JZ2440开发板无法挂载服务器上面的网络根文件系统

/etc/exports 加的是/work/nfs_root *(rw,sync,no_root_squash)，nfs服务已经开启了。


这是开发板的报错
IP-Config: Complete:
      device=eth0, addr=139.129.51.2, mask=255.255.255.0, gw=139.129.51.111,
     host=139.129.51.2, domain=, nis-domain=(none),
     bootserver=139.129.51.183, rootserver=139.129.51.183, rootpath=
Looking up port of RPC 100003/2 on 139.129.51.183
rpcbind: server 139.129.51.183 not responding, timed out
Root-NFS: Unable to get nfsd port number from server, using default
Looking up port of RPC 100005/1 on 139.129.51.183
rpcbind: server 139.129.51.183 not responding, timed out
Root-NFS: Unable to get mountd port number from server, using default
mount: server 139.129.51.183 not responding, timed out
Root-NFS: Server returned error -5 while mounting /work/nfs_root/first2_fs
VFS: Unable to mount root fs via NFS, trying floppy.
VFS: Cannot open root device "nfs" or unknown-block(2,0)
Please append a correct "root=" boot option; here are the available partitions:
1f00        256 mtdblock0 (driver?)
1f01        128 mtdblock1 (driver?)
1f02       2048 mtdblock2 (driver?)
1f03     259712 mtdblock3 (driver?)
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(2,0)  
2016-03-04 09:23:43