serial-unix: on linux, use the termios2 api to support arbitrary baud rates

[codysch]

Picocom includes some documentation on it: https://github.com/npat-efault/picocom/blob/master/termios2.txt

Assuming underlying adapter support, using termios2 allows more flexible baud rate selection.

[TechnoSam]

I'm not sure when this was added, but there is a BaudOther available.

@dcuddeback I tried to use a baud rate of 187500 on OS X and I got an "Invalid argument" error. Is this what you mean by

Non-standard baud rates may not be supported on all systems.

I may be able to change the baud rate of of my chip, but its an embedded platform and I'd prefer to not use a bunch of clock dividers to get a standard rate.

This may work on Raspbian Linux, which is my eventual target platform, but for now I'm having trouble getting the serial pins to behave, so I'm not sure when I'll get to test that.

[dcuddeback]

@TechnoSam Each platform has a different set of baud rates that it supports via B* constants, with Windows being the lowest common denominator. The BaudNNN constants in serial-core represent the baud rates that are well supported across platforms, and BaudOther can be used to access some platform-specific baud rates. The list that's currently supported are the ones defined in the platforms' termios.h header files. You can see that list here:

serial-rs/serial-unix/src/tty.rs

Lines 408 to 461 in cb28b14

	let baud = match baud_rate {
	core::BaudOther(50) => B50,
	core::BaudOther(75) => B75,
	core::Baud110 => B110,
	core::BaudOther(134) => B134,
	core::BaudOther(150) => B150,
	core::BaudOther(200) => B200,
	core::Baud300 => B300,
	core::Baud600 => B600,
	core::Baud1200 => B1200,
	core::BaudOther(1800) => B1800,
	core::Baud2400 => B2400,
	core::Baud4800 => B4800,
	#[cfg(any(target_os = "macos", target_os = "freebsd", target_os = "openbsd"))]
	core::BaudOther(7200) => B7200,
	core::Baud9600 => B9600,
	#[cfg(any(target_os = "macos", target_os = "freebsd", target_os = "openbsd"))]
	core::BaudOther(14400) => B14400,
	core::Baud19200 => B19200,
	#[cfg(any(target_os = "macos", target_os = "freebsd", target_os = "openbsd"))]
	core::BaudOther(28800) => B28800,
	core::Baud38400 => B38400,
	core::Baud57600 => B57600,
	#[cfg(any(target_os = "macos", target_os = "freebsd", target_os = "openbsd"))]
	core::BaudOther(76800) => B76800,
	core::Baud115200 => B115200,
	core::BaudOther(230400) => B230400,
	#[cfg(any(target_os = "linux", target_os = "freebsd"))]
	core::BaudOther(460800) => B460800,
	#[cfg(target_os = "linux")]
	core::BaudOther(500000) => B500000,
	#[cfg(target_os = "linux")]
	core::BaudOther(576000) => B576000,
	#[cfg(any(target_os = "linux", target_os = "freebsd"))]
	core::BaudOther(921600) => B921600,
	#[cfg(target_os = "linux")]
	core::BaudOther(1000000) => B1000000,
	#[cfg(target_os = "linux")]
	core::BaudOther(1152000) => B1152000,
	#[cfg(target_os = "linux")]
	core::BaudOther(1500000) => B1500000,
	#[cfg(target_os = "linux")]
	core::BaudOther(2000000) => B2000000,
	#[cfg(target_os = "linux")]
	core::BaudOther(2500000) => B2500000,
	#[cfg(target_os = "linux")]
	core::BaudOther(3000000) => B3000000,
	#[cfg(target_os = "linux")]
	core::BaudOther(3500000) => B3500000,
	#[cfg(target_os = "linux")]
	core::BaudOther(4000000) => B4000000,
	core::BaudOther(_) => return Err(super::error::from_raw_os_error(EINVAL)),
	};

.

Those are platform-specific baud rates, but not arbitrary baud rates. The plan is to support arbitrary baud rates, too, but that's not implemented yet. Arbitrary baud rates are also platform-specific. Linux (this ticket) is probably the easiest to support, because it provides the TCSETS2 and TCGETS2 ioctls which are very similar to tcsetattr() and tcgetattr() functions from the POSIX-portable termios API.

For Raspian, you would want to follow this ticket. For OS X, #37 is a complementary ticket.