概述

目前在kernel驱动代码中，都不再建议直接使用printk直接添加打印信息，而是使用dev_info，dev_dbg，dev_err之类的函数代替，虽然这些dev_xxx函数的本质还是使用printk打印的，但是相比起printk：

• 支持打印模块信息、dev信息
• 支持动态调试(dynamic debug)方式

下面简述下这几个dev_xxx函数的基本使用规则，以及动态调试使用方式。

• dev_info()：启动过程、或者模块加载过程等“通知类的”信息等，一般只会通知一次，例如probe函数;
• dev_dbg()：一般使用在普通错误，如-EINVAL、-ENOMEM等errno发生处，用于调试;
• dev_err()：一般使用在严重错误，尤其是用户无法得到errno的地方，或者程序员不容易猜测系统哪里出了问题的地方;

动态调试使用方法

打开内核动态调试开关，make menuconfig选中CONFIG_DYNAMIC_DEBUG以及CONFIG_DEBUG_FS
Linux启动后，使用命令行挂载上dbgfs

mkdir /mnt/dbg 
mount -t debugfs none /mnt/dbg 

使用下面方式控制你想输出dev_dbg()信息
1.控制某个文件所有dev_dbg()，echo -n "file xxx.c +p" > /mnt/dbg/dynamic_debug/control
2.控制某个函数所有dev_dbg()，echo -n "func xxx +p" > /mnt/dbg/dynamic_debug/control
运行程序，使用dmesg则可以看到相应dev_dbg()的输出信息
当调试结束，不再想输出dev_dbg()信息了，使用下面命令关闭即可
1.echo -n "file xxx.c -p" > /mnt/dbg/dynamic_debug/control
2.echo -n "func xxx -p" > /mnt/dbg/dynamic_debug/control

例子

echo -n "file ca_dsc_core.c +p" > /mnt/dbg/dynamic_debug/control 则打印ca_dsc_core.c所有的dev_dbg()信息 
echo -n "func ca_dsc_read +p" > /mnt/dbg/dynamic_debug/control 则打印ca_dsc_read()函数所有dev_dbg()信息 

动态打印调试的基本原理

当编译选项CONFIG_DYNAMIC_DEBUG打开的时候，在编译阶段，kernel会把所有使用dev_dbg()的信息记录在一个table中，这些信息我们可以从/mnt/dbg/dynamic_debug/control解析出来：

# cat /mnt/dbg/dynamic_debug/control 
... ... 
... ... 
drivers/alidrivers/modules/alidsc/ca_dsc_core.c:800 [alidsc]ca_dsc_probe_dt =_ "get dev-index error12" 
drivers/alidrivers/modules/alidsc/ca_dsc_core.c:789 [alidsc]ca_dsc_probe_dt =_ "get clk error12" 
drivers/alidrivers/modules/alidsc/ca_dsc_core.c:292 [alidsc]ca_dsc_read =p "read: session#%d read returned %d bytes12" 
drivers/alidrivers/modules/alidsc/ca_dsc_core.c:234 [alidsc]ca_dsc_read =p "read: session#%d read request: %zd bytes12" 
drivers/alidrivers/modules/alidsc/ca_dsc_core.c:435 [alidsc]ca_dsc_vm_fault =_ "dsc_vm_fault: buffer#%d release %d bytes for session#%d12" 
drivers/alidrivers/modules/alidsc/ca_dsc_core.c:112 [alidsc]ca_dsc_open =_ "dsc_se: failed register se12" 
drivers/alidrivers/modules/alidsc/ca_dsc_core.c:755 [alidsc]ca_dsc_splice_write =_ "splice_write: session#%d dsc_from_pipe %d bytes12" 
drivers/alidrivers/modules/alidsc/ca_dsc_core.c:729 [alidsc]ca_dsc_splice_write =_ "splice_write: session#%d count %zd bytes12" 
drivers/alidrivers/modules/alidsc/ca_dsc_core.c:619 [alidsc]ca_dsc_splice_read =_ "splice_read: session#%d ret %zd bytes12" 
drivers/alidrivers/modules/alidsc/ca_dsc_core.c:532 [alidsc]ca_dsc_splice_read =_ "splice_read: session#%d request %zd bytes12" 
drivers/alidrivers/modules/alidsc/ca_dsc_core.c:871 [alidsc]ca_dsc_probe =_ "Get DSC handler error!12" 
drivers/alidrivers/modules/alidsc/ca_dsc_core.c:820 [alidsc]ca_dsc_probe =_ "Failed to parse DT12" 
drivers/alidrivers/modules/alidsc/ca_dsc_core.c:923 [alidsc]ca_dsc_remove =_ "get clk error12" 
drivers/alidrivers/modules/alidsc/ca_dsc_core.c:396 [alidsc]ca_dsc_write =_ "write: session#%d ret %zd12" 
drivers/alidrivers/modules/alidsc/ca_dsc_core.c:325 [alidsc]ca_dsc_write =_ "write: session#%d count %zd12" 
... ... 
... ... 
net/ipv4/ping.c:965 [ping]ping_rcv =_ "no socket, dropping12" 
net/ipv4/ping.c:960 [ping]ping_rcv =_ "rcv on socket %p12" 
net/ipv4/ping.c:953 [ping]ping_rcv =_ "ping_rcv(skb=%p,id=%04x,seq=%04x)12" 
net/ipv4/ping.c:932 [ping]ping_queue_rcv_skb =_ "ping_queue_rcv_skb -> failed12" 
net/ipv4/ping.c:929 [ping]ping_queue_rcv_skb =_ "ping_queue_rcv_skb(sk=%p,sk->num=%d,skb=%p)12" 
net/ipv4/ping.c:921 [ping]ping_recvmsg =_ "ping_recvmsg -> %d12" 
net/ipv4/ping.c:840 [ping]ping_recvmsg =_ "ping_recvmsg(sk=%p,sk->num=%u)12" 
net/ipv4/ping.c:693 [ping]ping_v4_sendmsg =_ "ping_v4_sendmsg(sk=%p,sk->num=%u)12" 
net/ipv4/ping.c:197 [ping]ping_lookup =_ "found: %p: num=%d, daddr=%pI4, dif=%d12" 
net/ipv4/ping.c:189 [ping]ping_lookup =_ "iterate12" 
net/ipv4/ping.c:176 [ping]ping_lookup =_ "try to find: num = %d, daddr = %pI4, dif = %d12" 
net/ipv4/ping.c:505 [ping]ping_err =_ "err on socket %p12" 
net/ipv4/ping.c:502 [ping]ping_err =_ "no socket, dropping12" 
net/ipv4/ping.c:498 [ping]ping_err =_ "ping_err(proto=0x%x,type=%d,code=%d,id=%04x,seq=%04x)12" 
net/ipv4/ping.c:304 [ping]ping_check_bind_addr =_ "ping_check_bind_addr(sk=%p,addr=%pI4,port=%d)12" 
net/ipv4/ping.c:445 [ping]ping_bind =_ "ping_v4_bind -> %d12" 
net/ipv4/ping.c:423 [ping]ping_bind =_ "after bind(): num = %d, dif = %d12" 
net/ipv4/ping.c:286 [ping]ping_close =_ "isk->refcnt = %d12" 
net/ipv4/ping.c:285 [ping]ping_close =_ "ping_close(sk=%p,sk->num=%u)12" 
net/ipv4/ping.c:153 [ping]ping_unhash =_ "ping_unhash(isk=%p,isk->num=%u)12" 
net/ipv4/ping.c:146 [ping]ping_hash =_ "ping_hash(sk->port=%u)12" 
net/ipv4/ping.c:67 [ping]ping_hashfn =_ "hash(%d) = %d12" 
net/ipv4/ping.c:130 [ping]ping_get_port =_ "was not hashed12" 
net/ipv4/ping.c:127 [ping]ping_get_port =_ "found port/ident = %d12" 

以其中一条为例子：

drivers/alidrivers/modules/alidsc/ca_dsc_core.c:800 [alidsc]ca_dsc_probe_dt =_ "get dev-index error12" 则不会打印 
drivers/alidrivers/modules/alidsc/ca_dsc_core.c:800 [alidsc]ca_dsc_probe_dt =p "get dev-index error12" 则会打印 

所以在应用层，用户就可以通过使用echo来控制dynamic_debug/control文件，进而控制是否打印某个dev_dbg()信息!

dev_dbg()对于分析某些内核子系统或者驱动流程也十分有意义，例如，使能net/ipv4/ping.c的调试开关，则可以观测ping的运行原理。

代码分析

从代码角度，也很容易看出dev_dbg()的设计：

 include/linux/device.h include/linux/dynamic_debug.h lib/dynamic_debug.c 

    //使能CONFIG_DYNAMIC_DEBUG后则根据control信息动态打印 
    #if defined(CONFIG_DYNAMIC_DEBUG) 
     #define dev_dbg(dev, format, ...)        
     do {            
      dynamic_dev_dbg(dev, format, ##__VA_ARGS__);  
     } while (0) 
    //使能DEBUG，则打印整个kernel的dev_dbg信息 
    #elif defined(DEBUG) 
     #define dev_dbg(dev, format, arg...)   
      dev_printk(KERN_DEBUG, dev, format, ##arg) 
    //都不使能，dev_dbg不打印任何东西 
    #else 
     #define dev_dbg(dev, format, arg...)     
     ({         
      if (0)        
       dev_printk(KERN_DEBUG, dev, format, ##arg);  
      0;        
     }) 
    #endif 

下面的dynamic_dev_dbg()实现显然可以看出，打印是根据descriptor的标志位_DPRINTK_FLAGS_PRINT进行打印的，而标志位是可以通过dbgfs进行控制的。

#define dynamic_dev_dbg(dev, fmt, ...)     
do {         
 DEFINE_DYNAMIC_DEBUG_METADATA(descriptor, fmt);   
 if (unlikely(descriptor.flags & _DPRINTK_FLAGS_PRINT))  
  __dynamic_dev_dbg(&descriptor, dev, fmt,  
      ##__VA_ARGS__);   
} while (0) 

好处

• 开发版本，打开CONFIG_DYNAMIC_DEBUG和CONFIG_DEBUG_FS，配合dbgfs动态观测和调试内核代码;
• 正式版本，关闭CONFIG_DYNAMIC_DEBUG和CONFIG_DEBUG_FS，所有dbgfs以及dev_dbg信息都从编译阶段自动移除;