关键词：时钟、PLL、Mux、Divider、Gate、clk_summary等。
时钟和电源是各种设备的基础设施，整个时钟框架可以抽象为几种基本的元器件：负责提供晶振
Linux内核提供了良好的CCF(Common Clock Framework)，框架的两端一个是provider，一个是consumer。
provider指的是提供时钟模块，包括晶振、PLL、Mux、Divider、Gate等，consumer指的是使用这些时钟的模块。
1. Linux时钟框架基础
相关文档对时钟框架做了详细的介绍：《Linux common clock framework(1)_概述》、《Linux common clock framework(2)_clock provider》、《Linux common clock framework(3)_实现逻辑分析》以及《Common Clock Framework系统结构》。
这里简单罗列一下相关知识。
1.1 编写时钟provider驱动
provider包含基本硬件元素：Oscillator/Crystal-提供时钟晶振、PLL-倍频、Mux-多路选择、Divider-分频器、Gate-控制开关，还有Fixed-Divider-固定分频器。
这些硬件都可以抽象成一种类型的时钟，所有类型的时钟都可以通过struct clk_hw描述。
struct clk_hw { struct clk_core *core; struct clk *clk; const struct clk_init_data *init; }; struct clk_core { const char *name; const struct clk_ops *ops; struct clk_hw *hw; struct module *owner; struct clk_core *parent; const char **parent_names; struct clk_core **parents; u8 num_parents; u8 new_parent_index; unsigned long rate; unsigned long req_rate; unsigned long new_rate; struct clk_core *new_parent; struct clk_core *new_child; unsigned long flags; bool orphan; unsigned int enable_count; unsigned int prepare_count; unsigned long min_rate; unsigned long max_rate; unsigned long accuracy; int phase; struct hlist_head children; struct hlist_node child_node; struct hlist_head clks; unsigned int notifier_count; #ifdef CONFIG_DEBUG_FS struct dentry *dentry; struct hlist_node debug_node; #endif struct kref ref; }; struct clk_init_data { const char *name; const struct clk_ops *ops; const char * const *parent_names; u8 num_parents; unsigned long flags; };
struct clk_ops { int (*prepare)(struct clk_hw *hw); void (*unprepare)(struct clk_hw *hw); int (*is_prepared)(struct clk_hw *hw); void (*unprepare_unused)(struct clk_hw *hw); int (*enable)(struct clk_hw *hw); void (*disable)(struct clk_hw *hw); int (*is_enabled)(struct clk_hw *hw); void (*disable_unused)(struct clk_hw *hw); unsigned long (*recalc_rate)(struct clk_hw *hw, unsigned long parent_rate); long (*round_rate)(struct clk_hw *hw, unsigned long rate, unsigned long *parent_rate); int (*determine_rate)(struct clk_hw *hw, struct clk_rate_request *req); int (*set_parent)(struct clk_hw *hw, u8 index); u8 (*get_parent)(struct clk_hw *hw); int (*set_rate)(struct clk_hw *hw, unsigned long rate, unsigned long parent_rate); int (*set_rate_and_parent)(struct clk_hw *hw, unsigned long rate, unsigned long parent_rate, u8 index); unsigned long (*recalc_accuracy)(struct clk_hw *hw, unsigned long parent_accuracy); int (*get_phase)(struct clk_hw *hw); int (*set_phase)(struct clk_hw *hw, int degrees); void (*init)(struct clk_hw *hw); int (*debug_init)(struct clk_hw *hw, struct dentry *dentry); };
clk_register()将描述时钟的struct clk_hw注册，转化成strcut clk变量。
但在实际使用中，对不同类型的时钟往往调用其对应的封装函数。
对于上面提到的硬件在下面都能找到对应的注册函数，其中包括一个composite设备作为一个组合注册。
struct clk *clk_register(struct device *dev, struct clk_hw *hw) int clk_hw_register(struct device *dev, struct clk_hw *hw)
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 * const *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_fractional_divider(struct device *dev, const char *name, const char *parent_name, unsigned long flags, void __iomem *reg, u8 mshift, u8 mwidth, u8 nshift, u8 nwidth, u8 clk_divider_flags, spinlock_t *lock); struct clk *clk_register_composite(struct device *dev, const char *name, const char * const *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);
最后调用of_clk_add_provider()将注册的时钟加入到OF框架中。
int of_clk_add_provider(struct device_node *np, struct clk *(*clk_src_get)(struct of_phandle_args *args, void *data), void *data);
1.2 consumer使用时钟
其他设备需要使用时钟，可以再驱动中后去时钟也可以在设备DTS中引用时钟。
struct clk *clk_get(struct device *dev, const char *id); struct clk *devm_clk_get(struct device *dev, const char *id); int clk_enable(struct clk *clk); void clk_disable(struct clk *clk); unsigned long clk_get_rate(struct clk *clk); void clk_put(struct clk *clk); void devm_clk_put(struct device *dev, struct clk *clk); long clk_round_rate(struct clk *clk, unsigned long rate); int clk_set_rate(struct clk *clk, unsigned long rate); bool clk_has_parent(struct clk *clk, struct clk *parent); int clk_set_rate_range(struct clk *clk, unsigned long min, unsigned long max); int clk_set_min_rate(struct clk *clk, unsigned long rate); int clk_set_max_rate(struct clk *clk, unsigned long rate); int clk_set_parent(struct clk *clk, struct clk *parent); struct clk *clk_get_parent(struct clk *clk); struct clk *clk_get_sys(const char *dev_id, const char *con_id); int clk_prepare(struct clk *clk); void clk_unprepare(struct clk *clk); static inline int clk_prepare_enable(struct clk *clk) static inline void clk_disable_unprepare(struct clk *clk)
struct clk *of_clk_get(struct device_node *np, int index); struct clk *of_clk_get_by_name(struct device_node *np, const char *name); struct clk *of_clk_get_from_provider(struct of_phandle_args *clkspec);
2. 如何实现一款芯片的时钟框架
对一款芯片配置时钟框架，首先拿到时钟框架图，上面会有详细的Mux关系、是否有Divider、是否是Fixed Divider、是否有gate等等。
将这些器件找到对应的Linux时钟框架抽象，将整张时钟框架图抽象成Linux时钟框架识别的属性结构。
然后还需要每一个器件的寄存器解释。
在有了这些准备工作之后，工作氛围两部分：编写器件抽象驱动，比如Fixed clock、Gate、Divider等；按照时钟框架图编写DTS文件，寄存器参照规格书，compatible和驱动对应。
2.1 编写类型时钟驱动
首先通过CLK_OF_DECLARE()将字符串和xx2000_divider_setup()进行关联，然后在xx2000_divider_setup进行时钟的注册。
static void xx2000_divider_setup(struct device_node *node) { void __iomem *reg; struct resource res; struct clk *clk; unsigned int bit_shift = 0, bit_width = 0; const char *clk_name = NULL; const char *parent_name; int ret = 0; if(!node) return; reg = of_io_request_and_map(node, 0, of_node_full_name(node));-----------------------------------将寄存器映射，后续对divider的设置以及读取都需要此寄存器。 if(IS_ERR(reg)) { pr_err("%s <%s> must have a reg property.\n", __func__, node->name); return; } if(of_property_read_u32(node, "bit-shift", &bit_shift)) {----------------------------------------操作divider需要知道配置divider的位偏移及位宽。然后根据频率选择divider的值，设置到寄存器中。获取时钟频率也通过读取寄存器值进行计算。 pr_err("%s <%s> must have a bit-shift property.\n", __func__, node->name); goto err_unmap; } if(of_property_read_u32(node, "bit-width", &bit_width)) { pr_err("%s <%s> must have a bit-width property.\n", __func__, node->name); goto err_unmap; } parent_name = of_clk_get_parent_name(node, 0);----------------------------------------------------获取父时钟名称。 if(!parent_name) { pr_err("%s <%s> must have a parent.\n", __func__, node->name); goto err_unmap; } of_property_read_string(node, "clock-output-names", &clk_name); clk = clk_register_divider(NULL, clk_name, parent_name, 0, reg, bit_shift, bit_width, 0, NULL);---注册divider时钟，必须要有的参数有reg、bit_shift、bit_width，以及本身的名称。 if(IS_ERR(clk)) { pr_err("%s Failed to register <%s>.\n", __func__, node->name); goto err_unmap; } ret = of_clk_add_provider(node, of_clk_src_simple_get, clk);--------------------------------------将注册的时钟加入到OF框架。 if(ret) { pr_err("%s Failed to add <%s>.\n", __func__, node->name); goto err_unregister; } return; err_unregister: clk_unregister_divider(clk); err_unmap: iounmap(reg); of_address_to_resource(node, 0, &res); release_mem_region(res.start, resource_size(&res)); return; } CLK_OF_DECLARE(xx2000_clk_divider, "xx2000,clk-divider", xx2000_divider_setup);
2.2 编写DTS文件
有了上面的时钟框架图、时钟寄存器规格书和驱动，就可以按部就班的按照时钟框架图一步一步编写DTS。
- 编写fixed clock的晶振、PLL等；
- 编写多路复用Mux和分频器Divider，需要配置寄存器以及寄存器的bit-shift和bit-width。
具体的DTS配置，参考如下：
cpu_core_clk: cpu-core-clk {---------------------------------cpu_core_clk是在其他设备中clocks指向的名称。
#clock-cells = <0>;--------------------------------------0表示只有一个输出，1表示多余一个输出。
compatible = "xx2000,clk-divider";-----------------------如果有特殊需求，还需要编写自己的驱动。这里通过此字符串进行匹配。
reg = ;---------------------------------配置此事中的寄存器地址以及大小。
bit-shift = <0>;-----------------------------------------对于divider类型需要知道配置bit在寄存器中的偏移以及bit位宽。
bit-width = <5>; clocks = <&cpu_mux 0>;-----------------------------------clocks指向父时钟。 clock-output-names = "cpu_core_clk";---------------------本时钟输出名称，在consumer时钟中可以使用此名称来获得该时钟的struct clk结构体。 };
3. 对时钟框架进行验证
3.1 clk_summary验证时钟树
通过读取/sys/kernel/debug/clk/clk_summary信息，和时钟框图对照，可以验证DTS配置正确与否。
clock enable prepare_cnt rate accuracy phase ---------------------------------------------------------------------------------------- ddr_pll 0 0 1200000000 0 0 nn_pll 0 0 750000000 0 0 video_pll 0 0 1100000000 0 0 sdio0_mux 0 0 1100000000 0 0 sdio0_cclk_divider 0 0 39285715 0 0 sdio0_cclk 0 0 39285715 0 0... cpu_pll 0 0 1000000000 0 0 cpu_mux 0 0 1000000000 0 0 cpu_core_clk 0 0 1000000000 0 0 cpu_bus_clk 0 0 500000000 0 0 cpu_apb_clk 0 0 250000000 0 0 ddr_cpu_port_clk 0 0 500000000 0 0 rtc_clk 0 0 32768 0 0 tsen_mux 0 0 32768 0 0 tsen_clk 0 0 32768 0 0 ref_clk 0 0 24000000 0 0 wdt_clk 0 0 24000000 0 0 timer3_clk 0 0 24000000 0 0 timer2_clk 0 0 24000000 0 0 timer1_clk 0 0 24000000 0 0 timer0_clk 0 0 24000000 0 0 ref_clk_750_fixed_factor 0 0 32000 0 0 usb_suspend_clk 0 0 32000 0 0
3.2 验证时钟实际输出
在/sys/kernel/debug/clk目录下，每个时钟都有自己的目录。
在clk_debug_create_one()函数中，对divider和gate类型时钟创建相应的节点用于控制硬件。
static int clk_debug_create_one(struct clk_core *core, struct dentry *pdentry) { struct dentry *d; @@ -2182,6 +2290,7 @@ static int clk_debug_create_one(struct clk_core *core, struct dentry *pdentry) if (ret) goto err_out; } + xx2000_clk_create(core); ret = 0; goto out;
下面根据struct clk_core所对应的struct clk_ops来判断时钟的类型，gate创建xx2000_gate，divider创建xx2000_rate节点。
static ssize_t xx2000_gate_read(struct file *filp, char __user *buffer, size_t count, loff_t *ppos) { struct clk_core *pdata = filp->private_data; unsigned int value; char tmp[32]; size_t size; value = __clk_is_enabled(pdata->hw->clk); size = sprintf(tmp, "%u\n", value); printk("%s value=%u\n", __func__, value); return simple_read_from_buffer(buffer, count, ppos, tmp, size); } static ssize_t xx2000_gate_write(struct file *filp, const char __user *buffer, size_t count, loff_t *ppos) { struct clk_core *pdata = filp->private_data; unsigned int value; int ret = 0; ret = kstrtouint_from_user(buffer, count, 0, &value); if (ret) return -EFAULT; printk("%s name=%s value=%u\n", __func__, pdata->name, value); if(value) clk_prepare_enable(pdata->hw->clk); else clk_disable_unprepare(pdata->hw->clk); return count; } static const struct file_operations xx2000_gate_ops = { .owner = THIS_MODULE, .open = simple_open, .read = xx2000_gate_read, .write = xx2000_gate_write, .release = single_release, }; static ssize_t xx2000_rate_read(struct file *filp, char __user *buffer, size_t count, loff_t *ppos) { struct clk_core *pdata = filp->private_data; unsigned long rate; char tmp[32]; size_t size; rate = clk_get_rate(pdata->hw->clk); size = sprintf(tmp, "%lu\n", rate); printk("%s value=%lu\n", __func__, rate); return simple_read_from_buffer(buffer, count, ppos, tmp, size); } static ssize_t xx2000_rate_write(struct file *filp, const char __user *buffer, size_t count, loff_t *ppos) { struct clk_core *pdata = filp->private_data; unsigned int rate; int ret = 0; ret = kstrtouint_from_user(buffer, count, 0, &rate); if (ret) return -EFAULT; printk("%s value=%u\n", __func__, rate); if(rate) clk_set_rate(pdata->hw->clk, rate); return count; } static const struct file_operations xx2000_rate_ops = { .owner = THIS_MODULE, .open = simple_open, .read = xx2000_rate_read, .write = xx2000_rate_write, .release = single_release, }; void xx2000_clk_create(struct clk_core *core) { const struct clk_ops *clk_ops = core->ops; //printk("%s %s %p %p %p %p\n", __func__, core->name ,clk_ops, &clk_gate_ops, &clk_mux_ops, &clk_divider_ops); if(clk_ops == &clk_gate_ops) { debugfs_create_file("xx2000_gate", S_IRUSR | S_IWUSR, core->dentry, core, &xx2000_gate_ops); } else if(clk_ops == &clk_mux_ops) { // debugfs_create_file("xx2000_mux", S_IRUSR | S_IWUSR, core->dentry, core, &xx2000_mux_ops); } else if(clk_ops == &clk_divider_ops) { debugfs_create_file("xx2000_rate", S_IRUSR | S_IWUSR, core->dentry, core, &xx2000_rate_ops); } }
选择合适的clk输出pin，对上面的不同时钟进行开关、频率选择。
可以通过clk_summary查看结果；还可以通过测量pin输出波形验证结果是否正确。
4. 小结
Linux提供了良好的时钟框架，wowotech.net对其进行了详细的总结。
在实际应用中，通过时钟框架图对时钟树进行抽象，结合时钟规格书配置时钟树；编写时钟驱动。
然后查看clk_summary，并进行验证；最后在相应的设备驱动中使用时钟。