驅動程式 - Linux Device Driver (LDD) - 使用範例 - Assembly (ARM) - GPIO Input

由於PocketBeagle板上的Power按鍵是透過PMIC連接，因此，需要連接另一個按鍵來使用，司徒選定27腳位，如下圖：


連接27腳位到+3.3V


在此篇範例中，司徒使用Timer做按鍵檢查，當按鍵被按下時，點亮LED，反之則熄滅LED，因此，需要使用Linux Kernel的GPIO操作函數以及Timer，針對GPIO的輸入應用，步驟如下：

1. gpio_request()
2. gpiod_direction_input()
3. gpiod_get_raw_value()
4. gpio_free()

ldd.S

    .global init_module
    .global cleanup_module

    .equ BUTTON,   27
    .equ USR3_LED, ((32 * 1) + 24)
    .equ PERIOD,   (100 / 4)

    .equ timer_list.entry,    0
    .equ timer_list.expires,  8
    .equ timer_list.function, 12
    .equ timer_list.data,     16
    .equ timer_list.flags,    20

    .struct 0
timer_list_s:
    i0: .struct . + 8
    i1: .struct . + 4
    i2: .struct . + 4
    i3: .struct . + 4
    i4: .struct . + 4
timer_list_e:
timer_list_l = timer_list_e - timer_list_s

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

    .section .data
led_name: .asciz "USR3"
btn_name: .asciz "BTN"
led_desc: .dcb 4
btn_desc: .dcb 4
timer:    .space timer_list_l

    .align 2
    .section .text
timer_handler:
    push {lr}
    ldr r0, =btn_desc
    ldr r0, [r0]
    bl gpiod_get_raw_value
    mov r1, r0

    ldr r0, =led_desc
    ldr r0, [r0]
    bl gpiod_set_raw_value

    ldr r0, =timer
    ldr r1, =jiffies
    ldr r1, [r1]
    add r1, #PERIOD
    bl mod_timer
    pop {pc}

init_module:
    push {r4, lr}

    mov r0, #BUTTON
    ldr r1, =btn_name
    bl gpio_request

    mov r0, #USR3_LED
    ldr r1, =led_name
    bl gpio_request

    mov r0, #BUTTON
    bl gpio_to_desc
    mov r4, r0
    ldr r1, =btn_desc
    str r4, [r1]

    mov r0, r4
    bl gpiod_direction_input

    mov r0, #USR3_LED
    bl gpio_to_desc
    mov r4, r0
    ldr r1, =led_desc
    str r4, [r1]

    mov r0, r4
    mov r1, #1
    bl gpiod_direction_output_raw

    ldr r0, =timer
    mov r1, #0
    mov r2, #0
    mov r3, #0
    bl init_timer_key

    ldr r0, =timer
    add r0, #timer_list.function
    ldr r1, =timer_handler
    str r1, [r0]

    ldr r0, =timer
    add r0, #timer_list.data
    mov r1, #0
    str r1, [r0]

    ldr r0, =timer
    ldr r2, =jiffies
    ldr r1, [r2]
    add r1, #PERIOD
    ldr r3, =timer_handler
    bl mod_timer

    mov r0, #0
    pop {r4, pc}

cleanup_module:
    push {lr}
    ldr r0, =timer
    bl del_timer
    mov r0, #BUTTON
    bl gpio_free
    mov r0, #USR3_LED
    bl gpio_free
    pop {pc}
    .end

init_module: 初始化GPIO以及Timer
timer_handler: 取得按鍵狀態並且設定到USR3 LED
cleanup_module: 釋放GPIO以及Timer資源

完成