cpu: add BACKUP_RAM attribute

[benpicco]

Contribution description

Many platforms provide a special memory section that is retained during Deep Sleep when all other state is lost.
RIOT already has support for putting data in this memory, but writing __attribute__((section(".backup.bss"))) is clunky and hard to memorize.
Even worse, it's not consistent across platforms.

This introduces an easy to use define that makes placing things in this memory much easier to look at.
It also allows applications to fall back to using .noinit if no backup memory is available by doing

#ifndef BACKUP_RAM
#define BACKUP_RAM __attribute__((section(".noinit")))
#endif

Then memory might not be retained during Deep Sleep, but there is a good chance it stays the same across reboots, which might be the next best thing.

Testing procedure

Only introduces a define for readability, but you can run tests/periph_backup_ram.

Issues/PRs references

suggested in #13519 (comment)

[gschorcht]

I'm wondering whether it would make sense to have a default definition as fallback?

[benpicco]

I don't think there could be a good fallback.
There is just no way to emulate that 'memory not being lost' property.

Using .noinit is a clutch with some severe implications that the application should actively decide for.

[gschorcht]

Are you sure that BACKUP_RAM is really the correct name. A backup RAM is generally understood to be a RAM that is powered by a backup battery and that is retained even when the main power supply is off. Here we are talking more about a piece of RAM that is only retained during DeepSleep or Reboot.

[gschorcht]

BTW, we should also have a shortcut for __attribute__((section(".noinit"))).

[benpicco]

Are you sure that BACKUP_RAM is really the correct name. A backup RAM is generally understood to be a RAM that is powered by a backup battery and that is retained even when the main power supply is off.

I'm open to suggestions for a better name, however from my understanding it makes no difference for the software how that memory is wired up electrically.
The defining property of BACKUP_RAM is that it's retained when all other CPU state is lost.

BTW, we should also have a shortcut for __attribute__((section(".noinit"))).

I agree.

[gschorcht]

@benpicco Please squash

[gschorcht]

ACK