Problem when using fatfs example in a out_of_tree_driver with the file ff.h

[Tobi15]

Describe the bug

ff.h file missing when trying to access from an out_of_tree_driver

To Reproduce

I get t he sample "samples/application_development/out_of_tree_driver"
Then, I modified the driver with the "samples/subsys/fs/fatfs". I created an init() method in the C file and put the method signature in the H file.

In the main, I called this method init().

Run : "west build ..."

Expected behavior

The project can build properly

Screenshots or console output

fatal error: ff.h: No such file or directory
14 | #include <ff.h>
| ^~~~~~
compilation terminated.
ninja: build stopped: subcommand failed.

The error comes from the include in the driver H file.

Source code

fat_fs_with_sd_driver.zip

[carlescufi]

@nordicjm @de-nordic any hints?

[nordicjm]

A driver added to zephyr externally is a driver and is added as an external module, it should have access to the base zephyr features, fat_fs is not a base zephyr feature, it's a module. Because they are both modules, you cannot reference it, and why would a driver expose a fat filesystem? The driver should expose a base device which is then mounted as a filesystem - be it fat or littlefs or any other filesystem, which can then be mounted from application code.

The reason for there being a split of modules that can be used and those that can't, from an external module, is because some modules configuration is in the zephyr tree, and some is outside of it. fatfs is outside of it, segger is inside it (which is why you can enable RTT output and it will work from a module)

[de-nordic]

The <ff.h> header is not part of zephyr interface unless you enable FAT FS support.
The FAT FS module adds its include paths with CMakeLists.txt line:
https://github.com/zephyrproject-rtos/fatfs/blob/a30531af3a95a9a3ea7d771ea8a578ebfed45514/CMakeLists.txt#L5
and ELMFAT is added to FS interface with line

zephyr/subsys/fs/CMakeLists.txt

Line 19 in 3cef8e6

target_link_libraries_ifdef(CONFIG_FAT_FILESYSTEM_ELM FS INTERFACE ELMFAT)

under condition that CONFIG_FAT_FILESYSTEM_ELM=y
If your driver links zephyr_interface then you should enable the FAT FS driver module, to get access to ff.h, but this will also enable default FAT FS driver, so probably cause collision with what you are doing.

Anyway: I do not understand why SD card driver requires FAT file system access. SD card driver should provide disk interface that can then be used to mount FAT FS of that interface.

[de-nordic]

I allowed myself to remove "bug" label, because this is not a bug.

[Tobi15]

Thank you for your answers! I didn't understand it before. I changed the way I solve my problem by using a library to do my work.
Best regard!