tcgetatattr错误后cfgetispeed

Question

我在termios.h基础上编写了一个基于Linux的串口程序（应该支持B921600），这里是代码：

#include <stdio.h>
#include <termios.h>
#include <unistd.h>
#include <fcntl.h>      //O_RDWR | O_NOCTTY

int main (void)
{
    int hd;
    struct termios setting;
    speed_t speed_i, speed_o;

    hd = open("/dev/ttyS0", O_RDWR | O_NOCTTY);
    setting.c_cflag &= ~CBAUD;
    setting.c_cflag &= ~CBAUDEX;

    if(tcgetattr(hd, &setting) != 0)
    {
        printf("open error\n");
    }

    cfsetispeed(&setting, B921600);
    cfsetospeed(&setting, B921600);
    setting.c_cflag |= B921600;

    if(tcsetattr(hd, TCSANOW, &setting) != 0)
    {
        //ERROR;
    }
    speed_i = cfgetispeed(&setting);
    speed_o = cfgetospeed(&setting);
    printf("out baudrate: %x\n",speed_o);
    printf("in baudrate: %x\n",speed_i);

    if(tcgetattr(hd, &setting) != 0)
    {
        //ERROR;
    }
    speed_i = cfgetispeed(&setting);
    speed_o = cfgetospeed(&setting);
    printf("out baudrate: %x\n",speed_o);
    printf("in baudrate: %x\n",speed_i);

    return 0;
}

之前所有都正确：

if(tcsetattr(hd, TCSANOW, &setting) != 0)
{
    //ERROR;
}
speed_i = cfgetispeed(&setting);
speed_o = cfgetospeed(&setting);
printf("out baudrate: %x\n",speed_o);
printf("in baudrate: %x\n",speed_i);

然后我做tcgetattr，之后，速度被读作所有1002，这是我的测试日志：

out baudrate: 1007
in baudrate: 1007
out baudrate: 1002
in baudrate: 1002

有什么问题？似乎波特率高于115200无法设置？