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。