android kernel控制台初始化過程

對於我們的android平台，控制台被定義到了串口1上，因此初始化過程就是把控制台的輸出配置到串口1上

對kernel控制台初始化是在掛載文件系統之前，由於沒有串口的設備文件，不能通過打開設備文件來訪問串口，只能直接訪問硬件，更類似與裸機的訪問方式。

下面正式來看

板子初始化的過程
android\kernel_imx\arch\arm\mach-mx6\board-mx6q_sabresd.c
[cpp]
MACHINE_START(MX6Q_SABRESD, "Freescale i.MX 6Quad/DualLite/Solo Sabre-SD Board") 
/* Maintainer: Freescale Semiconductor, Inc. */ 
.boot_params = MX6_PHYS_OFFSET + 0x100, 
.fixup = fixup_mxc_board, 
.map_io = mx6_map_io, 
.init_irq = mx6_init_irq, 
.init_machine = mx6_sabresd_board_init, 
.timer = &mx6_sabresd_timer, 
.reserve = mx6q_sabresd_reserve, 
MACHINE_END 

MACHINE_START(MX6Q_SABRESD, "Freescale i.MX 6Quad/DualLite/Solo Sabre-SD Board")
/* Maintainer: Freescale Semiconductor, Inc. */
.boot_params = MX6_PHYS_OFFSET + 0x100,
.fixup = fixup_mxc_board,
.map_io = mx6_map_io,
.init_irq = mx6_init_irq,
.init_machine = mx6_sabresd_board_init,
.timer = &mx6_sabresd_timer,
.reserve = mx6q_sabresd_reserve,
MACHINE_END

這其中有個時鐘初始化mx6_sabresd_timer我們來看它的定義
[cpp]
static struct sys_timer mx6_sabresd_timer = { 
.init   = mx6_sabresd_timer_init, 
}; 
static void __init mx6_sabresd_timer_init(void) 
{ 
struct clk *uart_clk; 
#ifdef CONFIG_LOCAL_TIMERS  
twd_base = ioremap(LOCAL_TWD_ADDR, SZ_256); 
BUG_ON(!twd_base); 
#endif  
mx6_clocks_init(32768, 24000000, 0, 0); 
 
 
uart_clk = clk_get_sys("imx-uart.0", NULL); 
early_console_setup(UART1_BASE_ADDR, uart_clk); 
} 

static struct sys_timer mx6_sabresd_timer = {
.init   = mx6_sabresd_timer_init,
};
static void __init mx6_sabresd_timer_init(void)
{
struct clk *uart_clk;
#ifdef CONFIG_LOCAL_TIMERS
twd_base = ioremap(LOCAL_TWD_ADDR, SZ_256);
BUG_ON(!twd_base);
#endif
mx6_clocks_init(32768, 24000000, 0, 0);

uart_clk = clk_get_sys("imx-uart.0", NULL);
early_console_setup(UART1_BASE_ADDR, uart_clk);
}

可以看到這裡調用了early_console_setup(UART1_BASE_ADDR, uart_clk);
這個函數就是文件系統掛載之前控制台的初始化函數。下面我就開始分析這個函數
android\kernel_imx\arch\arm\plat-mxc\cpu.c
[cpp]
/**
 * early_console_setup - setup debugging console
 *
 * Consoles started here require little enough setup that we can start using
 * them very early in the boot process, either right after the machine
 * vector initialization, or even before if the drivers can detect their hw.
 *
 * Returns non-zero if a console couldn't be setup.
 * This function is developed based on
 * early_console_setup function as defined in arch/ia64/kernel/setup.c
 * 這個注釋裡寫的很清楚，在設備驅動執行之前，為了調試錯誤的需要我們
 * 需要在啟動的最初就初始化控制台
 */ 
void __init early_console_setup(unsigned long base, struct clk *clk) 
{ 
#ifdef CONFIG_SERIAL_IMX_CONSOLE  
mxc_early_serial_console_init(base, clk); 
#endif  
} 
這裡調用mxc_early_serial_console_init(base, clk); 
android\kernel_imx\drivers\tty\serial、mxc_uart_early.c 
int __init mxc_early_serial_console_init(unsigned long base, struct clk *clk) 
{ 
mxc_early_device.clk = clk; 
mxc_early_device.port.mapbase = base; 
 
 
register_console(&mxc_early_uart_console); 
return 0; 
} 

/**
 * early_console_setup - setup debugging console
 *
 * Consoles started here require little enough setup that we can start using
 * them very early in the boot process, either right after the machine
 * vector initialization, or even before if the drivers can detect their hw.
 *
 * Returns non-zero if a console couldn't be setup.
 * This function is developed based on
 * early_console_setup function as defined in arch/ia64/kernel/setup.c
 * 這個注釋裡寫的很清楚，在設備驅動執行之前，為了調試錯誤的需要我們
 * 需要在啟動的最初就初始化控制台
 */
void __init early_console_setup(unsigned long base, struct clk *clk)
{
#ifdef CONFIG_SERIAL_IMX_CONSOLE
mxc_early_serial_console_init(base, clk);
#endif
}
這裡調用mxc_early_serial_console_init(base, clk);
android\kernel_imx\drivers\tty\serial、mxc_uart_early.c
int __init mxc_early_serial_console_init(unsigned long base, struct clk *clk)
{
mxc_early_device.clk = clk;
mxc_early_device.port.mapbase = base;

register_console(&mxc_early_uart_console);
return 0;
}

這裡可以看到register_console(&mxc_early_uart_console);就是注冊一個設備到控制台中，
在最開始注冊的這個設備肯定是裸機的訪問方式的，因此我們重點來看這個設備
[cpp]
static struct console mxc_early_uart_console __initdata = { 
.name = "ttymxc", 
.write = early_mxcuart_console_write, 
.setup = mxc_early_uart_setup, 
.flags = CON_PRINTBUFFER | CON_BOOT, 
.index = -1, 
}; 

static struct console mxc_early_uart_console __initdata = {
.name = "ttymxc",
.write = early_mxcuart_console_write,
.setup = mxc_early_uart_setup,
.flags = CON_PRINTBUFFER | CON_BOOT,
.index = -1,
};

這個設備提供的設備訪問接口
.write = early_mxcuart_console_write,是串口的發送函數
.setup = mxc_early_uart_setup,是串口的初始化函數
.flags = CON_PRINTBUFFER | CON_BOOT,是控制台標志，CON_BOOT表明這事一個boot的控制台設備
也就是說是掛載設備文件之前的控制台設備

下面我們來分析初始化函數和 數據發送函數
初始化函數

[cpp]
static int __init mxc_early_uart_setup(struct console *console, char *options) 
{ 
 
struct mxc_early_uart_device *device = &mxc_early_device; 
struct uart_port *port = &device->port; 
int length; 
 
if (device->port.membase || device->port.iobase)   
return -ENODEV; 
 
 
/* Enable Early MXC UART Clock */ 
clk_enable(device->clk);//初始化總線時鐘  
 
 
port->uartclk = 5600000; 
port->iotype = UPIO_MEM; 
port->membase = ioremap(port->mapbase, SZ_4K);//串口寄存器內存映射  
 
 
if (options) { 
device->baud = simple_strtoul(options, NULL, 0); 
length = min(strlen(options), sizeof(device->options)); 
strncpy(device->options, options, length); 
} else { 
device->baud = probe_baud(port); 
snprintf(device->options, sizeof(device->options), "%u", 
device->baud); 
} 
printk(KERN_INFO 
      "MXC_Early serial console at MMIO 0x%x (options '%s')\n", 
      port->mapbase, device->options); 
return 0; 
} 

static int __init mxc_early_uart_setup(struct console *console, char *options)
{

struct mxc_early_uart_device *device = &mxc_early_device;
struct uart_port *port = &device->port;
int length;

if (device->port.membase || device->port.iobase) 
return -ENODEV;

/* Enable Early MXC UART Clock */
clk_enable(device->clk);//初始化總線時鐘

port->uartclk = 5600000;
port->iotype = UPIO_MEM;
port->membase = ioremap(port->mapbase, SZ_4K);//串口寄存器內存映射

if (options) {
device->baud = simple_strtoul(options, NULL, 0);
length = min(strlen(options), sizeof(device->options));
strncpy(device->options, options, length);
} else {
device->baud = probe_baud(port);
snprintf(device->options, sizeof(device->options), "%u",
device->baud);
}
printk(KERN_INFO
      "MXC_Early serial console at MMIO 0x%x (options '%s')\n",
      port->mapbase, device->options);
return 0;
}
其實從這個初始化函數裡看出，它做了很多向mxc_early_device結構體中填入數據的工作，而這些數據

找遍所有代碼也沒有用到，因此這些事沒有意義的，官方代碼給的這點並不太好。但是由於uboot中我們已經初始化了

串口因此這裡就算沒有任何初始化其實串口也可以是使用。

這裡真正有用的就兩句話
clk_enable(device->clk);//初始化總線時鐘
port->membase = ioremap(port->mapbase, SZ_4K);//串口寄存器內存映射
但是在寄存器映射結束後沒有進行任何串口寄存器初始化，這也很奇怪，我們仔細查找發現，
寄存器初始化代碼寫在了數據發送函數裡，具體為什麼我們來分析發送函數
early_mxcuart_console_write

[cpp]
/*!
 * This function is called to write the console messages through the UART port.
 *
 * @param   co    the console structure
 * @param   s     the log message to be written to the UART
 * @param   count length of the message
 */ 
void __init early_mxcuart_console_write(struct console *co, const char *s, 
u_int count) 
{ 
struct uart_port *port = &mxc_early_device.port; 
unsigned int status, oldcr1, oldcr2, oldcr3, cr2, cr3; 
 
 
/*
* First save the control registers and then disable the interrupts
*/ 
oldcr1 = readl(port->membase + MXC_UARTUCR1);   //讀取當前三個串口控制寄存器的值  
oldcr2 = readl(port->membase + MXC_UARTUCR2); 
oldcr3 = readl(port->membase + MXC_UARTUCR3); 
cr2 = 
   oldcr2 & ~(MXC_UARTUCR2_ATEN | MXC_UARTUCR2_RTSEN | //初始化串口寄存器數值  
      MXC_UARTUCR2_ESCI); 
cr3 = 
   oldcr3 & ~(MXC_UARTUCR3_DCD | MXC_UARTUCR3_RI | 
      MXC_UARTUCR3_DTRDEN); 
writel(MXC_UARTUCR1_UARTEN, port->membase + MXC_UARTUCR1);  //使能串口  
writel(cr2, port->membase + MXC_UARTUCR2);  //配置寄存器  
writel(cr3, port->membase + MXC_UARTUCR3); 
 
 
/* Transmit string */ 
uart_console_write(port, s, count, mxcuart_console_write_char); //發送數據  
 
 
/*
* Finally, wait for the transmitter to become empty等待發送完成
*/ 
do { 
status = readl(port->membase + MXC_UARTUSR2); 
} while (!(status & MXC_UARTUSR2_TXDC)); 
 
 
/*
* Restore the control registers
*/ 
writel(oldcr1, port->membase + MXC_UARTUCR1);//恢復串口寄存器數值  
writel(oldcr2, port->membase + MXC_UARTUCR2); 
writel(oldcr3, port->membase + MXC_UARTUCR3); 
} 

/*!
 * This function is called to write the console messages through the UART port.
 *
 * @param   co    the console structure
 * @param   s     the log message to be written to the UART
 * @param   count length of the message
 */
void __init early_mxcuart_console_write(struct console *co, const char *s,
u_int count)
{
struct uart_port *port = &mxc_early_device.port;
unsigned int status, oldcr1, oldcr2, oldcr3, cr2, cr3;

/*
* First save the control registers and then disable the interrupts
*/
oldcr1 = readl(port->membase + MXC_UARTUCR1);   //讀取當前三個串口控制寄存器的值
oldcr2 = readl(port->membase + MXC_UARTUCR2);
oldcr3 = readl(port->membase + MXC_UARTUCR3);
cr2 =
   oldcr2 & ~(MXC_UARTUCR2_ATEN | MXC_UARTUCR2_RTSEN | //初始化串口寄存器數值
      MXC_UARTUCR2_ESCI);
cr3 =
   oldcr3 & ~(MXC_UARTUCR3_DCD | MXC_UARTUCR3_RI |
      MXC_UARTUCR3_DTRDEN);
writel(MXC_UARTUCR1_UARTEN, port->membase + MXC_UARTUCR1);  //使能串口
writel(cr2, port->membase + MXC_UARTUCR2);  //配置寄存器
writel(cr3, port->membase + MXC_UARTUCR3);

/* Transmit string */
uart_console_write(port, s, count, mxcuart_console_write_char); //發送數據

/*
* Finally, wait for the transmitter to become empty等待發送完成
*/
do {
status = readl(port->membase + MXC_UARTUSR2);
} while (!(status & MXC_UARTUSR2_TXDC));

/*
* Restore the control registers
*/
writel(oldcr1, port->membase + MXC_UARTUCR1);//恢復串口寄存器數值
writel(oldcr2, port->membase + MXC_UARTUCR2);
writel(oldcr3, port->membase + MXC_UARTUCR3);
}

從這個函數看書，它首先保存了串口控制寄存器的值，然後初始化成符合控制台的，發送完數據後，又恢復了原來的數據
這樣做的目的就是，如果我們加載了串口的驅動，那麼很有可能打亂了控制台的配置，而系統啟動以後
我們還不能動串口驅動的配置，因此最好的辦法就是，每次發送數據都重新配置串口，發送完後再恢復以前的配置。
到了這裡控制台初始化的第一部分已經完成了。我們可以知道沒有文件系統，控制台是怎麼工作的。