examples/filesystem: Update README and increase default stack size

[crasbe]

Contribution description

This pull request updates the filesystem example. In a previous commit e657590 the behaviour of the filesystem example was changed, but the README was not updated, making it hard for new users to apply it to an actual board.
Therefore, the README was extended.

Furthermore, the Makefile was extended to add an option to increase stack size for larger flash memories to avoid Kernel Panics due to a stack overflow.

Testing procedure

The changes were tested with a Nordic Semiconductor nRF52840-DK development board. The behaviour of the native implementation is unchanged and reflects the behavior shown in the README.

The behaviour with a real board should equally reflect the behavior shown in the README in the section about the nrf52840dk board.

Issues/PRs references

fixes #20373

[benpicco]

Thank you for taking care of this!
Some notes, as a presence of a MTD device is not enough, we also need a mountpoint.

[crasbe]

Thank you for the feedback, I think all suggestions are now applied to the PR.

[crasbe]

I think the only remaining failed tests are the commit tests now, which require the commits to be sqashed. But the guidelines say to not do that unless requested by a maintainer. So should I do anything beyond this point?

[benpicco]

Thank you, please squash directly.

[riot-ci]

Murdock results

✔️ PASSED

4b33e3e examples/filesystem: Increase main stack size

Success	Failures	Total	Runtime
1	0	1	11s

Artifacts

[crasbe]

The build fails because of the "nucleo-l011k4" target: https://ci.riot-os.org/details/3a64dfbefb1a493abb7fce7dd25ced4e/builds/examples:filesystem

The Makefile.include specifies the stack size to 512 Bytes, which is less than would be required for the filesystem example (at least with littlefs). https://github.com/RIOT-OS/RIOT/blob/master/boards/nucleo-l011k4/Makefile.include#L5
The STM32L011K4 microcontroller only has 2kB of RAM, so it is likely that this example won't run either way on it.

I don't know why the target is not ignored by the CI, it is present in the Makefile.ci exclusion list for insufficient memory already: https://github.com/RIOT-OS/RIOT/blob/master/examples/filesystem/Makefile.ci#L9
All other targets with insufficient memory are built successfully.

Other examples redefine the stack size as well, but they define THREAD_STACKSIZE_MAIN. Perhaps that's what this example should do as well?

[crasbe]

I can confirm that the example still works on the Nordic nRF52840-DK with THREAD_STACKSIZE_MAIN=2048 instead of THREAD_STACKSIZE_DEFAULT=2048.
The stack size of main is only 128 bytes bigger when using THREAD_STACKSIZE_DEFAULT.

With THREAD_STACKSIZE_MAIN=2048;

> ps
	pid | name                 | state    Q | pri | stack  ( used) ( free) | base addr  | current     
	  - | isr_stack            | -        - |   - |    512 (  164) (  348) | 0x20000000 | 0x200001c8
	  1 | main                 | running  Q |   7 |   2048 ( 1424) (  624) | 0x20000400 | 0x20000b84 
	    | SUM                  |            |     |   2560 ( 1588) (  972)

With THREAD_STACKSIZE_DEFAULT=2048:

> ps
	pid | name                 | state    Q | pri | stack  ( used) ( free) | base addr  | current     
	  - | isr_stack            | -        - |   - |    512 (  164) (  348) | 0x20000000 | 0x200001c8
	  1 | main                 | running  Q |   7 |   2560 (  676) ( 1884) | 0x20000400 | 0x20000d84 
	    | SUM                  |            |     |   3072 (  840) ( 2232)

[benpicco]

Ah right it’s actually enough to bump the main thread stack since that’s the only one doing file operations. (Ok, it’s probably the only thread in this example, but still)

[crasbe]

It looks like this fixed it. Nice :)