使用mdev在/dev/下創建子目錄放置設備節點

最近在嵌入式Linux板子上調試USB聲卡，使用mdev創建設備節點時默認直接在 /dev/目錄下創建， 如

1. controlC0 pcmC0D0c seq sequencer2
   
2. mixer pcmC0D0p sequencer timer

但是用戶空間的程序alsa-lib, alsa-utils都是去 /dev/snd/目錄下找這些設備節點， 怎麼能讓mdev把設備節點創建在子目錄下呢？
在busybox源代碼的 doc/mdev.txt 文檔裡面找到以下說明：

1. You can rename/move device nodes by using the next optional field.
   
2. <device regex> <uid>:<gid> <permissions> [=path]
   
3. So if you want to place the device node into a subdirectory, make sure the path
   
4. has a trailing /. If you want to rename the device node, just place the name.
   
5. hda 0:3 660 =drives/
   
6. This will move "hda" into the drives/ subdirectory.
   
7. hdb 0:3 660 =cdrom
   
8. This will rename "hdb" to "cdrom".
   
9. 
   
10. Similarly, ">path" renames/moves the device but it also creates
    
11. a direct symlink /dev/DEVNAME to the renamed/moved device.

所以， 我們只要在 /etc/mdev.conf配置文件裡面加入幾行就可以了：

1. controlC[0-9] 0:0 0660 =snd/
2. pcm.* 0:0 0660 =snd/
3. seq.* 0:0 0660 =snd/
4. mix.* 0:0 0660 =snd/
5. timer 0:0 0660 =snd/

這樣再運行mdev， ALSA相關的設備節點就都創建在 /dev/snd/ 目錄下了。

後記： 內核裡面 struct class 裡面的 devnode 項跟設備節點所在目錄好像也有關系， 有空再研究一下。