问题：

NFS配置没问题：NFS服务开启与使用、目录挂载与卸载
使用 NFS 挂载文件系统的时候报错：

[  101.800373] VFS: Unable to mount root fs via NFS, trying floppy.
[  101.807335] VFS: Cannot open root device "nfs" or unknown-block(2,0): error -6
[  101.814718] Please append a correct "root=" boot option; here are the available partitions:
[  101.823160] 0100           65536 ram0  (driver?)
[  101.827880] 0101           65536 ram1  (driver?)
[  101.832573] 0102           65536 ram2  (driver?)
[  101.837347] 0103           65536 ram3  (driver?)
[  101.841988] 0104           65536 ram4  (driver?)
[  101.846695] 0105           65536 ram5  (driver?)
[  101.851335] 0106           65536 ram6  (driver?)
[  101.856027] 0107           65536 ram7  (driver?)
[  101.860667] 0108           65536 ram8  (driver?)
[  101.865333] 0109           65536 ram9  (driver?)
[  101.869974] 010a           65536 ram10  (driver?)
[  101.874720] 010b           65536 ram11  (driver?)
[  101.879446] 010c           65536 ram12  (driver?)
[  101.884194] 010d           65536 ram13  (driver?)
[  101.888922] 010e           65536 ram14  (driver?)
[  101.893669] 010f           65536 ram15  (driver?)
[  101.898410] b300        15273984 mmcblk0  driver: mmcblk
[  101.903768]   b301        15269888 mmcblk0p1 9be55837-01
[  101.909104] b308         7634944 mmcblk1  driver: mmcblk
[  101.914503]   b309          131072 mmcblk1p1 5edd22ea-01
[  101.919862]   b30a         7493632 mmcblk1p2 5edd22ea-02
[  101.925247] b320             512 mmcblk1rpmb  (driver?)
[  101.930494] b318            4096 mmcblk1boot1  (driver?)
[  101.935857] b310            4096 mmcblk1boot0  (driver?)
[  101.941192] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(2,0)
[  101.949470] ---[ end Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(2,0)
[  112.942790] random: nonblocking pool is initialized

原因：
以前使用的16.04版本的ubuntu没问题，现在更换了18.04.4版本，开始报错。
从Ubuntu17.04开始，nfs默认只支持协议3和协议4，而kernel中默认支持协议2。

1. 修改 NFS 配置文件：

sudo vim /etc/default/nfs-kernel-server

2.在末尾添加：

RPCNFSDOPTS="--nfs-version 2,3,4 --debug --syslog"

3.重启 NFS 服务：

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

注：如果kernel版本较高支持nfs协议3的话，可以在Uboot传到Kernel的bootargs参数中加入’nfsvers=3’,使kernel使用nfs协议3。

Environment size: 1420/4092 bytes
S5000C#
arch=arm
baudrate=115200
board=s5000c
board_name=s5000c
boot_fdt=booti 0x90000000 - 0x95000000
boot_usb_fdt=booti 0x90000000 - 0x95000000
bootargs=console=ttyAMA1,115200 earlycon=pl011,0x20001000 root=/dev/sda2 rootdelay=10 rw
bootcmd=run distro_bootcmd
bootdelay=3
cpu=armv8
distro_bootcmd=run load_kernel; run load_fdt; run boot_fdt
ethaddr=11:22:33:44:55:66
ext4_read_check=ext4load scsi 0:1 0x92000000 file_bake.txt
ext4_read_test=ext4load scsi 0:1 0x90000000 dtb/ft2004.dtb
ext4_write_test=ext4write scsi 0:1 0x90000000 /file_bake.txt 0x2DA8
fat_read_check=fatload usb 0:1 0x92000000 file_bake.txt
fat_read_test=fatload usb 0:1 0x90000000 md5sum.txt
fat_write_test=fatwrite usb 0:1 0x90000000 /file_bake.txt 0x5D4A
fdtcontroladdr=f9b40a70
fileaddr=95000000
filesize=87b7
gatewayip=192.168.1.1
ipaddr=192.168.1.250
load_fdt=ext4load scsi 0:1 0x95000000 s5000c-64c.dtb
load_kernel=ext4load scsi 0:1 0x90000000 Image
load_usb_fdt=fatload usb 0:1 0x95000000 s5000c-64c.dtb
load_usb_kernel=fatload usb 0:1 0x90000000 Image
loadaddr=0x90000000
netmask=255.255.255.0
serverip=192.168.1.110
soc=s5000c
start_autoscript=if usb start; then run start_usb_autoscript; fi; if scsi info; then run start_ssd_autoscript; fi;
start_ssd_autoscript=run load_kernel; run load_fdt; run boot_fdt
start_usb_autoscript=run load_usb_kernel; run load_usb_fdt; run boot_usb_fdt
stderr=serial
stdin=serial,usbkbd
stdout=serial
vendor=phytium

一：在uboot下的defconfig 打开如下配置

CONFIG_DM=y
CONFIG_DM_GPIO=y
CONFIG_DWAPB_GPIO=y
CONFIG_CMD_GPIO=y

二：重新编译u-boot后会生成cmd：gpio

（板子上电时连续按回车键）进入到板端uboot cmdline下执行” gpio status -a ” 查看板端对应的gpio numbe

三：利用 uboot gpio 命令操作GPIO 做测试

• gpio c 0 ; 将第0根PIN清零（拉低）
• gpio s 0 ; 将第0根PIN设为output同时拉高

四：gpio 操作demo

直接以下demo code添加到uboot/cmd路径下，再在uboot/cmd/Makefile中添加编译选项，编译完成后可以直接操作gpio

#include <command.h>
#include <linux/string.h>
#include <linux/kernel.h>
#include <linux/delay.h>
#include <asm/gpio.h>

int do_gpio_test(struct cmd_tbl *cmdtp, int flag, int argc, char * const argv[])
{
    if (argc < 2) {
        printf("usage: gpio_test [requ/out/on/off]\n");
        return 0;
    }

    if (strcmp("requ", argv[1]) == 0) {
        gpio_request(126, "ir_a");
        gpio_request(127, "ir_b");
        mdelay(10);
    } else if (strcmp("out", argv[1]) == 0) {
        gpio_direction_output(126, 1);
        gpio_direction_output(127, 1);
        mdelay(10);
    } else if (strcmp("on", argv[1]) == 0) {
        gpio_set_value(126, 0);
        gpio_set_value(127, 1);
        mdelay(100);
        gpio_set_value(126, 1);
        gpio_set_value(127, 1);
    } else if (strcmp("off", argv[1]) == 0) {
        gpio_set_value(126, 1);
        gpio_set_value(127, 0);
        mdelay(100);
        gpio_set_value(126, 1);
        gpio_set_value(127, 1);
    }

    return 0;
}

U_BOOT_CMD(
    gpio_test, 4, 1, do_gpio_test,
    "u-boot gpio cmd test",
    "gpio - just for test\n"
);

五、测试

编译完成升级后，进入到uboot会有gpio_test命令
gpio_test requ
初始化gpio
gpio_test out
设置gpio direction
gpio_test on
设置 ir_cur (的两根PIN) 状态