initramfs在编译内核的同时被编译并与内核生成一个映像文件，可以压缩也可以不压缩，但是目前只支持cpio包格式。它是根文件系统制作和制作的一种非常简单的方法，也可以通过执行这个文件系统中的程序引导真正的文件系统，这样加载根文件系统的工作就不是内核的工作，而是initramfs的工作。由于initramfs使用cpio包格式，所以很容易将一个单一的文件、目录、node编译链接到系统中去，这样很简单的系统中使用起来很方便，不需要另外挂接文件系统。
但是因为cpio包实际是文件、目录、节点的描述语言包，为了描述一个文件、目录、节点，要增加很多额外的描述文字开销，特别是对于目录和节点，本身很小额外添加的描述文字却很多，这样使得cpio包比相应的image文件大很多。
由于制作initramfs文件系统启动时会在根文件系统中执行第一个init程序，它对uboot传过来的参数init=/linuxrc不予理睬，所以需要在上面制作的根文件系统里的根目录加入init程序，否则无法启动。init程序和linuxrc程序一样都是符号链接文件，它们都是指向/bin/busybox程序。
[fanmaolin@Centeros ~]$ cd fl2440/kernel/linux-3.0
[fanmaolin@Centeros linux-3.0]$ export TERM=vt100
[fanmaolin@Centeros linux-3.0]$ make menuconfig
General setup --->
[*] Initial RAM filesystem and RAM disk (initramfs/initrd) support
(/home/fanmaolin/fl2440/roofts/roofts) Initramfs source file(s)
make
添加uboot对initramfs支持
set bkr 'tftp 30008000 linuxrom-s3c2440.bin;nand erase 100000 800000;nand write 30008000 100000 800000'
set bootcmd_initramdisk 'nand read 30008000 100000 800000;bootm 30008000'
set bootargs 'console=ttyS0,115200 mem=64M init=/linuxrc rw loglevel=7'
set bootcmd 'run bootcmd_initramdisk'
save
[fl2440@lingyun]# run bkr
dm9000 i/o: 0x20000300, id: 0x90000a46
DM9000: running in 16 bit mode
MAC: 08:00:3e:26:0a:5b
could not establish link
operating at 100M full duplex mode
Using dm9000 device
TFTP from server 192.168.1.2; our IP address is 192.168.1.168
Filename 'linuxrom-s3c2440.bin'.
Load address: 0x30008000
Loading: T T T T T T #################################################################
#################################################################
#################################################################
#################################################################
###########
done
Bytes transferred = 3967816 (3c8b48 hex)
NAND erase: device 0 offset 0x100000, size 0x800000
Erasing at 0x8e0000 -- 100% complete.
OK
NAND write: device 0 offset 0x100000, size 0x800000
8388608 bytes written: OK
[fl2440@lingyun]# boot
NAND read: device 0 offset 0x100000, size 0x400000
4194304 bytes read: OK
## Booting kernel from Legacy Image at 30008000 ...
Image Name: Linux kernel
Created: 2017-03-13 19:19:41 UTC
Image Type: ARM Linux Kernel Image (uncompressed)
Data Size: 3967752 Bytes = 3.8 MiB
Load Address: 30008000
Entry Point: 30008040
Verifying Checksum ... OK
XIP Kernel Image ... OK
OK
Starting kernel ...
Uncompressing Linux... done, booting the kernel.
Linux version 3.0.0 (fanmaolin@Centeros.ocaldomain) (gcc version 4.5.4 (Buildroot 2012.08) ) #8 Tue Mar 14 03:19:22 CST 2017
CPU: ARM920T [41129200] revision 0 (ARMv4T), cr=c0007177
CPU: VIVT data cache, VIVT instruction cache
Machine: SMDK2440
Memory policy: ECC disabled, Data cache writeback
CPU S3C2440A (id 0x32440001)
S3C24XX Clocks, Copyright 2004 Simtec Electronics
S3C244X: core 405.600 MHz, memory 101.400 MHz, peripheral 50.700 MHz
CLOCK: Slow mode (1.500 MHz), fast, MPLL on, UPLL on
Built 1 zonelists in Zone order, mobility grouping on. Total pages: 16256
Kernel command line: console=ttyS0,115200 mem=64M ubi.mtd=2 root=ubi0:rootfs rootwait rootfstype=ubifs rw
PID hash table entries: 256 (order: -2, 1024 bytes)
Dentry cache hash table entries: 8192 (order: 3, 32768 bytes)
Inode-cache hash table entries: 4096 (order: 2, 16384 bytes)
Memory: 64MB = 64MB total
Memory: 56904k/56904k available, 8632k reserved, 0K highmem
Virtual kernel memory layout:
vector : 0xffff0000 - 0xffff1000 ( 4 kB)
fixmap : 0xfff00000 - 0xfffe0000 ( 896 kB)
DMA : 0xffc00000 - 0xffe00000 ( 2 MB)
vmalloc : 0xc4800000 - 0xf6000000 ( 792 MB)
lowmem : 0xc0000000 - 0xc4000000 ( 64 MB)
modules : 0xbf000000 - 0xc0000000 ( 16 MB)
.init : 0xc0008000 - 0xc032a000 (3208 kB)
.text : 0xc032a000 - 0xc0771c3c (4384 kB)
.data : 0xc0772000 - 0xc07966e0 ( 146 kB)
.bss : 0xc0796704 - 0xc07d156c ( 236 kB)
SLUB: Genslabs=13, HWalign=32, Order=0-3, MinObjects=0, CPUs=1, Nodes=1
NR_IRQS:85
irq: clearing pending ext status 00000800
irq: clearing subpending status 00000003
irq: clearing subpending status 00000002
Console: colour dummy device 80x30
console [ttyS0] enabled
Calibrating delay loop... 202.54 BogoMIPS (lpj=506368)
pid_max: default: 32768 minimum: 301
Mount-cache hash table entries: 512
CPU: Testing write buffer coherency: ok
gpiochip_add: gpios 288..303 (GPIOK) failed to register
gpiochip_add: gpios 320..334 (GPIOL) failed to register
gpiochip_add: gpios 352..353 (GPIOM) failed to register
NET: Registered protocol family 16
S3C Power Management, Copyright 2004 Simtec Electronics
S3C2440: Initialising architecture
S3C2440: IRQ Support
S3C244X: Clock Support, DVS off
bio: create slab <bio-0> at 0
usbcore: registered new interface driver usbfs
usbcore: registered new interface driver hub
usbcore: registered new device driver usb
s3c-i2c s3c2440-i2c: slave address 0x10
s3c-i2c s3c2440-i2c: bus frequency set to 99 KHz
s3c-i2c s3c2440-i2c: i2c-0: S3C I2C adapter
Advanced Linux Sound Architecture Driver Version 1.0.24.
NET: Registered protocol family 2
IP route cache hash table entries: 1024 (order: 0, 4096 bytes)
TCP established hash table entries: 2048 (order: 2, 16384 bytes)
TCP bind hash table entries: 2048 (order: 1, 8192 bytes)
TCP: Hash tables configured (established 2048 bind 2048)
TCP reno registered
UDP hash table entries: 256 (order: 0, 4096 bytes)
UDP-Lite hash table entries: 256 (order: 0, 4096 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.
JFFS2 version 2.2. (NAND) ? 2001-2006 Red Hat, Inc.
ROMFS MTD (C) 2007 Red Hat, Inc.
msgmni has been set to 111
io scheduler noop registered
io scheduler deadline registered
io scheduler cfq registered (default)
Console: switching to colour frame buffer device 60x53
fb0: s3c2410fb frame b-Boot 2010.09 (Mar 08 2017 - 10:15:52)
DRAM: 64 MiB
NAND: 256 MiB
In: serial
Out: serial
Err: serial
Net: dm9000
Hit any key to stop autoboot: 0
NAND read: device 0 offset 0x100000, size 0x400000
4194304 bytes read: OK
## Booting kernel from Legacy Image at 30008000 ...
Image Name: Linux kernel
Created: 2017-03-13 19:19:41 UTC
Image Type: ARM Linux Kernel Image (uncompressed)
Data Size: 3967752 Bytes = 3.8 MiB
Load Address: 30008000
Entry Point: 30008040
Verifying Checksum ... OK
XIP Kernel Image ... OK
OK
Starting kernel ...
Uncompressing Linux... done, booting the kernel.
Linux version 3.0.0 (fanmaolin@Centeros.ocaldomain) (gcc version 4.5.4 (Buildroot 2012.08) ) #8 Tue Mar 14 03:19:22 CST 2017
CPU: ARM920T [41129200] revision 0 (ARMv4T), cr=c0007177
CPU: VIVT data cache, VIVT instruction cache
Machine: SMDK2440
Memory policy: ECC disabled, Data cache writeback
CPU S3C2440A (id 0x32440001)
S3C24XX Clocks, Copyright 2004 Simtec Electronics
S3C244X: core 405.600 MHz, memory 101.400 MHz, peripheral 50.700 MHz
CLOCK: Slow mode (1.500 MHz), fast, MPLL on, UPLL on
Built 1 zonelists in Zone order, mobility grouping on. Total pages: 16256
Kernel command line: console=ttyS0,115200 mem=64M ubi.mtd=2 root=ubi0:rootfs rootwait rootfstype=ubifs rw
PID hash table entries: 256 (order: -2, 1024 bytes)
Dentry cache hash table entries: 8192 (order: 3, 32768 bytes)
Inode-cache hash table entries: 4096 (order: 2, 16384 bytes)
Memory: 64MB = 64MB total
Memory: 56904k/56904k available, 8632k reserved, 0K highmem
Virtual kernel memory layout:
vector : 0xffff0000 - 0xffff1000 ( 4 kB)
fixmap : 0xfff00000 - 0xfffe0000 ( 896 kB)
DMA : 0xffc00000 - 0xffe00000 ( 2 MB)
vmalloc : 0xc4800000 - 0xf6000000 ( 792 MB)
lowmem : 0xc0000000 - 0xc4000000 ( 64 MB)
modules : 0xbf000000 - 0xc0000000 ( 16 MB)
.init : 0xc0008000 - 0xc032a000 (3208 kB)
.text : 0xc032a000 - 0xc0771c3c (4384 kB)
.data : 0xc0772000 - 0xc07966e0 ( 146 kB)
.bss : 0xc0796704 - 0xc07d156c ( 236 kB)
SLUB: Genslabs=13, HWalign=32, Order=0-3, MinObjects=0, CPUs=1, Nodes=1
NR_IRQS:85
irq: clearing subpending status 00000002
Console: colour dummy device 80x30
console [ttyS0] enabled
Calibrating delay loop... 202.54 BogoMIPS (lpj=506368)
pid_max: default: 32768 minimum: 301
Mount-cache hash table entries: 512
CPU: Testing write buffer coherency: ok
gpiochip_add: gpios 288..303 (GPIOK) failed to register
gpiochip_add: gpios 320..334 (GPIOL) failed to register
gpiochip_add: gpios 352..353 (GPIOM) failed to register
NET: Registered protocol family 16
S3C Power Management, Copyright 2004 Simtec Electronics
S3C2440: Initialising architecture
S3C2440: IRQ Support
S3C244X: Clock Support, DVS off
bio: create slab <bio-0> at 0
usbcore: registered new interface driver usbfs
usbcore: registered new interface driver hub
usbcore: registered new device driver usb
s3c-i2c s3c2440-i2c: slave address 0x10
s3c-i2c s3c2440-i2c: bus frequency set to 99 KHz
s3c-i2c s3c2440-i2c: i2c-0: S3C I2C adapter
Advanced Linux Sound Architecture Driver Version 1.0.24.
NET: Registered protocol family 2
IP route cache hash table entries: 1024 (order: 0, 4096 bytes)
TCP established hash table entries: 2048 (order: 2, 16384 bytes)
TCP bind hash table entries: 2048 (order: 1, 8192 bytes)
TCP: Hash tables configured (established 2048 bind 2048)
TCP reno registered
UDP hash table entries: 256 (order: 0, 4096 bytes)
UDP-Lite hash table entries: 256 (order: 0, 4096 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.
JFFS2 version 2.2. (NAND) ? 2001-2006 Red Hat, Inc.
ROMFS MTD (C) 2007 Red Hat, Inc.
msgmni has been set to 111
io scheduler noop registered
io scheduler deadline registered
io scheduler cfq registered (default)
Console: switching to colour frame buffer device 60x53
fb0: s3c2410fb frame buffer device
s3c2440-uart.0: ttyS0 at MMIO 0x50000000 (irq = 70) is a S3C2440
s3c2440-uart.1: ttyS1 at MMIO 0x50004000 (irq = 73) is a S3C2440
s3c2440-uart.2: ttyS2 at MMIO 0x50008000 (irq = 76) is a S3C2440
brd: module loaded
S3C24XX NAND Driver, (c) 2004 Simtec Electronics
s3c24xx-nand s3c2440-nand: Tacls=3, 29ns Twrph0=7 69ns, Twrph1=3 29ns
s3c24xx-nand s3c2440-nand: NAND soft ECC
NAND device: Manufacturer ID: 0xec, Chip ID: 0xda (Samsung NAND 256MiB 3,3V 8-bit)
Scanning device for bad blocks
Bad eraseblock 651 at 0x000005160000
Bad eraseblock 825 at 0x000006720000
Bad eraseblock 1667 at 0x00000d060000
Bad eraseblock 1787 at 0x00000df60000
Bad eraseblock 1879 at 0x00000eae0000
Creating 5 MTD partitions on "NAND":
0x000000000000-0x000000100000 : "bootloader"
ftl_cs: FTL header not found.
0x000000100000-0x000001000000 : "linux"
ftl_cs: FTL header not found.
0x000001000000-0x000003800000 : "rootfs"
ftl_cs: FTL header not found.
0x000003800000-0x000009c00000 : "apps"
ftl_cs: FTL header not found.
0x000000000000-0x000010000000 : "data"
ftl_cs: FTL header not found.
UBI: attaching mtd2 to ubi0
UBI: physical eraseblock size: 131072 bytes (128 KiB)
UBI: logical eraseblock size: 129024 bytes
UBI: smallest flash I/O unit: 2048
UBI: sub-page size: 512
UBI: VID header offset: 512 (aligned 512)
UBI: data offset: 2048
UBI: max. sequence number: 30
UBI: attached mtd2 to ubi0
UBI: MTD device name: "rootfs"
UBI: MTD device size: 40 MiB
UBI: number of good PEBs: 320
UBI: number of bad PEBs: 0
UBI: number of corrupted PEBs: 0
UBI: max. allowed volumes: 128
UBI: wear-leveling threshold: 4096
UBI: number of internal volumes: 1
UBI: number of user volumes: 1
UBI: available PEBs: 0
UBI: total number of reserved PEBs: 320
UBI: number of PEBs reserved for bad PEB handling: 3
UBI: max/mean erase counter: 2/0
UBI: image sequence number: 1869742398
UBI: background thread "ubi_bgt0d" started, PID 675
dm9000 Ethernet Driver, V1.31
ohci_hcd: USB 1.1 'Open' Host Controller (OHCI) Driver
s3c2410-ohci s3c2410-ohci: S3C24XX OHCI
s3c2410-ohci s3c2410-ohci: new USB bus registered, assigned bus number 1
s3c2410-ohci s3c2410-ohci: irq 42, io mem 0x49000000
hub 1-0:1.0: USB hub found
hub 1-0:1.0: 2 ports detected
usbcore: registered new interface driver libusual
mousedev: PS/2 mouse device common for all mice
S3C24XX RTC, (c) 2004,2006 Simtec Electronics
i2c /dev entries driver
S3C2410 Watchdog Timer, (c) 2004 Simtec Electronics
s3c2410-wdt s3c2410-wdt: watchdog inactive, reset disabled, irq enabled
cpuidle: using governor ladder
sdhci: Secure Digital Host Controller Interface driver
sdhci: Copyright(c) Pierre Ossman
usbcore: registered new interface driver usbhid
usbhid: USB HID core driver
ALSA device list:
No soundcards found.
TCP cubic registered
NET: Registered protocol family 17
Registering the dns_resolver key type
drivers/rtc/hctosys.c: unable to open rtc device (rtc0)
Freeing init memory: 3208K
usb 1-1: new full speed USB device number 2 using s3c2410-ohci
hub 1-1:1.0: USB hub found
hub 1-1:1.0: 4 ports detected
Copyright (C) 2017 fanmaolin<fanmaolin@gmail.com>
root login: root
>: ls
apps data etc init linuxrc proc sbin tmp var
bin dev info lib mnt root sys usr
>:
大功告成
要注意内核编译时路径的选择