[riot-notifications] [RIOT-OS/RIOT] sys/stdio_uart: Add warning about use in ISR (#13985)

Kaspar Schleiser notifications at github.com
Sat May 2 22:42:25 CEST 2020

I'm not sure I agree on this.

To me, even simple outputs from UART are an extremely often used tool.
IMO debugging alone makes stdio from ISR an important use case.

```An UART implementation might choose to not guard an atomic sequence of
    interactions with the hardware by disabling IRQs but using a mutex, e.g.
    because the sequence takes long. Locking a mutex from interrupt
    context will result in a deadlock, if the mutex is already locked.```

If some implementations do this, can they be changed?

```If an ISR is triggered from a power saving mode, some peripherals or
    clock domains might still be offline during that ISR; including the UART
    peripheral. This is a valid implementation choice to allow time critical
    low power scenarios being covered by RIOT.```

They might still be offline *after* the interrupt that caused a wakeup.

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