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

Kaspar Schleiser notifications at github.com
Sun May 3 22:05:39 CEST 2020

> Let's compare the situation to LTO, which is also not officially supported

Do we accept code that does not work with LTO? I don't think so, we consider that a bug.
So, LTO is kinda "supported", there are just corner cases where it breaks stuff. (are we aware of any that haven't been worked around?).
Also, LTO is an optimization, which puts it into a different class of features. Applications usually work with or without LTO, the latter being somewhat faster or smaller. Only in corner cases (non-LTO doesn't fit or non-LTO is too slow) non-working LTO poses an actual problem.

> Note that the alternative to not officially supporting STDIO over UART from ISR is to _do officially support_ it. And there are systems on which this is just not feasible.

Why is it not feasible?

> Won't other clock domains be equally affected to this? If e.g. an SPI transfer is occasionally unreliable after a wake up

Depends. SPI and I2C are clocked by one side, so if that clock is not as fast as it should be, it doesn't really matter in practice. UART is special in that sense.
HF timers might also pose a problem shortly after wakeup.

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