目标板通过nfs挂载根文件系统

目标板挂载根文件系统的方法有两种（这里所说的服务端就是ubuntu，Ubuntu已经成功安装了nfs服务，并且保证服务端与目标板ping 通）

第一种：等待开发板启动之后去挂载，此时文件系统从Flash中启动，然后手动的通过命令去挂载服务端的文件系统

首先修改配置文件/etc/export，在export文件中最后一行加入：[文件系统的目录]  *(rw,sync,no_subtree_check,no_root_squash)

/home/linux/root_fs/first_fs/  *(rw,sync,no_subtree_check,no_root_squash)///home/linux/root_fs/first_fs/ 为文件系统的目录

整个export文件为

# /etc/exports: the access control list for filesystems which may be exported
#        to NFS clients.  See exports(5).
#
# Example for NFSv2 and NFSv3:
# /srv/homes       hostname1(rw,sync,no_subtree_check) hostname2(ro,sync,no_subtree_check)
#
# Example for NFSv4:
# /srv/nfs4        gss/krb5i(rw,sync,fsid=0,crossmnt,no_subtree_check)
# /srv/nfs4/homes  gss/krb5i(rw,sync,no_subtree_check)
/source/rootfs *(rw,sync,no_subtree_check,no_root_squash)
/home/linux/root_fs/first_fs/  *(rw,sync,no_subtree_check,no_root_squash)

export文件

设置好之后，重新启动nfs服务

sudo /etc/init.d/nfs-kernel-server restart

在服务器段自己测试挂载是否成功

sudo mount -t nfs 192.168.1.24:/home/linux/root_fs/first_fs/ /mnt/

在服务端/mnt/目录下查看挂载到的文件，在服务端调通之后，就进行重要环节，

在目标板上通过nfs进行挂载，命令如下：

mount -t nfs -o nolock 192.168.1.24:/home/linux/root_fs/first_fs/ /mnt/

第二种方法：直接从nfs启动，这样服务端所生成的目标文件，目标端可以直接执行，不需要挂载

这种做法就是在u-boot启动之后，设置nfs挂载参数。首先必须掌握nfs命令，

root=/dev/nfs

  This is necessary to enable the pseudo-NFS-device. Note that it‘s not a
  real device but just a synonym to tell the kernel to use NFS instead of
  a real device.

nfsroot=[<server-ip>:]<root-dir>[,<nfs-options>]

  If the `nfsroot‘ parameter is NOT given on the command line,
  the default "/tftpboot/%s" will be used.

  <server-ip>    Specifies the IP address of the NFS server.
        The default address is determined by the `ip‘ parameter
        (see below). This parameter allows the use of different
        servers for IP autoconfiguration and NFS.

  <root-dir>    Name of the directory on the server to mount as root.
        If there is a "%s" token in the string, it will be
        replaced by the ASCII-representation of the client‘s
        IP address.

  <nfs-options>    Standard NFS options. All options are separated by commas.
        The following defaults are used:
            port        = as given by server portmap daemon
            rsize        = 4096
            wsize        = 4096
            timeo        = 7
            retrans        = 3
            acregmin    = 3
            acregmax    = 60
            acdirmin    = 30
            acdirmax    = 60
            flags        = hard, nointr, noposix, cto, ac

ip=<client-ip>:<server-ip>:<gw-ip>:<netmask>:<hostname>:<device>:<autoconf>

  This parameter tells the kernel how to configure IP addresses of devices
  and also how to set up the IP routing table. It was originally called
  `nfsaddrs‘, but now the boot-time IP configuration works independently of
  NFS, so it was renamed to `ip‘ and the old name remained as an alias for
  compatibility reasons.

  If this parameter is missing from the kernel command line, all fields are
  assumed to be empty, and the defaults mentioned below apply. In general
  this means that the kernel tries to configure everything using
  autoconfiguration.

  The <autoconf> parameter can appear alone as the value to the `ip‘
  parameter (without all the ‘:‘ characters before) in which case auto-
  configuration is used.

  <client-ip>    IP address of the client.

          Default:  Determined using autoconfiguration.

  <server-ip>    IP address of the NFS server. If RARP is used to determine
        the client address and this parameter is NOT empty only
        replies from the specified server are accepted.

        Only required for for NFS root. That is autoconfiguration
        will not be triggered if it is missing and NFS root is not
        in operation.

        Default: Determined using autoconfiguration.
                 The address of the autoconfiguration server is used.

  <gw-ip>    IP address of a gateway if the server is on a different subnet.

        Default: Determined using autoconfiguration.

  <netmask>    Netmask for local network interface. If unspecified
        the netmask is derived from the client IP address assuming
        classful addressing.

        Default:  Determined using autoconfiguration.

  <hostname>    Name of the client. May be supplied by autoconfiguration,
          but its absence will not trigger autoconfiguration.

          Default: Client IP address is used in ASCII notation.

  <device>    Name of network device to use.

        Default: If the host only has one device, it is used.
             Otherwise the device is determined using
             autoconfiguration. This is done by sending
             autoconfiguration requests out of all devices,
             and using the device that received the first reply.

  <autoconf>    Method to use for autoconfiguration. In the case of options
                which specify multiple autoconfiguration protocols,
        requests are sent using all protocols, and the first one
        to reply is used.

        Only autoconfiguration protocols that have been compiled
        into the kernel will be used, regardless of the value of
        this option.

                  off or none: don‘t use autoconfiguration (default)
          on or any:   use any protocol available in the kernel
          dhcp:        use DHCP
          bootp:       use BOOTP
          rarp:        use RARP
          both:        use both BOOTP and RARP but not DHCP
                       (old option kept for backwards compatibility)

                Default: any

nfs命令格式

在u-boot界面，输入print就可以显示如下参数

bootargs=noinitrd root=/dev/mtdblock3 init=/linuxrc console=ttySAC0 

然后根据nfs命令格式，具体格式解析要看nfs命令介绍。

nfsroot=[<server-ip>:]<root-dir>[,<nfs-options>]  ip=<client-ip>:<server-ip>:<gw-ip>:<netmask>:<hostname>:<device>:<autoconf>

将启动参数设置为

set bootargs noinitrd root=/dev/nfs nfsroot=192.168.1.24:/home/linux/root_fs/first_fs/ ip=192.168.1.12:192.168.1.24:192.168.1.1:255.255.255.0::eth0:off init=/linuxrc console=ttySAC0

当文件系统启动之后，服务器端的文件会共享过来。