USB Media Transfer Protocol (MTP)

[yoursunny]

There are use cases in which the computer uploads a file into the Raspberry Pi Pico, which then causes an action on the microcontroller side.
For example, the computer could upload the lighting patterns of an LED matrix, which is applied to WS2812 NeoPixels.

While the FatFSUSB library could allow such use cases, it must write to the onboard flash partition, and the computer must umount the drive before the microcontroller could access the file.
The reason is that, USB mass storage mode exposes a block device, and FAT filesystem requires exclusive access to the block device.

The Media Transfer Protocol (MTP) allows a USB device to expose its filesystem instead of a block device.
The protocol operates at file level, in which the device can choose which files to present to the computer, and which files are writable.

Ideally, the MTP implementation in arduino-pico should allow two operation modes:

• Expose a local partition that is either LittleFS or FatFS, but allow callbacks to hide files or disallow writing to certain files.
• Decouple from any existing filesystem, but use callbacks to obtain file listing, read files, and write files.

[dhalbert]

A bit of an aside: MTP is not supported by Apple operating systems. (There have been a few third-party implementations on and off.) This was a business decision long ago against non-iPod music players. We would have loved to be able to use for CircuitPython, for instance, but that would have locked out Mac users.

[earlephilhower]

I also think MTP is not yet supported by TinyUSB, meaning this is a big hill to surmount: hathach/tinyusb#1079