解决linux内核配置成runtime后，无法启动的问题（自己添加的lcd代码的问题导致）
内核版本：linux-3.4.99
bootloader:u-boot-2012-04-01
开发板：mini2440
参考视频：韦东山第三期视频
在韦东山第三期视频中电源管理项目的runtime一节中，有讲到在lcd_init（）函数开头注册lcd_device,lcd_driver后，linux内核启动会出现：
## Booting kernel from Legacy Image at 30000000 ...
Image Name: Linux-3.4.99
Image Type: ARM Linux Kernel Image (uncompressed)
Data Size: 2862672 Bytes = 2.7 MiB
Load Address: 30008000
Entry Point: 30008000
Verifying Checksum ... OK
Loading Kernel Image ... OK
OK
Using machid 0x16a from environment
Starting kernel ...
Uncompressing Linux... done, booting the kernel.
之后内核就停在这里，当时韦老师直接就把错误告诉了我们（lcd_device,lcd_driver应该在lcd的寄存器初始后再调用），实际如果在调试过程中，如果出现这种问题，应该怎么办呢？
此时串口已经没有打印信息，上网百度，谷歌后，关于“Uncompressing Linux... done, booting the kernel.”的问题，网上基本归结为两类：
1.uboot传入给linux的参数不对（机器码等等）。
2.串口配置不对。
照着这个思路，网上溜一圈，出现了各种解决方法，盲目的去找哪种方法显然是不科学的。
那我就慢慢的从linux内核开始，一步步去定位出错的地方：
1.首先确定linux_kernel有没有启动：在booting the kernel 之后Kernel 最先执行的是start_kernel() 函数，确认start_kernel() 有否执行就是在其开始代码段添加printascii(“start_kernel …”) ，如果串口没有打印出start_kernel …，说明start_kernel() 没有运行。测试代码如下（init/main.c）：
extern void printascii(const char*); // Modify
asmlinkage void __init start_kernel(void)
{
char * command_line;
extern struct kernel_param __start___param[], __stop___param[];
printascii("[tony]:start linux_kernel …"); // Modify
smp_setup_processor_id();
…
编译内核，下载，发现：
## Booting kernel from Legacy Image at 30000000 ...
Image Name: Linux-3.4.99
Image Type: ARM Linux Kernel Image (uncompressed)
Data Size: 2862672 Bytes = 2.7 MiB
Load Address: 30008000
Entry Point: 30008000
Verifying Checksum ... OK
Loading Kernel Image ... OK
OK
Using machid 0x16a from environment
Starting kernel ...
Uncompressing Linux... done, booting the kernel.
[tony]:start linux_kernel
好了，可以确定启动了start_kernel()
2.接下来,使用printascii() 函数打印printk() 缓存信息 :
extern void printascii(const char*); // Modify
static char printk_buf[1024]; // Modify
asmlinkage int printk(const char *fmt, ...)
{
va_list args;
int r;
va_start(args, fmt);
r = vprintk(fmt, args);
va_end(args);
printascii(printk_buf); // Modify
return r;
}
…
static int recursion_bug;
static int new_text_line = 1;
//static char printk_buf[1024]; // Modify
编译内核，下载，重启：
## Booting kernel from Legacy Image at 30000000 ...
Image Name: Linux-3.4.99
Image Type: ARM Linux Kernel Image (uncompressed)
Data Size: 2862272 Bytes = 2.7 MiB
Load Address: 30008000
Entry Point: 30008000
Verifying Checksum ... OK
Loading Kernel Image ... OK
OK
Using machid 0x16a from environment
Starting kernel ...
Uncompressing Linux... done, booting the kernel.
[tony]:start linux_kernel<6>Booting Linux on physical CPU 0
<5>Linux version 3.4.99 (root@tony-virtual-machine) (gcc version 4.3.2 (Sourcery G++ Lite 2008q3-72) ) #49 Mon Nov 24 19:08:40 CST 2014
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)
<6>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
<7>On node 0 totalpages: 16384
<7>free_area_init_node: node 0, pgdat c0592f80, node_mem_map c05bb000
<7> Normal zone: 128 pages used for memmap
<7> Normal zone: 0 pages reserved
<7> Normal zone: 16256 pages, LIFO batch:3
<7>pcpu-alloc: s0 r0 d32768 u32768 alloc=1*32768<c>
<7>pcpu-alloc: <c>[0] <c>0 <c>
Built 1 zonelists in Zone order, mobility grouping on. Total pages: 16256
<5>Kernel command line: noinitrd root=/dev/nfs nfsroot=192.168.10.219:/work/tony_fs ip=192.168.10.244:192.168.10.219:192.168.10.1:255.255.255.0::eth0:off console=ttySAC0,115200
<6>PID hash table entries: 256 (order: -2, 1024 bytes)
<6>Dentry cache hash table entries: 8192 (order: 3, 32768 bytes)
<6>Inode-cache hash table entries: 4096 (order: 2, 16384 bytes)
<6>Memory: 64MB = 64MB total
<5>Memory: 59040k/59040k available, 6496k reserved, 0K highmem
<5>Virtual kernel memory layout:
vector : 0xffff0000 - 0xffff1000 ( 4 kB)
fixmap : 0xfff00000 - 0xfffe0000 ( 896 kB)
vmalloc : 0xc4800000 - 0xff000000 ( 936 MB)
lowmem : 0xc0000000 - 0xc4000000 ( 64 MB)
modules : 0xbf000000 - 0xc0000000 ( 16 MB)
.text : 0xc0008000 - 0xc0527d40 (5248 kB)
.init : 0xc0528000 - 0xc0551000 ( 164 kB)
.data : 0xc0552000 - 0xc0593860 ( 263 kB)
.bss : 0xc0593884 - 0xc05badfc ( 158 kB)
<6>NR_IRQS:85
irq: clearing subpending status 00000002
<7>timer tcon=00500000, tcnt a509, tcfg 00000200,00000000, usec 00001e4c
<6>sched_clock: 32 bits at 200 Hz, resolution 5000000ns, wraps every 4294967291ms
<6>Console: colour dummy device 80x30
<6>Calibrating delay loop... <c>201.52 BogoMIPS (lpj=503808)
<6>pid_max: default: 32768 minimum: 301
<6>Mount-cache hash table entries: 512
<6>CPU: Testing write buffer coherency: ok
<6>Setting up static identity map for 0x303cfc78 - 0x303cfcd0
<6>gpiochip_add: registered GPIOs 0 to 23 on device: GPIOA
<6>gpiochip_add: registered GPIOs 32 to 47 on device: GPIOB
<6>gpiochip_add: registered GPIOs 64 to 79 on device: GPIOC
<6>gpiochip_add: registered GPIOs 96 to 111 on device: GPIOD
<6>gpiochip_add: registered GPIOs 128 to 143 on device: GPIOE
<6>gpiochip_add: registered GPIOs 160 to 167 on device: GPIOF
<6>gpiochip_add: registered GPIOs 192 to 207 on device: GPIOG
<6>gpiochip_add: registered GPIOs 224 to 234 on device: GPIOH
<6>gpiochip_add: registered GPIOs 256 to 271 on device: GPIOJ
<6>NET: Registered protocol family 16
S3C Power Management, Copyright 2004 Simtec Electronics
S3C2440: Initialising architecture
S3C2440: IRQ Support
S3C244X: Clock Support, DVS off
S3C24XX DMA Driver, Copyright 2003-2006 Simtec Electronics
DMA channel 0 at c4804000, irq 33
DMA channel 1 at c4804040, irq 34
DMA channel 2 at c4804080, irq 35
DMA channel 3 at c48040c0, irq 36
<6>bio: create slab <bio-0> at 0
<5>SCSI subsystem initialized
<6>usbcore: registered new interface driver usbfs
<6>usbcore: registered new interface driver hub
<6>usbcore: registered new device driver usb
<6>s3c-i2c s3c2440-i2c: slave address 0x10
<6>s3c-i2c s3c2440-i2c: bus frequency set to 99 KHz
<6>s3c-i2c s3c2440-i2c: i2c-0: S3C I2C adapter
<6>Linux video capture interface: v2.00
<6>Advanced Linux Sound Architecture Driver Version 1.0.25.
<6>cfg80211: Calling CRDA to update world regulatory domain
<6>NET: Registered protocol family 2
<6>IP route cache hash table entries: 1024 (order: 0, 4096 bytes)
<6>TCP established hash table entries: 2048 (order: 2, 16384 bytes)
<6>TCP bind hash table entries: 2048 (order: 1, 8192 bytes)
<6>TCP: Hash tables configured (established 2048 bind 2048)
<6>TCP: reno registered
<6>UDP hash table entries: 256 (order: 0, 4096 bytes)
<6>UDP-Lite hash table entries: 256 (order: 0, 4096 bytes)
<6>NET: Registered protocol family 1
<6>RPC: Registered named UNIX socket transport module.
<6>RPC: Registered udp transport module.
<6>RPC: Registered tcp transport module.
<6>RPC: Registered tcp NFSv4.1 backchannel transport module.
<4>NetWinder Floating Point Emulator V0.97 (extended precision)
<6>jffs2: version 2.2. (NAND) (SUMMARY) 漏 2001-2006 Red Hat, Inc.
<6>ROMFS MTD (C) 2007 Red Hat, Inc.
<7>yaffs: yaffs built Nov 24 2014 17:02:32 Installing.
<6>msgmni has been set to 115
<6>io scheduler noop registered
<6>io scheduler deadline registered
<6>io scheduler cfq registered (default)
drivers/video/mini2440_t35.c lcd_suspend 130
<1>Unable to handle kernel NULL pointer dereference at virtual address 00000000
<1>pgd = c0004000
<1>[00000000] *pgd=00000000
<0>Internal error: Oops: 5 [#1] ARM
<d>Modules linked in:
CPU: 0 Not tainted (3.4.99 #49)
PC is at lcd_enable+0x18/0x94
LR is at lcd_suspend+0x28/0x3c
pc : [<c01842cc>] lr : [<c01843b0>] psr: 40000093
sp : c381fe40 ip : 00001342 fp : 00000000
r10: c056d2c4 r9 : c05649d8 r8 : 00000000
r7 : 00000000 r6 : 00000000 r5 : 40000013 r4 : 00000000
r3 : 00000000 r2 : 00000008 r1 : 00000000 r0 : c05b1ed8
Flags: nZcv IRQs off FIQs on Mode SVC_32 ISA ARM Segment kernel
Control: c000717f Table: 30004000 DAC: 00000017
<0>Process swapper (pid: 1, stack limit = 0xc381e270)
<0>Stack: (0xc381fe40 to 0xc3820000)
<0>fe40: c056d218 c056d218 00000000 c01b77e8 00000000 c01bb890 c0594a94 00000000
<0>fe60: c056d294 c381e000 24af4412 00000000 c381feb0 c00e0a70 c381feb0 c381feb0
<0>fe80: c0571700 60000013 24af4412 00000000 00000000 00000000 22222222 60000013
<0>fea0: 00000004 00000000 c056d38c c05b3bf4 c05938a0 00000000 00000000 c01bcc64
<0>fec0: c056d218 c01b7d2c c056d218 c01bbe74 60000013 c056d218 00000000 c01bc008
<0>fee0: 00000001 c01b39d4 c056d218 c056d24c c056d38c c01b3ac4 c38e9ba0 c05938a0
<0>ff00: 00000000 c01b3b50 00000000 c381ff18 c056d38c c01b2360 c38047b8 c38e9c30
<0>ff20: c05500c4 c05500c4 c056d38c c056d38c c05718c8 c01b2aa4 c04ab1b4 00000000
<0>ff40: c056d218 c05500c4 c056d38c 00000007 c381e000 000000bb c01843c4 c01b4134
<0>ff60: c05500c4 c0547d5c 00000007 c381e000 000000bb c01843d8 c05500c4 c0547d5c
<0>ff80: 00000007 c381e000 000000bb c0008560 00000006 00000006 c0504d2c c0490fa8
<0>ffa0: 00000000 c05500c4 c0547d5c 00000007 c05938a0 000000bb c05281ac c0547d68
<0>ffc0: 00000000 c052830c 00000006 00000006 c05281ac 00000000 00000000 c0528218
<0>ffe0: c00162e8 00000013 00000000 00000000 00000000 c00162e8 00000100 00000000
[<c01842cc>] (lcd_enable+0x18/0x94) from [<00000000>] ( (null))
<0>Code: e3853080 e121f003 e5904004 e5903070 (e594c000)
<4>---[ end trace 2d1ab6a7025a992b ]---
<0>Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
好了，之前没打印出来的出错信息都出来。
drivers/video/mini2440_t35.c lcd_suspend 130
<1>Unable to handle kernel NULL pointer dereference at virtual address 00000000
很快就定位到lcd代码的错误。