驅動程式 - Linux Device Driver (LDD) - 使用範例 - Assembly (ARM) - Char Device - Device Node

參考資訊：
https://linux-kernel-labs.github.io/refs/heads/master/labs/device_drivers.html

User Application可以透過檔案操作方式跟驅動程式溝通，在Linux Kernel中，將檔案分成字元驅動程式(Char)和區塊驅動程式(Block)兩大類，主要區別在於傳輸方式，可以把字元驅動程式想像成每次傳輸都是以1個Byte為單位，而區塊驅動程式則是多個Bytes為單位，如：512 Bytes，User Application可以透過open()、read()、write()、ioctl()、close()跟字元驅動程式溝通，那User Application怎麼知道路徑在哪呢？答案是Symbolic Link(/dev/xxx)，那Symbolic Link又如何連結到字元驅動程式呢？答案是Major、Minor號碼，Major、Minor號碼是驅動程式的裝置索引，字元驅動程式在被系統載入時，會註冊Major和Minor號碼，使用者可以透過mknod工具，去建立一個Symbolic Link(名稱可以由使用者隨意取)並且綁定到字元驅動程式的這個Major、Minor號碼，完成連接的管道

字元驅動程式建立步驟：

1. alloc_chrdev_region()
2. cdev_init()
3. cdev_add()
4. cdev_del()
5. unregister_chrdev_region()

alloc_chrdev_region()可以取得空閒的裝置號碼，如果想要自訂Major號碼，可以使用register_chrdev_region()

ldd.S

    .global init_module
    .global cleanup_module

    .section .modinfo, "ae"
__UNIQUE_ID_0: .asciz "license=GPL"
__UNIQUE_ID_1: .asciz "author=Steward Fu"
__UNIQUE_ID_2: .asciz "description=Linux Driver"

    .equ cdev.kobj,  0
    .equ cdev.owner, 36
    .equ cdev.ops,   40
    .equ cdev.list,  44
    .equ cdev.dev,   52
    .equ cdev.count, 56

    .struct 0
cdev_s:
    c0: .struct . + 36
    c1: .struct . + 4
    c2: .struct . + 4
    c3: .struct . + 8
    c4: .struct . + 4
    c5: .struct . + 4
cdev_e:
cdev_l = cdev_e - cdev_s

    .equ file_operations.owner,             0
    .equ file_operations.llseek,            4
    .equ file_operations.read,              8
    .equ file_operations.write,             12
    .equ file_operations.read_iter,         16
    .equ file_operations.write_iter,        20
    .equ file_operations.iterate,           24
    .equ file_operations.iterate_shared,    28
    .equ file_operations.poll,              32
    .equ file_operations.unlocked_ioctl,    36
    .equ file_operations.compat_ioctl,      40
    .equ file_operations.mmap,              44
    .equ file_operations.open,              48
    .equ file_operations.flush,             52
    .equ file_operations.release,           56
    .equ file_operations.fsync,             60
    .equ file_operations.fasync,            64
    .equ file_operations.lock,              68
    .equ file_operations.sendpage,          72
    .equ file_operations.get_unmapped_area, 76
    .equ file_operations.check_flags,       80
    .equ file_operations.setfl,             84
    .equ file_operations.flock,             88
    .equ file_operations.splice_write,      92
    .equ file_operations.splice_read,       96
    .equ file_operations.setlease,          100
    .equ file_operations.fallocate,         104
    .equ file_operations.show_fdinfo,       108
    .equ file_operations.mmap_capabilities, 112
    .equ file_operations.copy_file_range,   116
    .equ file_operations.clone_file_range,  120
    .equ file_operations.dedupe_file_range, 124

    .struct 0
file_operations_s:
    i0: .struct . + 4
    i1: .struct . + 4
    i2: .struct . + 4
    i3: .struct . + 4
    i4: .struct . + 4
    i5: .struct . + 4
    i6: .struct . + 4
    i7: .struct . + 4
    i8: .struct . + 4
    i9: .struct . + 4
    i10: .struct . + 4
    i11: .struct . + 4
    i12: .struct . + 4
    i13: .struct . + 4
    i14: .struct . + 4
    i15: .struct . + 4
    i16: .struct . + 4
    i17: .struct . + 4
    i18: .struct . + 4
    i19: .struct . + 4
    i20: .struct . + 4
    i21: .struct . + 4
    i22: .struct . + 4
    i23: .struct . + 4
    i24: .struct . + 4
    i25: .struct . + 4
    i26: .struct . + 4
    i27: .struct . + 4
    i28: .struct . + 4
    i29: .struct . + 4
    i30: .struct . + 4
    i31: .struct . + 4
file_operations_e:
file_operations_l = file_operations_e - file_operations_s

    .section .data
base:      .dcb 4
mycdev:    .space cdev_l
myfops:    .space file_operations_l
chr_name:  .asciz "myfile"
fmt:       .asciz "major:%d, minor:%d\n"
open_msg:  .asciz "myopen\n"
close_msg: .asciz "myclose\n"

    .align 2
    .section .text
myopen:
    push {lr}
    ldr r0, =open_msg
    bl printk
    mov r0, #0
    pop {pc}

myclose:
    push {lr}
    ldr r0, =close_msg
    bl printk
    mov r0, #0
    pop {pc}

init_module:
    push {r4, lr}

    ldr r0, =myfops
    add r1, r0, #file_operations.open
    ldr r2, =myopen
    str r2, [r1]
    add r1, r0, #file_operations.release
    ldr r2, =myclose
    str r2, [r1]

    ldr r0, =base
    mov r1, #0
    mov r2, #1
    ldr r3, =chr_name
    bl alloc_chrdev_region
    ldr r4, =base

    ldr r0, =mycdev
    ldr r1, =myfops
    bl cdev_init

    ldr r0, =mycdev
    ldr r1, [r4]
    mov r2, #1
    bl cdev_add

    ldr r0, =fmt
    ldr r1, [r4]
    ubfx r2, r1, #0, #0x14
    mov r1, r1, asr#20
    bl printk

    mov r0, #0
    pop {r4, pc}

cleanup_module:
    push {lr}
    ldr r0, =mycdev
    bl cdev_del
    ldr r0, =base
    ldr r0, [r0]
    mov r1, #1
    bl unregister_chrdev_region
    pop {pc}
    .end

init_module: 建立字元驅動程式
myopen: 對應User Application的open()
myclose: 對應User Application的close()
cleanup_module: 刪除字元驅動程式

安裝驅動

# insmod /boot/main.ko 
    major:243, minor:0

P.S. Linux驅動程式使用Major、Minor號碼當作裝置索引

make node

# mknod /dev/myfile c 243 0

# ls -al /dev/myfile
    crw-r--r--    1 root     root      243,   0 Jan  1 00:00 /dev/myfile

P.S. 建立Symbolic符號，供User Application開啟使用

開啟裝置

# echo "" > /dev/myfile
    myopen
    sh: write error: Invalid argument myclose