Improve memory exhaustion warnings/errors

[Sophist-UK]

Firmware typically pre-allocates RAM memory where it can, and then remaining memory is used for dynamic allocation and the stack.

Currently excessive RAM usage is only a warning and not an error, which means that you are not alerted by a build failure but instead need to scan the PIO RUN output to see whether RAM usage is too high for the build. But in any case, typically firmware requires a certain amount of RAM to be available after the pre-allocation that is calculated by PIO, and you need to alerted when the remaining memory falls below this required free-memory amount.

This PR does two things:

1. RAM pre-allocation greater than available memory is now treated as an error and not a warning.
2. It introduces a RAMFREE option that can be used to define a minimum amount of memory required to remain free (for dynamic allocation and stack usage) after pre-allocation, with an error if pre-allocation results in less than this amount free.

[CLAassistant]

All committers have signed the CLA.

[Sophist-UK]

P.S. This simple PR has been coded on Github and I have not tested it. Sorry - but I don't have the skills or time to create a local dev environment and test it.

[ivankravets]

ram_free

You can override this value via board_upload.maximum_ram_size (RAM) and board_upload.maximum_size (Flash)

[Sophist-UK]

@ivankravets said:

You can override this value via board_upload_maximum_ram_size (RAM)

1. I am not sure how to provide this. In the platformio.ini [env] section?
2. Since I have automated builds for a couple of different boards with different amounts of memory from the same source code, if I am specifying maximum ram size I need to be able to set this dynamicaly - any advice here would also be useful.

(P.S. Flash is all or nothing - it is RAM which is more nuanced.)

[Sophist-UK]

In the platformio.ini [env] section?

I tried this and it didn't work.

[ivankravets]

Could you provide your INI file on how you declared these lines?

[Sophist-UK]

platformio.ini.txt

[ivankravets]

Sorry, this is a my typo. See updated #4886 (comment)

The JSON path must be split with . dot. https://github.com/platformio/platform-atmelavr/blob/develop/boards/megaatmega2560.json#L33

[Sophist-UK]

@ivankravets I tried the dotted parameter name in the same place in the ini file and it didn't reduce the target ram size from 8k to 6.5K as it should have. Sorry.

The documentation page is here and it says exactly what you suggested, so it should have worked. {Sigh}

Edit: I moved it from a sub-[env] section to the main [env] section and it worked. 👍 Thanks for pointing this out and helping me get there.

However whilst it is a potential second class alternative, it does not meet the use case of this PR.