This applies to newlib platforms at least, possibly others.

Actual behavior

stdio_init is called from _init which is run after board_init. This has the downside that it is not possible to use printf or DEBUG from board_init or from the init function of periph drivers which are initialized by periph_init, e.g. SPI, since those run before _init.

Desired behavior

stdio_init should be called as early as possible, for stdio_uart this would likely be immediately after clocking has been set up, (which is a prerequisite for generating the proper baud rate)

References

#10615 (comment)