如果clk沒有parent，那麼clk有標誌CLK_IS_ROOT，就加入到clk_root_list，否則加入到clk_orphan_list

static HLIST_HEAD(clk_root_list);--------clk的parent連結串列
static HLIST_HEAD(clk_orphan_list);------clk的孤兒連結串列

• 設定clk的rate：

  if (clk->ops->recalc_rate)
   clk->rate = clk->ops->recalc_rate(clk->hw,
           __clk_get_rate(clk->parent));
  else if (clk->parent)
   clk->rate = clk->parent->rate;
  else
   clk->rate = 0;
  

2clk_register_HARDWARE

2.1註冊函式

不同hardware的註冊函式如下：
(include/linux/clk-provider.h)

struct clk *clk_register_fixed_rate(struct device *dev, const char *name,
        const char *parent_name, unsigned long flags,
        unsigned long fixed_rate);

struct clk *clk_register_gate(struct device *dev, const char *name,
        const char *parent_name, unsigned long flags,
        void __iomem *reg, u8 bit_idx,
        u8 clk_gate_flags, spinlock_t *lock);

struct clk *clk_register_divider(struct device *dev, const char *name,
        const char *parent_name, unsigned long flags,
        void __iomem *reg, u8 shift, u8 width,
        u8 clk_divider_flags, spinlock_t *lock);

struct clk *clk_register_mux(struct device *dev, const char *name,
        const char **parent_names, u8 num_parents, unsigned long flags,
        void __iomem *reg, u8 shift, u8 width,
        u8 clk_mux_flags, spinlock_t *lock);


struct clk *clk_register_fixed_factor(struct device *dev, const char *name,
        const char *parent_name, unsigned long flags,
        unsigned int mult, unsigned int div);

struct clk *clk_register_composite(struct device *dev, const char *name,
        const char **parent_names, int num_parents,
        struct clk_hw *mux_hw, const struct clk_ops *mux_ops,
        struct clk_hw *rate_hw, const struct clk_ops *rate_ops,
        struct clk_hw *gate_hw, const struct clk_ops *gate_ops,
        unsigned long flags);

2.2clk_register_gate

struct clk *clk_register_gate(struct device *dev, const char *name,
        const char *parent_name, unsigned long flags,
        void __iomem *reg, u8 bit_idx,
        u8 clk_gate_flags, spinlock_t *lock)
{
    struct clk_gate *gate;
    struct clk *clk;
    struct clk_init_data init;

    if (clk_gate_flags & CLK_GATE_HIWORD_MASK) {
        if (bit_idx > 16) {
            pr_err("gate bit exceeds LOWORD field\n");
            return ERR_PTR(-EINVAL);
        }
    }

    /* allocate the gate */
    gate = kzalloc(sizeof(struct clk_gate), GFP_KERNEL);
    if (!gate) {
        pr_err("%s: could not allocate gated clk\n", __func__);
        return ERR_PTR(-ENOMEM);
    }

    init.name = name;
    init.ops = &clk_gate_ops;
    init.flags = flags | CLK_IS_BASIC;
    init.parent_names = (parent_name ? &parent_name: NULL);
    init.num_parents = (parent_name ? 1 : 0);

    /* struct clk_gate assignments */
    gate->reg = reg;
    gate->bit_idx = bit_idx;
    gate->flags = clk_gate_flags;
    gate->lock = lock;
    gate->hw.init = &init;

    clk = clk_register(dev, &gate->hw);

    if (IS_ERR(clk))
        kfree(gate);

    return clk;
}

從程式碼可以看出，不同的hardware註冊函式，主要是根據其需要分別初始化不同的clk_hw，最終都要呼叫clk_register來進行註冊

3幾個clk-API的執行流程

clk的API有許多，具體參考include/linux/clk.h，下面介紹兩個典型的：使能clk和設定clk-rate

3.1 clk-enable

int clk_enable(struct clk *clk)
{
    unsigned long flags;
    int ret;

    flags = clk_enable_lock();
    ret = __clk_enable(clk);
    clk_enable_unlock(flags);

    return ret;
}

static int __clk_enable(struct clk *clk)
{
    int ret = 0;

    if (!clk)
        return 0;

    if (WARN_ON(clk->prepare_count == 0))
        return -ESHUTDOWN;

    if (clk->enable_count == 0) {
        ret = __clk_enable(clk->parent);

        if (ret)
            return ret;

        if (clk->ops->enable) {
            ret = clk->ops->enable(clk->hw);--------最終呼叫ops->enable函式
            if (ret) {
                __clk_disable(clk->parent);
                return ret;
            }
        }
    }

    clk->enable_count++;
    return 0;
}

3.2 clk-set-rate

ref.

linux3.10