clk_get函數實現，Linux內核時鐘框架

（1）  對應外設時鐘的開啟

struct clk=clk_get(NULL,"adc");

clk.enable();

之後adc對應的時鐘位就能時能。

struct clk *clk_get(struct device *dev, const char *id)
{
    ..........
    list_for_each_entry(p, &clocks, list) {
        if (p->id == idno &&
            strcmp(id, p->name) == 0 &&
            try_module_get(p->owner)) {
            clk = p;
            break;
        }
    }

    .............................................
    return clk;
}

clk_get從一個時鐘list鏈表中以字符id名稱來查找一個時鐘clk結構體並且返回，最後調用clk.enable()，來時能對應的外設時鐘源。

（2）  list的設置和對應list中的成員。

MACHINE_START(MINI2440, "FriendlyARM Mini2440 development board")
    .phys_io    = S3C2410_PA_UART,
    .io_pg_offst    = (((u32)S3C24XX_VA_UART) >> 18) & 0xfffc,
    .boot_params    = S3C2410_SDRAM_PA + 0x100,

    .init_irq    = s3c24xx_init_irq,
    .map_io        = mini2440_map_io,
    .init_machine    = mini2440_machine_init,
    .timer        = &s3c24xx_timer,
MACHINE_END

static void __init mini2440_map_io(void)
{
    s3c24xx_init_io(mini2440_iodesc, ARRAY_SIZE(mini2440_iodesc));
    s3c24xx_init_clocks(12000000);
    s3c24xx_init_uarts(mini2440_uartcfgs, ARRAY_SIZE(mini2440_uartcfgs));
}

void __init s3c24xx_init_io(struct map_desc *mach_desc, int size)
{
    unsigned long idcode = 0x0;
   ...........
    arm_pm_restart = s3c24xx_pm_restart;

    s3c_init_cpu(idcode, cpu_ids, ARRAY_SIZE(cpu_ids));
}

void __init s3c_init_cpu(unsigned long idcode,
             struct cpu_table *cputab, unsigned int cputab_size)
{
    cpu = s3c_lookup_cpu(idcode, cputab, cputab_size);
   ......
    cpu->map_io();

static struct cpu_table cpu_ids[] __initdata = {
    .......
    {
        .idcode        = 0x32410002,
        .idmask        = 0xffffffff,
        .map_io        = s3c2410_map_io,
        .init_clocks    = s3c2410_init_clocks,
        .init_uarts    = s3c2410_init_uarts,
        .init        = s3c2410a_init,
        .name        = name_s3c2410a
    },
    .......
}

在static void __init mini2440_map_io(void)中調用s3c24xx_init_io(mini2440_iodesc, ARRAY_SIZE(mini2440_iodesc))後返回了struct cpu_table cpu_ids中的

    {
        .idcode        = 0x32410002,
        .idmask        = 0xffffffff,
        .map_io        = s3c2410_map_io,
        .init_clocks    = s3c2410_init_clocks,
        .init_uarts    = s3c2410_init_uarts,
        .init        = s3c2410a_init,
        .name        = name_s3c2410a
    }

cpu_table

之後運行s3c24xx_init_clocks(12000000)後又執行(cpu->init_clocks)(xtal)，對應就是s3c2410_init_clocks

void __init s3c244x_init_clocks(int xtal)
{
    /* initialise the clocks here, to allow other things like the
     * console to use them, and to add new ones after the initialisation
     */

    s3c24xx_register_baseclocks(xtal); //完成祖宗級別時鐘的注冊 
    s3c244x_setup_clocks();//填充祖宗級別時鐘結構，方便以後調用
    s3c2410_baseclk_add();//添加一些外設時鐘結構到list中，並且關閉它們方便省電
}

int __init s3c24xx_register_baseclocks(unsigned long xtal)
{
    printk(KERN_INFO "S3C24XX Clocks, (c) 2004 Simtec Electronics\n");

    clk_xtal.rate = xtal;

    /* register our clocks */

    if (s3c24xx_register_clock(&clk_xtal) < 0)
        printk(KERN_ERR "failed to register master xtal\n");

    if (s3c24xx_register_clock(&clk_mpll) < 0)
        printk(KERN_ERR "failed to register mpll clock\n");

    if (s3c24xx_register_clock(&clk_upll) < 0)
        printk(KERN_ERR "failed to register upll clock\n");

    if (s3c24xx_register_clock(&clk_f) < 0)
        printk(KERN_ERR "failed to register cpu fclk\n");

    if (s3c24xx_register_clock(&clk_h) < 0)
        printk(KERN_ERR "failed to register cpu hclk\n");

    if (s3c24xx_register_clock(&clk_p) < 0)
        printk(KERN_ERR "failed to register cpu pclk\n");

    return 0;
}

以上的作用就是將一個個的時鐘結構體鏈接成雙向鏈表的，以後好使用clk_get在時鐘結構體鏈表中查找id字符匹配的clk結構體並返回。