Add your own code to the kernel:

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How to view the kernel source code:
    1. Delete linux-3.5 -for-look/arch/arm/ Code for everything about Samsung. Because this kernel supports a lot of chips, we learned arm. When querying, it prevents the code of other platforms from being queried, so simplify the code.
    2.ctags -R .
    3.vim -t memcpy
    4.vim command line mode: cstag memcpy
    5.ctrl + ] : enter
    6.ctrl + o : exit

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Add your own code in the kernel:
    1. The driver can be directly compiled into the kernel.
    2. Or compile it into a module, and then install it when needed.
    3.mkdir linux-3.5/Driver/mydriver
    4.Create a few files: my.c Makefile
    code in my.c:
          1 #include <linux/init.h>
          2 #include <linux/module.h>
          3 # include <linux/sched.h>
          4 //Executed during the kernel startup process, and the function type executed during the kernel startup process is fixed: int test_init(void): The return value and line >>
            parameters are fixed.
          5 //init segment: This function is placed in the init segment. The advantage is that after the code is executed, the function body is released, saving some memory. >> can only be called
            once. Don't call it later.
          6 static __init int test_init(void)
          7 {
          8 int i;
          9 //If the resource application fails, will the beggar make a corresponding error
         10 //      
         11 //return -ENOMEM; memory application failed
         12 for(i=0;i< 20;i++)
         13 {
         14 printk("hello kernel\n");
         15 }
         16 //printf("") //>> 1
         17 return 0;
         18
         19 }
         20 //Put the test_init function into the kernel startup process and execute
         21 module_init(test_init);
         22 MODULE_LICENSE("GPL"); //This is the code that follows the GPL open source rules, and whoever uses the code we wrote in the future must also Follow the GPL rules.
         23 //The following three lines are optional
         24 MODULE_AUTHOR("LIUYE");
         25 MODULE_VERSION("V1.0");    
            26 MODULE_DESCRIPTION("test for class");

    Makefile:
        obj-y += my.o
    vim . ./Makefile The information of the lower Makefile is added to the upper layer of the makefile. The makefile is a hierarchical compilation structure. The upper layer malefile records the lower layer makefile information in order to compile correctly.
        obj-y += mydriver/ There must be "/" here, because this is a directory
    and you can exit to linux-3.5/ make -j4
        [liuye@LiuYe linux-3.5]$>make -j4
          CHK include/linux/ version.h
          CHK include/generated/utsrelease.h
        make[1]: 'include/generated/mach-types.h' is up to date.
          CALL    scripts/checksyscalls.sh
          CHK     include/generated/compile.h
          CC      drivers/mydriver/my.o
          LD      drivers/mydriver/built-in.o
          LD      drivers/built-in.o
          LINK    vmlinux
          LD      vmlinux.o
          MODPOST vmlinux.o
        WARNING: modpost: Found 9 section mismatch(es).
        To see full details build your kernel with:
        'make CONFIG_DEBUG_SECTION_MISMATCH=y'
          GEN     .version
          CHK     include/generated/compile.h
          UPD     include/generated/compile.h
          CC      init/version.o
          LD init/built-in.o
          KSYM .tmp_kallsyms1.o
          KSYM .tmp_kallsyms2.o
          LD vmlinux
          SYSMAP System.map
          OBJCOPY arch/arm/boot/Image
          Kernel: arch/arm/boot/Image is ready
          GZIP arch/arm/boot/ compressed/piggy.gzip
          AS arch/arm/boot/compressed/piggy.gzip.o
          LD arch/arm/boot/compressed/vmlinux
          OBJCOPY arch/arm/boot/zImage
          Kernel: arch/arm/boot/zImage is ready
          Building modules , stage 2.
          MODPOST 3 modules
    can see that our code is compiled and linked. Then replace the zImage to the SD card, flash the machine, just flash the zImage.
        [ 3.225000] usbhid: USB HID core driver
        [    3.225000] hello kernel
        [    3.225000] hello kernel
        [    3.225000] hello kernel
        [    3.225000] hello kernel
        [    3.225000] hello kernel
        [    3.225000] hello kernel
        [    3.225000] hello kernel
        [    3.225000] hello kernel
        [    3.225000] hello kernel
        [    3.225000] hello kernel
        [    3.225000] hello kernel
        [    3.225000] hello kernel
        [    3.225000] hello kernel
        [    3.225000] hello kernel
        [    3.225000] hello kernel
        [    3.225000] hello kernel
        [    3.225000] hello kernel
        [ 3.225000] hello kernel
        [ 3.225000] hello kernel
        [ 3.225000] hello kernel
        [ 3.225000] Samsung Audio Subsystem Driver, (c) 2011 Samsung Electronics
        [ 3.225000] audss_init: RCLK SRC[busclk]
        [ 3.225000] Samsung SRP driver, (c)2011 Samsung Electronics
        [ 3.310000] mmcblk0boot1: mmc0:0001 M4G1YC partition 2 2.00 MiB
    You can see that our code is already running during the kernel boot process.
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