Linux下为FTDI虚拟串口指定非标准波特率 [英] Specifying non-standard baud rate for FTDI virtual serial port under Linux

问题描述

我有一个 USB 设备，我试图通过 ftdi_sio 内核模块提供的虚拟串行端口与之通信.但是，我在将端口的波特率设置为 14400 时遇到了一些问题:

I have a USB device I'm trying to communicate with over a virtual serial port provided by the ftdi_sio kernel module. However, I'm having some trouble setting the baud rate of the port to 14400:

• termios.h 没有为 14400 指定常量，所以我不能使用 cfsetispeed 和 cfsetispeed.
• 在 ftdi_sio 内核模块的源代码中，波特基设置为 24000000，似乎没有办法更改它.这意味着我无法将自定义除数与 TIOCSSERIAL ioctl 一起使用，从而获得 14400 波特率.
• 模块源有一条注释，听起来像是将端口的 tty_struct 结构的 alt_speed 成员设置为 14400 可以满足我的要求，但没有鉴于现有接口，似乎没有任何方法可以将其设置为 14400.

• termios.h doesn't specify a constant for 14400, so I can't use cfsetispeed and cfsetospeed.
• In the source for the ftdi_sio kernel module, baud base is set to 24000000 and there doesn't seem to be a way to change it. This means I can't use a custom divisor with the TIOCSSERIAL ioctl and get a 14400 baud rate that way.
• The module source has a comment making it sound like setting the alt_speed member of the tty_struct structure for the port to 14400 would do what I want, but there doesn't seem to be any way to set it to 14400 given the existing interfaces.

有人对此有任何想法吗?通过破解内核模块很容易解决这个问题，但我真的在寻找一种不需要更改内核的解决方案.

Does anyone have any ideas about this? It'd be pretty easy to fix this by hacking up the kernel module, but I'm really looking for a solution that doesn't require kernel changes.

推荐答案

你不能改变波特基，我想它与硬件有关.所以弄乱模块对你没有任何好处.在您的第三点中，您只讨论了设置自定义波特率的第一种方法，您需要访问 tty->alt_speed.似乎没有直接从用户空间设置 tty 结构的接口，至少没有使用 ftdi_sio 驱动程序.
但是，评论中解释了另一种方法:

You can't change baud base, I suppose it is hardware related. So messing with the module won't do you any good. In your third point you only talk about the first method proposed for setting a custom baudrate, where you need to access the tty->alt_speed. It seems there is no interface to directly set tty struct from userspace, at least not with the ftdi_sio driver.
However, there is another method explained in the comments :

     * 3. You can also set baud rate by setting custom divisor as follows
     *    - set tty->termios->c_cflag speed to B38400
     *    - call TIOCSSERIAL ioctl with (struct serial_struct) set as
     *      follows:
     *      o flags & ASYNC_SPD_MASK == ASYNC_SPD_CUST
     *      o custom_divisor set to baud_base / your_new_baudrate

你试过了吗?

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