为什么NFS挂载会出现Unable to mount root fs via NFS, trying floppy？

内核版本linux-3.4.2，开发板ip地址为192.168.1.201   Ubuntu IP地址为192.168.1.108
TCP: cubic registered
NET: Registered protocol family 17
drivers/rtc/hctosys.c: unable to open rtc device (rtc0)
ALSA device list:
  No soundcards found.
VFS: Unable to mount root fs via NFS, trying floppy.
VFS: Cannot open root device "nfs" or unknown-block(2,0): error -6
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)

在u-boot能够ping通
SMDK2410 #  ping 192.168.1.100
ERROR: resetting DM9000 -> not responding
dm9000 i/o: 0x20000000, id: 0x90000a46
DM9000: running in 16 bit mode
MAC: 00:0c:29:4d:e4:f4
could not establish link
Using dm9000 device
host 192.168.1.100 is alive

参数设置
set bootargs noinitrd root=/dev/nfs nfsroot=192.168.1.108:/home/hj/work/nfs_root/sys ip=192.168.1.201:192.168.1.108:192.168.1.1:255.255.255.0::eth0:off init=/linuxrc console=ttySAC0,115200

@ubuntu:~/work/system/linux-3.4.2$ showmount -e
Export list for ubuntu:
/home/hj/work/nfs_root/sys *
在本地能够挂载
@ubuntu:~$ sudo mount -t nfs 192.168.1.108:/home/hj/work/nfs_root/sys /home/hj/Downloads/
@ubuntu:~$ cd ~/Downloads/
@ubuntu:~/Downloads$ ls
bin  dev  etc  lib  linuxrc  proc  ***in  usr
有人知道这是什么原因吗