Jetson Nano 系列之：C通过内存映射操作GPIO

目标

NVIDIA提供了python版本的GPIO库，但没有提供C版，并且python版本是通过文件句柄实现的，性能不行，需要C版本内存映射来操作GPIO。

寄存器地址映射规则

根据官方文档 Tegra_X1_TRM_DP07225001_v1.3p.pdf 第9章表32，A~Z、AA、BB、CC、DD、EE、FF共有32个port，每4个port为一个controller，每个port控制8个GPIO针(pin)。寄存器地址范围000~FFF共4096个字节，每4个port的地址是交错的，如下表中A、B、C、D所示。

步骤一：打开GPIO口

代码来自jetsonGPIO，gpio参数传入GPIO的设备地址，例如BCM=20/BOARD=38对应的设备地址是77，详见下表。

// Export the given gpio to userspace;
// Return: Success = 0 ; otherwise open file error
int gpioExport(unsigned int gpio)
{
    int fileDescriptor, length;
    char commandBuffer[MAX_BUF];
 
    fileDescriptor = open(SYSFS_GPIO_DIR "/export", O_WRONLY);
    if (fileDescriptor < 0)
    {
        char errorBuffer[128];
        snprintf(errorBuffer, sizeof(errorBuffer), "gpioExport unable to open gpio%d", gpio);
        perror(errorBuffer);
        return fileDescriptor;
    }
 
    length = snprintf(commandBuffer, sizeof(commandBuffer), "%d", gpio);
    if (write(fileDescriptor, commandBuffer, length) != length)
    {
        perror("gpioExport");
        return fileDescriptor;
    }
    close(fileDescriptor);
 
    return 0;
}

步骤二：设置GPIO口的读/写

代码来自jetsonGPIO，gpio参数传入GPIO的设备地址，详见下表。

// Set the direction of the GPIO pin
// Return: Success = 0 ; otherwise open file error
int gpioSetDirection(unsigned int gpio, unsigned int out_flag)
{
    int fileDescriptor;
    char commandBuffer[MAX_BUF];
 
    snprintf(commandBuffer, sizeof(commandBuffer), SYSFS_GPIO_DIR "/gpio%d/direction", gpio);
 
    fileDescriptor = open(commandBuffer, O_WRONLY);
    if (fileDescriptor < 0)
    {
        char errorBuffer[128];
        snprintf(errorBuffer, sizeof(errorBuffer), "gpioSetDirection unable to open gpio%d", gpio);
        perror(errorBuffer);
        return fileDescriptor;
    }
 
    if (out_flag)
    {
        if (write(fileDescriptor, "out", 4) != 4)
        {
            perror("gpioSetDirection");
            return fileDescriptor;
        }
    }
    else
    {
        if (write(fileDescriptor, "in", 3) != 3)
        {
            perror("gpioSetDirection");
            return fileDescriptor;
        }
    }
    close(fileDescriptor);
    return 0;
}

步骤三：建立mmap

代码来自jetson-nano-gpio-example。其中，GPIO_ADDRESS 见下表中的内存映射地址。

    //  read physical memory (needs root)
    int fd = open("/dev/mem", O_RDWR | O_SYNC);
    if (fd < 0) {
        fprintf(stderr, "usage : $ sudo %s (with root privilege)\n", argv[0]);
        exit(1);
    }
 
    //  map a particular physical address into our address space
    int pagesize = getpagesize();
    int pagemask = pagesize-1;
 
    //  This page will actually contain all the GPIO controllers, because they are co-located
    void *base = mmap(0, pagesize, PROT_READ | PROT_WRITE, MAP_SHARED, fd, (GPIO_ADDRESS & ~pagemask));
    if (base == NULL) {
        perror("mmap()");
        exit(1);
    }

内存映射表

该表是我根据jetson的地址规则计算的内存映射地址和设备地址。