C 使用mmap/dev/mem方法的ARM linux用户空间gpio操作（能够写入gpio寄存器，但无法读取）

PHYTEC提供的内核版本3.12.30-AM335x-PD15.1.1

如果我使用/sys/class/gpio方式，我可以看到按钮输入引脚（AM3359的gpio103）值从0变为1

在本次演习结束后 并执行以下命令，以使用/dev/mem方法读取gpio引脚：

`devmem2 0x481ae13c` 

（第3列gpio的基数为0x481ae000+0x13c数据输出偏移量）

无论按钮位置如何，我都会得到下面的输出

/dev/mem opened
Memory mapped at address 0xb6fd1000.
Read at address  0x481AE13C (0xb6fd113c): 0x00000000

还有下面给出的c程序，我使用/dev/mem切换了gpios；然而，我无法成功地阅读它们

在头文件中：

#define GPIO0_START_ADDRESS (0x44E07000)
#define GPIO1_START_ADDRESS (0x4804C000)
#define GPIO2_START_ADDRESS (0x481AC000)
#define GPIO3_START_ADDRESS (0x481AE000)

#define GPIO0_END_ADDRESS   (0x44e09000)
#define GPIO1_END_ADDRESS   (0x4804E000)
#define GPIO2_END_ADDRESS   (0x481AE000)
#define GPIO3_END_ADDRESS   (0x481B0000)

#define GPIO0_SIZE (GPIO0_END_ADDRESS - GPIO0_START_ADDRESS)
#define GPIO1_SIZE (GPIO1_END_ADDRESS - GPIO1_START_ADDRESS)
#define GPIO2_SIZE (GPIO2_END_ADDRESS - GPIO2_START_ADDRESS)
#define GPIO3_SIZE (GPIO3_END_ADDRESS - GPIO3_START_ADDRESS)

#define GPIO_SETDATAOUT (0x194)
#define GPIO_CLEARDATAOUT (0x190)
#define GPIO_DATAOUT (0x13c)

在程序中的某个地方：

    int fd = open("/dev/mem", O_RDWR);
    //int fd = open("/dev/mem", O_RDWR|O_SYNC);

    gpio0_address = mmap(0, GPIO0_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, GPIO0_START_ADDRESS);
    gpio1_address = mmap(0, GPIO1_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, GPIO1_START_ADDRESS);
    gpio2_address = mmap(0, GPIO2_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, GPIO2_START_ADDRESS);
    gpio3_address = mmap(0, GPIO3_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, GPIO3_START_ADDRESS);

    if(gpio0_address == MAP_FAILED){ printf("unable to map GPIO0 bank.\n"); }
    if(gpio1_address == MAP_FAILED){ printf("unable to map GPIO1 bank.\n"); }
    if(gpio2_address == MAP_FAILED){ printf("unable to map GPIO2 bank.\n"); }
    if(gpio3_address == MAP_FAILED){ printf("unable to map GPIO3 bank.\n"); }


    gpio0_dataout_address = gpio0_address + GPIO_DATAOUT;

    gpio0_setdataout_address = gpio0_address + GPIO_SETDATAOUT;

    gpio0_cleardataout_address = gpio0_address + GPIO_CLEARDATAOUT;

    gpio1_dataout_address = gpio1_address + GPIO_DATAOUT;

    gpio1_setdataout_address = gpio1_address + GPIO_SETDATAOUT;

    gpio1_cleardataout_address = gpio1_address + GPIO_CLEARDATAOUT;

    gpio2_dataout_address = gpio2_address + GPIO_DATAOUT;

    gpio2_setdataout_address = gpio2_address + GPIO_SETDATAOUT;

    gpio2_cleardataout_address = gpio2_address + GPIO_CLEARDATAOUT;

    gpio3_dataout_address = gpio3_address + GPIO_DATAOUT;

    gpio3_setdataout_address = gpio3_address + GPIO_SETDATAOUT;

    gpio3_cleardataout_address = gpio3_address + GPIO_CLEARDATAOUT;
    read_GPIO_bank_memory(3, &GPIO3_bank_memory);

读取功能：

   void read_GPIO_bank_memory(int bank, four_bytes* gpio_bank_dataout)
    {
        switch(bank)
        {
            case 0:

                gpio_bank_dataout->ALL = *gpio0_dataout_address;
                break;
            case 1:

                gpio_bank_dataout->ALL = *gpio1_dataout_address;

                break;
            case 2:

                gpio_bank_dataout->ALL = *gpio2_dataout_address;

                break;
            case 3:

                gpio_bank_dataout->ALL = *gpio3_dataout_address;

                break;
        }
    }

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您应该使用GPIO_数据输入寄存器（偏移量=138h）从GPIO引脚读取数据，而不是GPIO_数据输出。

毫不奇怪，数据输出寄存器用于输出。如果要读取输入值，请使用数据输入寄存器。名字里有个线索…谢谢你指出，我知道这一定是很愚蠢的事情，但我迷路了。