今天终于将dev_dbg信息输出了，为了以后方便查找，特记录下来：

1、修改缺省信息的宏定义级别(kernel\include\linux\printk.h文件内)：

/* printk's without a loglevel use this.. */
#define MESSAGE_LOGLEVEL_DEFAULT CONFIG_MESSAGE_LOGLEVEL_DEFAULT

/* We show everything that is MORE important than this.. */
#define CONSOLE_LOGLEVEL_SILENT  0 /* Mum's the word */
#define CONSOLE_LOGLEVEL_MIN	 1 /* Minimum loglevel we let people use */
#define CONSOLE_LOGLEVEL_QUIET	 4 /* Shhh ..., when booted with "quiet" */
#define CONSOLE_LOGLEVEL_DEFAULT 8 /* anything MORE serious than KERN_DEBUG */
#define CONSOLE_LOGLEVEL_DEBUG	10 /* issue debug messages */
#define CONSOLE_LOGLEVEL_MOTORMOUTH 15	/* You can't shut this one up */

//#define DEFAULT_CONSOLE_LOGLEVEL  7 /* anything MORE serious than KERN_DEBUG */

将值改为8

#define DEFAULT_CONSOLE_LOGLEVEL  8 /* anything MORE serious than KERN_DEBUG */

该行表示只有打印级别高于DEFAULT_CONSOLE_LOGLEVEL(值小于DEFAULT_CONSOLE_LOGLEVEL)的打印才会出现在终端上。

而 KERN_DEBUG也为7，所以我们的调试不会直接打印出来

kernel\include\linux\kern_levels.h

#define KERN_SOH	"\001"		/* ASCII Start Of Header */
#define KERN_SOH_ASCII	'\001'

#define KERN_EMERG	KERN_SOH "0"	/* system is unusable */
#define KERN_ALERT	KERN_SOH "1"	/* action must be taken immediately */
#define KERN_CRIT	KERN_SOH "2"	/* critical conditions */
#define KERN_ERR	KERN_SOH "3"	/* error conditions */
#define KERN_WARNING	KERN_SOH "4"	/* warning conditions */
#define KERN_NOTICE	KERN_SOH "5"	/* normal but significant condition */
#define KERN_INFO	KERN_SOH "6"	/* informational */
#define KERN_DEBUG	KERN_SOH "7"	/* debug-level messages */

2、在要输出dev_dbg信息的*.c文件的最前面添加：

#define DEBUG 1

因为dev_dbg的宏定义在kernel\include\linux\device.h，而这文件通常被其它文件引用，在本BSP内被kernel/include/linux/platform_device.h引用，因此必须在引用这头文件之前定义DEBUG。