无法从Linux字符设备读取

Question

我正在为加速度计实现 SPI 驱动程序。 SPI 部分已完成，但我无法从用户空间读取值。

static char charDevMessage[CD_BUFFER_SIZE] = { 0 };
...
static ssize_t char_dev_read(struct file *filep, char *buffer, size_t len, loff_t *position)
{

    int error_count = 0;
    struct xyz_values xyz;
    size_t size_requested;

    xyz = adxl345_get_xyz();
    memset(charDevMessage, 0, CD_BUFFER_SIZE);
    sprintf(charDevMessage, "%d,%d,%d", xyz.x, xyz.y, xyz.z);
    printk(KERN_INFO "MOB: %s, requested size: %d\n", charDevMessage, len);

    if (len >= CD_BUFFER_SIZE)
    {
        size_requested = CD_BUFFER_SIZE;
    }
    else
    {
        size_requested = len;
    }

    error_count = copy_to_user(buffer, charDevMessage, size_requested);      

    if (error_count == 0)          
    {
        printk(KERN_INFO "MOB: Sent %d characters to the user\n", size_requested);
        return (size_requested = 0);      

    }
    else
    {
        printk(KERN_INFO "MOB: Failed to send %d characters to the user\n", error_count);
        return -EFAULT;              

    }
}

当我安装驱动程序时，该节点已创建。但是当我尝试 cat 或尝试通过 python 读取时，它返回一个空字符串。

dmesg 表示已成功发送到用户空间:

[ 3094.495972] MOB: SPI Character device has been opened 1 time(s)
[ 3094.506075] MOB: -349,-512,511 , requested size:49
[ 3094.514487] MOB: Sent 256 characters to the user
[ 3094.522646] MOB: Character device successfully closed
[ 3120.658568] MOB: SPI Character device has been opened 2 time(s)
[ 3120.668609] MOB: 0,0,0 , requested size:48
[ 3120.676392] MOB: Sent 256 characters to the user
[ 3120.684740] MOB: Character device successfully closed

我做错了什么？

Answer 1

您误解了.read函数的概念:

读取器(用户空间)仅看到 .read 返回的值，并将其解释为已读取的字节数 .

至于 .read 的最后一个参数(在您的情况下是位置)，其解释完全取决于驱动程序的作者。 position 指向的值在文件打开时由内核初始化为 0。此后，内核本身不再修改它。

如果你想总是从头开始读取，你可以忽略 position 参数:

return size_requested;

或者，从语义上讲，您可以增加由位置指向的值，这样它将反射(reflect)读取的总字节数。但除此之外忽略它:

*position += size_requested;
return size_requested;