上一篇我们已经成功将 ARM Linux 4.7.3 的内核利用 U-BOOT 引导了起来。但是细心的你会发现，引导到后面，系统无法启动，出现内核恐慌 （Kernel Panic）。 原因是找不到文件系统。为了让内核成功启动，我们还需要构建一个根文件系统。为了后期开发的方便，我们采用 NFS 网络文件系统。

利用 BusyBox 构建 mini 根文件系统

• 什么是 BusyBox ？

BusyBox 是一个遵循GPL协议、以自由软件形式发行的应用程序。Busybox在单一的可执行文件中提供了精简的Unix工具集，可运行于多款POSIX环境的操作系统，例如Linux（包括Android、Hurd、FreeBSD等等。 由于BusyBox可执行文件尺寸小、并通常使用 Linux内核，这使得它非常适合使用于嵌入式系统。 此外，由于BusyBox功能强大，因此有些人将 BusyBox 称为“嵌入式Linux的瑞士军刀”。

• 下载最新 BusyBox 源码

$ wget -c http://busybox.net/downloads/busybox-1.25.0.tar.bz2 $ tar xvf busybox-1.25.0.tar.bz2 $ cd busybox-1.25.0

• 配置编译 BusyBox

$ make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- defconfig $ make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- menuconfig

BusyBox Setting >> Build Options 勾选编译成静态文件。

$ make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- $ make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- install $ cd _install $ mkdir proc sys dev etc etc/init.d $ touch etc/init.d/rcS $ cat << EOF > etc/init.d/rcS #!bin/sh mount -t proc none /proc mount -t sysfs none /sys /sbin/mdev -s EOF $ chmod +x etc/init.d/rcS $ sudo mkdir /rootfs $ sudo pax -r -w -p e . /rootfs $ sudo chmod 777 /rootfs -R $ sudo chown -R nobody:nobody /rootfs

到这里，根文件系统的内容已经准备好了，接下来配置 NFS 。

NFS 服务配置

$ sudo apt install nfs-kernel-server $ sudo touch /etc/exports $ sudo cat << EOF > /etc/exports /rootfs *(rw,sync,no_root_squash,no_subtree_check) EOF $ sudo service start nfs-kernel-server.service

OK, 到这里 NFS 服务配置完毕。

最后的准备

• 确保 Kernel 配置了 NFS 支持模块。

File systems ---> [*] Network File Systems ---> <*> NFS client support <*> NFS client support for NFS version 3 [*] NFS client support for the NFSv3 ACL protocol extension [*] Root file system on NFS

• 修改 U-BOOT 传递给内核的引导参数

想要开机就生效，请参看本系列文章（一）,在编译 U-BOOT 时配置。

setenv bootargs 'root=/dev/nfs rw nfsroot=192.168.0.3:/rootfs init=/linuxrc console=ttyAMA0 ip=192.168.0.5'

启动系统

$ sudo qemu-system-arm -M vexpress-a9 -m 128M -nographic -net nic -net tap,ifname=tap0 -kernel u-boot

U-Boot 2016.07 (Sep 16 2016 - 11:18:06 +0800) DRAM: 128 MiB WARNING: Caches not enabled Flash: 128 MiB MMC: MMC: 0 *** Warning - bad CRC, using default environment In: serial Out: serial Err: serial Net: smc911x-0 Hit any key to stop autoboot: 0 => setenv bootargs 'root=/dev/nfs rw nfsroot=192.168.0.3:/home/rain/rootfs init=/linuxrc console=ttyAMA0 ip=192.168.0.5' => tftp 0x62000000 kernel smc911x: MAC 52:54:00:12:34:56 smc911x: detected LAN9118 controller smc911x: phy initialized smc911x: MAC 52:54:00:12:34:56 Using smc911x-0 device TFTP from server 192.168.0.3; our IP address is 192.168.0.5 Filename 'kernel'. Load address: 0x62000000 Loading: ################################################################# ################################################################# ################################################################# ################################################################# ################################################################# ################################################################# ################################################################# ################################################################# ################################################################# ################################################################# ################################################ 1.6 MiB/s done Bytes transferred = 3571036 (367d5c hex) smc911x: MAC 52:54:00:12:34:56 => bootz 0x62000000 Kernel image @ 0x62000000 [ 0x000000 - 0x3643e8 ] Starting kernel ... Uncompressing Linux... done, booting the kernel. Booting Linux on physical CPU 0x0 Linux version 4.7.3 (rain@rain-pc) (gcc version 5.4.0 20160609 (Ubuntu/Linaro 5.4.0-6ubuntu1~16.04.1) ) #7 SMP Fri Sep 16 11:11:23 CST 2016 CPU: ARMv7 Processor [410fc090] revision 0 (ARMv7), cr=10c5387d CPU: PIPT / VIPT nonaliasing data cache, VIPT nonaliasing instruction cache Machine model: V2P-CA9 Memory policy: Data cache writeback CPU: All CPU(s) started in SVC mode. percpu: Embedded 13 pages/cpu @87eb8000 s23116 r8192 d21940 u53248 Built 1 zonelists in Zone order, mobility grouping on. Total pages: 32512 Kernel command line: root=/dev/nfs rw nfsroot=192.168.0.3:/home/rain/rootfs init=/linuxrc console=ttyAMA0 ip=192.168.0.5 log_buf_len individual max cpu contribution: 4096 bytes log_buf_len total cpu_extra contributions: 12288 bytes log_buf_len min size: 16384 bytes log_buf_len: 32768 bytes early log buf free: 14944(91%) PID hash table entries: 512 (order: -1, 2048 bytes) Dentry cache hash table entries: 16384 (order: 4, 65536 bytes) Inode-cache hash table entries: 8192 (order: 3, 32768 bytes) Memory: 121092K/131072K available (5261K kernel code, 161K rwdata, 1424K rodata, 1024K init, 155K bss, 9980K reserved, 0K cma-reserved) Virtual kernel memory layout: vector : 0xffff0000 - 0xffff1000 ( 4 kB) fixmap : 0xffc00000 - 0xfff00000 (3072 kB) vmalloc : 0x88800000 - 0xff800000 (1904 MB) lowmem : 0x80000000 - 0x88000000 ( 128 MB) modules : 0x7f000000 - 0x80000000 ( 16 MB) .text : 0x80008000 - 0x807875c0 (7678 kB) .init : 0x80800000 - 0x80900000 (1024 kB) .data : 0x80900000 - 0x809284a0 ( 162 kB) .bss : 0x8092a000 - 0x80950cc4 ( 156 kB) SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=4, Nodes=1 Hierarchical RCU implementation. Build-time adjustment of leaf fanout to 32. RCU restricting CPUs from NR_CPUS=8 to nr_cpu_ids=4. RCU: Adjusting geometry for rcu_fanout_leaf=32, nr_cpu_ids=4 NR_IRQS:16 nr_irqs:16 16 GIC CPU mask not found - kernel will fail to boot. GIC CPU mask not found - kernel will fail to boot. L2C: platform modifies aux control register: 0x02020000 -> 0x02420000 L2C: DT/platform modifies aux control register: 0x02020000 -> 0x02420000 L2C-310 enabling early BRESP for Cortex-A9 L2C-310 full line of zeros enabled for Cortex-A9 L2C-310 dynamic clock gating disabled, standby mode disabled L2C-310 cache controller enabled, 8 ways, 128 kB L2C-310: CACHE_ID 0x410000c8, AUX_CTRL 0x46420001 smp_twd: clock not found -2 sched_clock: 32 bits at 24MHz, resolution 41ns, wraps every 89478484971ns clocksource: arm,sp804: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 1911260446275 ns Console: colour dummy device 80x30 Calibrating local timer... 92.79MHz. Calibrating delay loop... 516.09 BogoMIPS (lpj=2580480) pid_max: default: 32768 minimum: 301 Mount-cache hash table entries: 1024 (order: 0, 4096 bytes) Mountpoint-cache hash table entries: 1024 (order: 0, 4096 bytes) CPU: Testing write buffer coherency: ok CPU0: thread -1, cpu 0, socket 0, mpidr 80000000 Setting up static identity map for 0x60100000 - 0x60100058 Brought up 1 CPUs SMP: Total of 1 processors activated (516.09 BogoMIPS). CPU: All CPU(s) started in SVC mode. devtmpfs: initialized VFP support v0.3: implementor 41 architecture 3 part 30 variant 9 rev 0 clocksource: jiffies: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 19112604462750000 ns NET: Registered protocol family 16 DMA: preallocated 256 KiB pool for atomic coherent allocations cpuidle: using governor ladder of_amba_device_create(): amba_device_add() failed (-19) for /memory-controller@100e0000 of_amba_device_create(): amba_device_add() failed (-19) for /memory-controller@100e1000 of_amba_device_create(): amba_device_add() failed (-19) for /watchdog@100e5000 of_amba_device_create(): amba_device_add() failed (-19) for /smb@04000000/motherboard/iofpga@7,00000000/sysctl@01000 of_amba_device_create(): amba_device_add() failed (-19) for /smb@04000000/motherboard/iofpga@7,00000000/wdt@0f000 hw-breakpoint: debug architecture 0x4 unsupported. Serial: AMBA PL011 UART driver 10009000.uart: ttyAMA0 at MMIO 0x10009000 (irq = 35, base_baud = 0) is a PL011 rev1 console [ttyAMA0] enabled 1000a000.uart: ttyAMA1 at MMIO 0x1000a000 (irq = 36, base_baud = 0) is a PL011 rev1 1000b000.uart: ttyAMA2 at MMIO 0x1000b000 (irq = 37, base_baud = 0) is a PL011 rev1 1000c000.uart: ttyAMA3 at MMIO 0x1000c000 (irq = 38, base_baud = 0) is a PL011 rev1 SCSI subsystem initialized usbcore: registered new interface driver usbfs usbcore: registered new interface driver hub usbcore: registered new device driver usb Advanced Linux Sound Architecture Driver Initialized. clocksource: Switched to clocksource arm,sp804 NET: Registered protocol family 2 TCP established hash table entries: 1024 (order: 0, 4096 bytes) TCP bind hash table entries: 1024 (order: 1, 8192 bytes) TCP: Hash tables configured (established 1024 bind 1024) UDP hash table entries: 256 (order: 1, 8192 bytes) UDP-Lite hash table entries: 256 (order: 1, 8192 bytes) NET: Registered protocol family 1 RPC: Registered named UNIX socket transport module. RPC: Registered udp transport module. RPC: Registered tcp transport module. RPC: Registered tcp NFSv4.1 backchannel transport module. hw perfevents: enabled with armv7_cortex_a9 PMU driver, 1 counters available futex hash table entries: 1024 (order: 4, 65536 bytes) workingset: timestamp_bits=29 max_order=15 bucket_order=0 squashfs: version 4.0 (2009/01/31) Phillip Lougher jffs2: version 2.2. (NAND) © 2001-2006 Red Hat, Inc. 9p: Installing v9fs 9p2000 file system support io scheduler noop registered (default) clcd-pl11x 10020000.clcd: PL111 rev2 at 0x10020000 clcd-pl11x 10020000.clcd: /clcd@10020000 hardware, 1024x768@59 display Console: switching to colour frame buffer device 128x48 clcd-pl11x 1001f000.clcd: PL111 rev2 at 0x1001f000 clcd-pl11x 1001f000.clcd: /smb@04000000/motherboard/iofpga@7,00000000/clcd@1f000 hardware, 640x480@59 display 40000000.flash: Found 2 x16 devices at 0x0 in 32-bit bank. Manufacturer ID 0x000000 Chip ID 0x000000 Intel/Sharp Extended Query Table at 0x0031 Using buffer write method 40000000.flash: Found 2 x16 devices at 0x0 in 32-bit bank. Manufacturer ID 0x000000 Chip ID 0x000000 Intel/Sharp Extended Query Table at 0x0031 Using buffer write method Concatenating MTD devices: (0): "40000000.flash" (1): "40000000.flash" into device "40000000.flash" libphy: smsc911x-mdio: probed Generic PHY 4e000000.etherne:01: attached PHY driver [Generic PHY] (mii_bus:phy_addr=4e000000.etherne:01, irq=-1) smsc911x 4e000000.ethernet eth0: MAC Address: 52:54:00:12:34:56 isp1760 4f000000.usb: bus width: 32, oc: digital isp1760 4f000000.usb: NXP ISP1760 USB Host Controller isp1760 4f000000.usb: new USB bus registered, assigned bus number 1 isp1760 4f000000.usb: Scratch test failed. isp1760 4f000000.usb: can't setup: -19 isp1760 4f000000.usb: USB bus 1 deregistered usbcore: registered new interface driver usb-storage mousedev: PS/2 mouse device common for all mice rtc-pl031 10017000.rtc: rtc core: registered pl031 as rtc0 mmci-pl18x 10005000.mmci: Got CD GPIO mmci-pl18x 10005000.mmci: Got WP GPIO mmci-pl18x 10005000.mmci: mmc0: PL181 manf 41 rev0 at 0x10005000 irq 31,32 (pio) ledtrig-cpu: registered to indicate activity on CPUs usbcore: registered new interface driver usbhid usbhid: USB HID core driver input: AT Raw Set 2 keyboard as /devices/platform/smb@04000000/smb@04000000:motherboard/smb@04000000:motherboard:iofpga@7,00000000/10006000.kmi/serio0/input/input0 aaci-pl041 10004000.aaci: ARM AC'97 Interface PL041 rev0 at 0x10004000, irq 30 aaci-pl041 10004000.aaci: FIFO 512 entries oprofile: using arm/armv7-ca9 NET: Registered protocol family 17 9pnet: Installing 9P2000 support Registering SWP/SWPB emulation handler rtc-pl031 10017000.rtc: setting system clock to 2016-09-19 05:56:32 UTC (1474264592) smsc911x 4e000000.ethernet eth0: SMSC911x/921x identified at 0x892a0000, IRQ: 28 input: ImExPS/2 BYD TouchPad as /devices/platform/smb@04000000/smb@04000000:motherboard/smb@04000000:motherboard:iofpga@7,00000000/10007000.kmi/serio1/input/input2 IP-Config: Guessing netmask 255.255.255.0 IP-Config: Complete: device=eth0, hwaddr=52:54:00:12:34:56, ipaddr=192.168.0.5, mask=255.255.255.0, gw=255.255.255.255 host=192.168.0.5, domain=, nis-domain=(none) bootserver=255.255.255.255, rootserver=192.168.0.3, rootpath= ALSA device list: #0: ARM AC'97 Interface PL041 rev0 at 0x10004000, irq 30 VFS: Mounted root (nfs filesystem) on device 0:12. Freeing unused kernel memory: 1024K (80800000 - 80900000) nfs: server 192.168.0.3 not responding, still trying nfs: server 192.168.0.3 OK Please press Enter to activate this console. / # ls bin dev etc linuxrc proc sbin sys usr / # ip a 1: lo:  mtu 65536 qdisc noqueue qlen 1 link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 inet 127.0.0.1/8 scope host lo valid_lft forever preferred_lft forever 2: eth0:  mtu 1500 qdisc pfifo_fast qlen 1000 link/ether 52:54:00:12:34:56 brd ff:ff:ff:ff:ff:ff inet 192.168.0.5/24 brd 192.168.0.255 scope global eth0 valid_lft forever preferred_lft forever / # uname -a Linux 192.168.0.5 4.7.3 #7 SMP Fri Sep 16 11:11:23 CST 2016 armv7l GNU/Linux / #