2410下clock源碼分析
Author:aaron
這篇文章主要使用2.6.22下2410方面關於clock的源碼來進行簡單的分析, 希望通過這篇文檔能對系統中的clock的使用問題有個瞭解.
寫這篇文檔除了參考了源碼外, 還要參考2410的data sheet, 畢竟代碼都是按照spec來寫的嘛. 我們先來看下2410下各種clock是如何產生的:
我們可以看到, 2410的時鐘源可以有兩種: 由晶振產生或由外部CLOCK直接提供, 這可以由OM[3:2]來選擇, 一般都是硬件定死的, 圖中FCLK爲CPU使用, HCLK爲在AHB總線一側的設備使用, PCLK爲APB總線一側的設備使用. 時鐘源進來後要通過MPLL電路來產生FCLK,HCLK,PCLK, 因此很明顯, 我們要通過設置MPLL方面的寄存器來控制輸出的時鐘頻率.
更詳細的關於clock方面的描述請參考2410的data sheet. 下面我們就開始來分析源碼了
分析代碼當然從初始化開始了, s3c24xx_init_clocks 就是它的初始化函數了,
Arch/arm/plat-s3c24xx/cpu.c:
/* s3c24xx_init_clocks
*
* Initialise the clock subsystem and associated information from the
* given master crystal value.
*
* xtal = 0 -> use default PLL crystal value (normally 12MHz)
* != 0 -> PLL crystal value in Hz
*/
void __init s3c24xx_init_clocks(int xtal)
{
if (xtal == 0)
xtal = 12*1000*1000; //看函數頭的註釋
if (cpu == NULL) //運行到這裏, 我們已經確定了我們板子上的cpu.
panic("s3c24xx_init_clocks: no cpu setup?/n");
if (cpu->init_clocks == NULL)
panic("s3c24xx_init_clocks: cpu has no clock init/n");
else
(cpu->init_clocks)(xtal); //調用具體型號cpu的初始化函數
}
這個函數是在smdk2410_map_io裏被調用的, 傳進來的參數是0, 可以看到主要的初始化由具體cpu來完成. 對於2410來說就是調用s3c2410_init_clocks()
arch/arm/mach-s3c2410/s3c2410.c:
void __init s3c2410_init_clocks(int xtal)
{
unsigned long tmp;
unsigned long fclk;
unsigned long hclk;
unsigned long pclk;
/* now we've got our machine bits initialised, work out what
* clocks we've got */
fclk = s3c2410_get_pll(__raw_readl(S3C2410_MPLLCON), xtal); //得到fclk,
tmp = __raw_readl(S3C2410_CLKDIVN); //獲取fclk, hclk, plk間的比例參數
/* work out clock scalings */
hclk = fclk / ((tmp & S3C2410_CLKDIVN_HDIVN) ? 2 : 1); //計算出hclk
pclk = hclk / ((tmp & S3C2410_CLKDIVN_PDIVN) ? 2 : 1); //計算出pclk
/* print brieft summary of clocks, etc */
printk("S3C2410: core %ld.%03ld MHz, memory %ld.%03ld MHz, peripheral %ld.%03ld MHz/n",
print_mhz(fclk), print_mhz(hclk), print_mhz(pclk));
/* initialise the clocks here, to allow other things like the
* console to use them
*/
s3c24xx_setup_clocks(xtal, fclk, hclk, pclk); //把clock註冊到系統中去
s3c2410_baseclk_add(); //把外設用到的clock也註冊進系統中去
}
通過設置寄存器S3C2410_CLKDIVN的值可以設置fclk, hclk, pclk之間的比例, 因此由其中的一個值就可以計算出其他兩個值了, 而這個比值的設置實在bootloader階段設置好的, 而fclk的獲取也是通過一個計算表達式得到的,
Mpll就是我們的FCLK, 根據這個公式, 函數s3c2410_get_pll就很好理解了, 這裏就不貼出來了.
我們來看s3c24xx_setup_clocks函數
arch/arm/plat-s3c24xx/clock.c:
/* initalise all the clocks */
int __init s3c24xx_setup_clocks(unsigned long xtal,
unsigned long fclk,
unsigned long hclk,
unsigned long pclk)
{
printk(KERN_INFO "S3C24XX Clocks, (c) 2004 Simtec Electronics/n");
/* initialise the main system clocks */
clk_xtal.rate = xtal; //時鐘源的頻率
clk_upll.rate = s3c2410_get_pll(__raw_readl(S3C2410_UPLLCON), xtal);
clk_mpll.rate = fclk;
clk_h.rate = hclk;
clk_p.rate = pclk;
clk_f.rate = fclk;
/* assume uart clocks are correctly setup */
/* register our clocks */
if (s3c24xx_register_clock(&clk_xtal) < 0) //註冊時鐘源
printk(KERN_ERR "failed to register master xtal/n");
if (s3c24xx_register_clock(&clk_mpll) < 0) //註冊mpll
printk(KERN_ERR "failed to register mpll clock/n");
if (s3c24xx_register_clock(&clk_upll) < 0) //註冊upll
printk(KERN_ERR "failed to register upll clock/n");
if (s3c24xx_register_clock(&clk_f) < 0) //註冊fclk
printk(KERN_ERR "failed to register cpu fclk/n");
if (s3c24xx_register_clock(&clk_h) < 0) //註冊hclk
printk(KERN_ERR "failed to register cpu hclk/n");
if (s3c24xx_register_clock(&clk_p) < 0) //註冊pclk
printk(KERN_ERR "failed to register cpu pclk/n");
return 0;
}
這個函數把所有的clock都註冊進系統, 以備以後使用.
arch/arm/plat-s3c24xx/clock.c:
/* initialise the clock system */
int s3c24xx_register_clock(struct clk *clk)
{
clk->owner = THIS_MODULE;
if (clk->enable == NULL)
clk->enable = clk_null_enable; //enable函數, 以後會用到
/* add to the list of available clocks */
mutex_lock(&clocks_mutex);
list_add(&clk->list, &clocks); //把clock註冊到clocks列表中去
mutex_unlock(&clocks_mutex);
return 0;
}
系統中存在一個clocks的列表, 系統中的所有用到的時鐘都會被註冊到該列表中去
arch/arm/plat-s3c24xx/clock.c:
static LIST_HEAD(clocks);
接着我們來看s3c2410_baseclk_add().
arch/arm/plat-s3c24xx/clock.c:
/* s3c2410_baseclk_add()
*
* Add all the clocks used by the s3c2410 or compatible CPUs
* such as the S3C2440 and S3C2442.
*
* We cannot use a system device as we are needed before any
* of the init-calls that initialise the devices are actually
* done.
*/
//上面的註釋很明瞭了吧
int __init s3c2410_baseclk_add(void)
{
unsigned long clkslow = __raw_readl(S3C2410_CLKSLOW); //快,慢時鐘寄存器
unsigned long clkcon = __raw_readl(S3C2410_CLKCON); //clock使能禁止寄存器
struct clk *clkp;
struct clk *xtal;
int ret;
int ptr;
clk_upll.enable = s3c2410_upll_enable; //登記使能函數
if (s3c24xx_register_clock(&clk_usb_bus) < 0) //註冊usb clock
printk(KERN_ERR "failed to register usb bus clock/n");
/* register clocks from clock array */
clkp = init_clocks;//初始化要註冊的clock 列表
for (ptr = 0; ptr < ARRAY_SIZE(init_clocks); ptr++, clkp++) {
/* ensure that we note the clock state */
clkp->usage = clkcon & clkp->ctrlbit ? 1 : 0; //該clock當前是否使能着,
ret = s3c24xx_register_clock(clkp); //註冊該clock到系統中去
if (ret < 0) {
printk(KERN_ERR "Failed to register clock %s (%d)/n",
clkp->name, ret);
}
}
/* We must be careful disabling the clocks we are not intending to
* be using at boot time, as subsytems such as the LCD which do
* their own DMA requests to the bus can cause the system to lockup
* if they where in the middle of requesting bus access.
*
* Disabling the LCD clock if the LCD is active is very dangerous,
* and therefore the bootloader should be careful to not enable
* the LCD clock if it is not needed.
*/
/* install (and disable) the clocks we do not need immediately */
clkp = init_clocks_disable; //又是一個clock列表
for (ptr = 0; ptr < ARRAY_SIZE(init_clocks_disable); ptr++, clkp++) {
ret = s3c24xx_register_clock(clkp); //註冊
if (ret < 0) {
printk(KERN_ERR "Failed to register clock %s (%d)/n",
clkp->name, ret);
}
s3c2410_clkcon_enable(clkp, 0); //禁止該clock.
}
/* show the clock-slow value */
xtal = clk_get(NULL, "xtal");
printk("CLOCK: Slow mode (%ld.%ld MHz), %s, MPLL %s, UPLL %s/n",
print_mhz(clk_get_rate(xtal) /
( 2 * S3C2410_CLKSLOW_GET_SLOWVAL(clkslow))),
(clkslow & S3C2410_CLKSLOW_SLOW) ? "slow" : "fast",
(clkslow & S3C2410_CLKSLOW_MPLL_OFF) ? "off" : "on",
(clkslow & S3C2410_CLKSLOW_UCLK_OFF) ? "off" : "on");
return 0;
}
該函數把外設要使用的clock也都註冊進了系統, 便於以後設備驅動使用clock的時候獲取.
那我們來看看這些外設的clock是如何定義的吧
arch/arm/plat-s3c24xx/clock.c:
struct clk clk_usb_bus = {
.name = "usb-bus",
.id = -1,
.rate = 0,
.parent = &clk_upll, //父節點,
};
Usb的clock. 它使用upll提供的clock
arch/arm/plat-s3c24xx/clock.c:
static struct clk init_clocks[] = {
{
.name = "lcd", //lcd控制器的clock
.id = -1,
.parent = &clk_h, //父節點爲hclk, 即該控制器掛在了AHB總線上
.enable = s3c2410_clkcon_enable, //使能函數
.ctrlbit = S3C2410_CLKCON_LCDC,
}, {
.name = "gpio", //gpio的clock
.id = -1,
.parent = &clk_p, //父節點爲pclk, 掛在APB上
.enable = s3c2410_clkcon_enable,
.ctrlbit = S3C2410_CLKCON_GPIO,
}, {
.name = "usb-host", //usb-host控制器
.id = -1,
.parent = &clk_h,
.enable = s3c2410_clkcon_enable,
.ctrlbit = S3C2410_CLKCON_USBH,
}, {
.name = "usb-device", //usb-device控制器
.id = -1,
.parent = &clk_h,
.enable = s3c2410_clkcon_enable,
.ctrlbit = S3C2410_CLKCON_USBD,
}, {
.name = "timers", //timers
.id = -1,
.parent = &clk_p,
.enable = s3c2410_clkcon_enable,
.ctrlbit = S3C2410_CLKCON_PWMT,
}, {
.name = "uart", //uart
.id = 0,
.parent = &clk_p,
.enable = s3c2410_clkcon_enable,
.ctrlbit = S3C2410_CLKCON_UART0,
}, {
.name = "uart",
.id = 1,
.parent = &clk_p,
.enable = s3c2410_clkcon_enable,
.ctrlbit = S3C2410_CLKCON_UART1,
}, {
.name = "uart",
.id = 2,
.parent = &clk_p,
.enable = s3c2410_clkcon_enable,
.ctrlbit = S3C2410_CLKCON_UART2,
}, {
.name = "rtc",
.id = -1,
.parent = &clk_p,
.enable = s3c2410_clkcon_enable,
.ctrlbit = S3C2410_CLKCON_RTC,
}, {
.name = "watchdog",
.id = -1,
.parent = &clk_p,
.ctrlbit = 0,
}, {
.name = "usb-bus-host",
.id = -1,
.parent = &clk_usb_bus,
}, {
.name = "usb-bus-gadget",
.id = -1,
.parent = &clk_usb_bus,
},
};
arch/arm/plat-s3c24xx/clock.c:
/* standard clock definitions */
static struct clk init_clocks_disable[] = {
{
.name = "nand",
.id = -1,
.parent = &clk_h,
.enable = s3c2410_clkcon_enable,
.ctrlbit = S3C2410_CLKCON_NAND,
}, {
.name = "sdi",
.id = -1,
.parent = &clk_p,
.enable = s3c2410_clkcon_enable,
.ctrlbit = S3C2410_CLKCON_SDI,
}, {
.name = "adc",
.id = -1,
.parent = &clk_p,
.enable = s3c2410_clkcon_enable,
.ctrlbit = S3C2410_CLKCON_ADC,
}, {
.name = "i2c",
.id = -1,
.parent = &clk_p,
.enable = s3c2410_clkcon_enable,
.ctrlbit = S3C2410_CLKCON_IIC,
}, {
.name = "iis",
.id = -1,
.parent = &clk_p,
.enable = s3c2410_clkcon_enable,
.ctrlbit = S3C2410_CLKCON_IIS,
}, {
.name = "spi",
.id = -1,
.parent = &clk_p,
.enable = s3c2410_clkcon_enable,
.ctrlbit = S3C2410_CLKCON_SPI,
}
};
所有這些clock都被註冊進了系統, 接着我們來看使能函數.
arch/arm/plat-s3c24xx/clock.c:
int s3c2410_clkcon_enable(struct clk *clk, int enable)
{
unsigned int clocks = clk->ctrlbit; //該位就是該clock在寄存器S3C2410_CLKCON中的使能位
unsigned long clkcon;
//接下來就是使能該位
clkcon = __raw_readl(S3C2410_CLKCON);
if (enable)
clkcon |= clocks;
else
clkcon &= ~clocks;
/* ensure none of the special function bits set */
clkcon &= ~(S3C2410_CLKCON_IDLE|S3C2410_CLKCON_POWER);
__raw_writel(clkcon, S3C2410_CLKCON); //寫回寄存器,即使能該clock.
return 0;
}
該函數主要是對寄存器S3C2410_CLKCON的操作, 可以參考2410的data sheet.
實際上寫到這裏clock的初始化基本完成了, 不過通過前面的代碼我們可以看到, 外設的clock都還沒使能了, 那在什麼時候被使能的呢? 呵呵當然是在設備驅動裏了, 我們以nand爲例來看下.
Drivers/mtd/nand/s3c2410.c:
static int s3c24xx_nand_probe(struct platform_device *pdev,
enum s3c_cpu_type cpu_type)
{
…..
info->clk = clk_get(&pdev->dev, "nand"); //從系統中獲取nand的clock
if (IS_ERR(info->clk)) {
dev_err(&pdev->dev, "failed to get clock");
err = -ENOENT;
goto exit_error;
}
clk_enable(info->clk); //使能該clock.
…..
}
看到了吧, 在nand的驅動裏通過clk_get來從初始化時註冊的clock列表中獲取nand的clock, 然後通過clk_enable()來使能該clock.
arch/arm/plat-s3c24xx/clock.c:
int clk_enable(struct clk *clk)
{
if (IS_ERR(clk) || clk == NULL)
return -EINVAL;
clk_enable(clk->parent); //先使能父clock
mutex_lock(&clocks_mutex);
if ((clk->usage++) == 0)
(clk->enable)(clk, 1); //使能自己, 這個enable函數就是初始化時登記的函數.
mutex_unlock(&clocks_mutex);
return 0;
}
我們可以看到要使能某個clock時必須要先使能父clock, 然後才能使能自己,
Clock的disable工作跟enable差不多, 不分析了.
通過這篇文章的分析, 我們至少知道了每個設備的clock是如何而來的, 又是如何被使能的這麼一個流程, 希望這篇文檔對大家有所幫助.