printascii，kernel low level debug

printk输出的log到串口上，配置串口用的是bootloader传过来的参数，例如console＝ttyS0。

printascii是独立于printk的打印log的方法。printascii是汇编实现的，直接往uart发送寄存器里扔数据。所以在kernel开始，解析参数之前的log，就可以用printascii去看log.

为了支持 printascii()函数，需要在Kernel裁减中（make menuconfig）添加Kernel hacking-? [*]Kernel low-level debugging functions的支持。

arch/arm/kernel/debug.S

ENTRY(printascii)

addruart r3

b 2f

1: waituart r2, r3

senduart r1, r3

busyuart r2, r3

teq r1, #'\n'

moveq r1, #'\r'

beq 1b

2: teq r0, #0

ldrneb r1, [r0], #1

teqne r1, #0

bne 1b

mov pc, lr

可以看到addruart就是uart寄存器的地址base。所以只需要配置addruart的宏定义就可以了。

arch/arm/mach-pxa/include/mach/debug-macro.S

.macro addruart,rx

mrc p15, 0, \rx, c1, c0

tst \rx, #1 @ MMU enabled?

moveq \rx, #0x40000000 @ physical

movne \rx, #io_p2v(0x40000000) @ virtual

orr \rx, \rx, #CONFIG_DEBUG_UART_OFFSET

.endm

0x40000000是寄存器base, CONFIG_DEBUG_UART_OFFSET就是UART的寄存器偏移。

实际寄存器地址为：

#define STRBR      (volatile unsigned long *)( 0x40700000 )  // Receive Buffer Register (read only)

#define STTHR      (volatile unsigned long *)( 0x40700000 )  // Transmit Holding Register (write only)

#define STIER      (volatile unsigned long *)( 0x40700004 )  // Interrupt Enable Register (read/write)

#define STIIR      (volatile unsigned long *)( 0x40700008 )  // Interrupt ID Register (read only)

#define STFCR      (volatile unsigned long *)( 0x40700008 )  // FIFO Control Register (write only)

#define STLCR      (volatile unsigned long *)( 0x4070000C )  // Line Control Register (read/write)

#define STMCR      (volatile unsigned long *)( 0x40700010 )  // Modem Control Register (read/write)

#define STLSR      (volatile unsigned long *)( 0x40700014 )  // Line Status Register (read only)

#define STMSR      (volatile unsigned long *)( 0x40700018 )  // Modem Status Register (read only)

#define STSPR      (volatile unsigned long *)( 0x4070001C )  // Scratch Pad Register (read/write)

#define STISR      (volatile unsigned long *)( 0x40700020 )  // slow Infrared Select Register (read/write)

#define STFOR      (volatile unsigned long *)( 0x40700024 )  // Receive FIFO Occupancy Register (read/write)

#define STDLL      (volatile unsigned long *)( 0x40700000 )  // baud divisor lower byte (read/write)

#define STDLH      (volatile unsigned long *)( 0x40700004 )

只要配好了这些，就printacsii就可以打印到STUART上了。