hi3559A 使用 nfs 加载 rootfs的错误请教

hi3559a 想使用nfs 进行 rootfs启动 ，nfsserver安装了， /etc/exports中加了 nfs rootfs 路径。
在uboot种设置了 启动参数 setenv bootargs 'mem=128M console=ttyAMA0,115200 root=/dev/nfs ip=10.2.0.173:10.2.0.178:10.2.0.1:255.255.255.0::eth0:off rw nfsroot=10.2.0.178:/home/gllou/his/Hi3559A/rootfs init=/linuxrc'。
内核配置也已经加了  Root file system on NFS 。

出现如下的错误信息
hi_gmac_v200 101c0000.ethernet eth0: Link is Up - 100Mbps/Full - flow control rx/tx
IPv6: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready
IP-Config: Complete:
     device=eth0, hwaddr=3e:98:81:bd:bb:17, ipaddr=10.2.0.173, mask=255.255.255.0, gw=10.2.0.1
     host=10.2.0.173, domain=, nis-domain=(none)
     bootserver=10.2.0.178, rootserver=10.2.0.178, rootpath=
uart-pl011 12100000.uart: no DMA platform data
RAMDISK: Couldn't find valid RAM disk image starting at 0.
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:
0100           65536 ram0  (driver?)
0101           65536 ram1  (driver?)
0102           65536 ram2  (driver?)
0103           65536 ram3  (driver?)
0104           65536 ram4  (driver?)
0105           65536 ram5  (driver?)
0106           65536 ram6  (driver?)
0107           65536 ram7  (driver?)
0108           65536 ram8  (driver?)
0109           65536 ram9  (driver?)
010a           65536 ram10  (driver?)
010b           65536 ram11  (driver?)
010c           65536 ram12  (driver?)
010d           65536 ram13  (driver?)
010e           65536 ram14  (driver?)
010f           65536 ram15  (driver?)
1f00          262144 mtdblock0  (driver?)
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(2,0)
CPU: 1 PID: 1 Comm: swapper/0 Not tainted 4.9.37_hi3559av100_big.LITTLE #2
Hardware name: Hisilicon HI3559AV100 DEMO Board (DT)
Call trace:
[] dump_backtrace+0x0/0x198
[] show_stack+0x14/0x20
[] dump_stack+0x94/0xb8
[] panic+0x118/0x268
[] mount_block_root+0x184/0x25c
[] mount_root+0x11c/0x134
[] prepare_namespace+0x16c/0x1a8
[] kernel_init_freeable+0x1c0/0x1e0
[] kernel_init+0x10/0x100
[] ret_from_fork+0x10/0x50
SMP: stopping secondary CPUs
Kernel Offset: disabled
Memory Limit: 128 MB
---[ end Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(2,0)