drivers: disk: add fakefat disk driver

[jfischer-no]

Fakefat disk emulates a disk formatted with the FAT file system.
The driver supports FAT16 and FAT32 emulation and multiple instances.
A backend can register at least 15 emulated files on a disk instance.
The file structure contain read and write callbacks that are called
when the filesystem driver accesses the volume. A backend can have
multiple files with read callbacks, but usually only one or zero with
write callbacks.

Fakefat disk can be used for testing or to map services such as
firmware updates to a file on the emulated disk.
Complementary to #53798

TODO:

• sample README.rst
• documentation
• tests

[jfischer-no]

@petejohanson You might find it useful for your fancy stuff.

[petejohanson]

@petejohanson You might find it useful for your fancy stuff.

Interesting.. possibly useful for some of our testing scenarios or development with the posix targets.

[desowin]

I don't expect this to work.

The "ffdisk" name is horrible because it looks like typo. Why are you using such ridiculously short names recently (that are completely unreadable)? What's wrong with just using "fakefatdisk"?

[desowin]

How this is not leading to absolutely bizzare and pretty much impossible to debug issues?

The idea of abusing FAT file system for specific uses that can work is https://github.com/microsoft/uf2. Note that UF2 operates on different principle and is pretty much limited to write only.

The fakefat in my opinion cannot really work reliably, because host pretty much has complete control over MSC disks.

I believe MTP is the solution for exporting fixed amount of files, not MSC.

[jfischer-no]

How this is not leading to absolutely bizzare and pretty much impossible to debug issues?

What issues and for what use cases? The write accesses can be ignored because we do not change e.g. the FAT, what is emulated anyway.

The idea of abusing FAT file system for specific uses that can work is https://github.com/microsoft/uf2. Note that UF2 operates on different principle and is pretty much limited to write only.

This PR uses a similar principle (also similar to virtual_fs in daplink). One of my intentions is to use it to provide similar functionality as the DAPLink firmware. And I am pretty sure you can read emulated files in UF2 (and daplink's virtual_fs).

The fakefat in my opinion cannot really work reliably, because host pretty much has complete control over MSC disks.

I believe MTP is the solution for exporting fixed amount of files, not MSC.

It works very reliably for what it was developed for. It is not there to export arbitrary file systems or to be something like MTP.

[tmon-nordic]

Virtual fs in daplink is actively checking for problems (e.g. data being sent sequential) while this implementation does not.

UF2 is operating on different principle. UF2 only assumes that files are written on sector boundary (i.e. file does not start in the middle of sector) and has plenty of padding.

[jfischer-no]

I don't expect this to work.

The "ffdisk" name is horrible because it looks like typo. Why are you using such ridiculously short names recently (that are completely unreadable)? What's wrong with just using "fakefatdisk"?

What exactly do you not expect to work? Or what is not working for you? Where did you find ffdisk? (Why would I use ridiculously long names?)

[tmon-nordic]

What exactly do you not expect to work? Or what is not working for you?

When the host reorders the files, e.g. deletes a file and then creates a new one things will break.

Why would I use ridiculously long names?

Code is written for humans. It is annoying when something that can be mistaken as a typo has some very special meaning behind it. In my opinion longer names are generally better because such names avoid ambiguity and are especially helpful for newcomers.