ESP32-S3 USB MSC and FatFs coexistence

[gregory.heifets]

We're working on an ESP32-S3 project that uses EMMC flash with FAT32 file system that runs on the ESP32.
The EMMC is also accessed by USB MSC when plugged into the PC.

As far as I understand, FatFs that runs on the ESP cannot coexist with MSC.
The idea is to unmount the EMMC and finalize the EMMC transactions gracefully before MSC takes control of the EMMC.
From what we've seen, the MSC takes over the EMMC at the moment the USB is plugged-in without any notification.

What is the best strategy to implement this switchover?
Thanks
Greg

[pacucha42]

Hi Greg,
thank you for the report - the question makes a good sense, now we are checking internally what the steps would be the best solution for you. We will let you know ASAP.

Regards,
Martin

[tore-espressif]

Hi Greg,
You are right. Current implementation notifies the user after the memory was unmounted from FatFs https://github.com/espressif/idf-extra- ... age.c#L323, which does not allow you to gracefully end your EMMC transactions.

As a first step, we will provide a hotfix, that will call the callback before the unmount (with possibility to deny the USB mount).

As a second step, we want to investigate options of parallel access to Virtual File System from both USB Host and your application at the same time.

I'll keep you posted

[ESP_saurabh.bansal]

Hello Greg,

The code v3.1.1 is released that notifies through callback before mount/unmount operation.

Code commit: https://github.com/espressif/idf-extra- ... 86c1209e3d

Thanks.

[mcclellansws]

As a second step, we want to investigate options of parallel access to Virtual File System from both USB Host and your application at the same time.

I'll keep you posted

Any progress with this?

[ESP_Sprite]

As a second step, we want to investigate options of parallel access to Virtual File System from both USB Host and your application at the same time.

I'll keep you posted

Any progress with this?

Don't expect this to work reliably over MSC. MSC offers a block device to the host, and the host can do whatever it wants with that block device; the only implicit rule is that it leaves a valid fatfs filesystem once it's done (unplugged). That means as long as it's plugged in, the flash can be in any random state, and fatfs won't be able to correctly interpret its contents (as it doesn't know the internal state of the host). There's other protocols, like mtp, that work on a file-based level and would be more stable.