Taking it futher: How would I disable the uart completely? Because uart is selected automatically?

I think we need a bit of documentation somewhere to explain how disables uart_stdio_rx and why, as it not very straightforward right now.

@dylad +1 ... go ahead ;) And also we should give a short overview, where uart is initialized. For stm32 it is in newlib, am I right here? But for other platforms it is in the board_init, e.g. telosb, z1 and a few others (including my platform ;)).

Are we waiting for documentation here? This is probably the duty of @kaspar030

@yegorich can you comment on this PR?

I've looked at various users of stdio_read(). Some users check the return value like newlib's syscalls.c, but others atmega_stdio.c or msp430_stdio.c just return a theoretically received character without checking stdio_read's return value.

I wonder whether it make sense to propagate MODULE_UART_STDIO_RX to cpu files, so that it looks like in cpu/fe310/nano/nanostubs.c :

_ssize_t _read(int fd, void *buffer, size_t count)
  {
  #if defined(MODULE_STDIO_UART)
      if (fd == STDIN_FILENO) {
          return stdio_read(buffer, count);
      }
      errno = EBADF;
      return -1;
  #else
      (void) fd;
      (void) buffer;
      (void) count;
      errno = ENODEV;
      return -1;
  #endif
  }

@MrKevinWeiss maybe something you would like to have a look at, too?

I'm not gonna work on this in the foreseeable future. Anyone interesting in taking over?

@kaspar030 I rebased your branch to a recent master, do you mind if I push it here, or do you prefer I open a new PR?

Closing in favor of #11310.