Fix for JLink debuggers

[korken89]

Explanation

When sending 1 character it returns -1 from syscall. This fix mitigates the issue, but it not correct.
This is probably not the correct way to solve it (might be a bug in the JLink debugger firmware), but it currently solves the issue.

---

To reproduce

JLink version info (this dev board https://www.decawave.com/product/dwm1001-development-board/):

Firmware: J-Link OB-STM32F072-128KB-CortexM compiled Jan  7 2019 14:08:04
Hardware: V1.00

Minimal example showing the problem:

#![no_main]
#![no_std]

// panic handler
extern crate panic_semihosting;
extern crate nrf52_hal_common;

use cortex_m_semihosting::hprint;
use cortex_m_rt::entry;

#[entry]
fn main() -> ! {
    hprint!("a").unwrap();

    loop{}
}

[rust-highfive]

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[korken89]

The JLink seems very unstable for semihosting, sometimes it returns -2 and -3 as well.
I would not accept this PR. The better fix is to use OpenOCD, it does not have this issue.

[nickray]

@korken89 Can you add some more info about this fix and its retraction? I'm playing around with an LPCXpresso55S69 and in my case (GDB + JLink) the hprintln is just being silently swallowed. OpenOCD isn't currently an option since it has no support (yet) AFAIK.

[korken89]

Sure! The fix checks the return value from the system call, and it was sometimes -1. So this was changed to be accepted.
This is however probably a bug in jlink.

The better fix, if one wants to do this, is to accept negative numbers plus some testing of what -1, -2 etc corresponds to.

[nickray]

Thanks! Seems I haven't hit your issue yet, I was just lacking monitor semihosting IOClient 2. 🤦‍♂️

[korken89]

Hehe, a easy thing to miss :) but if you at the same issue I had, please report it