samples: subsys: usb: mass: Use &flash0 storage_partition for USB mass storage

[joaodullius]

What would be the necessary changes for samples/subsys/usb/mass to use internal &flash0 storage_partition for the mouting the USB device partition?

From the Kconfig, it appears that it only supports RAM, NAND and SD.

Most boards (at least the nrfxxxxDKs) now have a storage_partition in the end of the &flash. I with to mount the USB there:

  storage_partition: partition@3a000 {
      label = "storage";
      reg = <0x0003a000 0x00006000>;

[de-nordic]

You would need to modify the flash disk driver here: https://github.com/zephyrproject-rtos/zephyr/blob/main/drivers/disk/flashdisk.c
to use DTS instead of taking info from Kconfig, or make DTS alternative source of flash disk configuration.

From the Kconfig, it appears that it only supports RAM, NAND and SD.

These are only labels that the ELM FAT driver recognizes as mount points when doing path lookup; you can mount SD at NAND and SD at RAM, it does not matter (https://github.com/zephyrproject-rtos/fatfs/blob/94fcd6bfb3801ac0a5e12ea2f52187e0a688b90e/include/ffconf.h#L204)

[joaodullius]

@de-nordic, can't I somehow point CONFIG_DISK_FLASH_DEV_NAME to the &flash0?

[de-nordic]

What is flash0? The CONFIG_DISK_FLASH_DEV_NAME points to flash control device; Do you mean that you want to point CONFIG_DISK_FLASH_DEV_NAME to a partition label?

[joaodullius]

Yes, to point to a flash partition on the internal flash of the device.
I'm targeting nrf52833dk_nrf52820 that has the following flash definition in the dts:

&flash0 {

    partitions {
	    compatible = "fixed-partitions";
	    #address-cells = <1>;
	    #size-cells = <1>;

	    boot_partition: partition@0 {
		    label = "mcuboot";
		    reg = <0x00000000 0xC000>;
	    };
	    slot0_partition: partition@C000 {
		    label = "image-0";
		    reg = <0x0000C000 0x17000>;
	    };
	    slot1_partition: partition@23000 {
		    label = "image-1";
		    reg = <0x00023000 0x17000>;
	    };
	    storage_partition: partition@3a000 {
		    label = "storage";
		    reg = <0x0003a000 0x00006000>;
	    };
    };
};

I was wondering if I could somehow from the Kconfig point to the storage_partition instead of the external flash.

[de-nordic]

Unfortunately not with current implementation, changes to the https://github.com/zephyrproject-rtos/zephyr/blob/main/drivers/disk/flashdisk.c and Kconfig are required.
Problem with Kconfig is that it can only take int, string or bool values, which means that passing DTS label/alias like flash0 would be visible as "flash0" and the DTS macros can not deal with strings to get values for partition size and offset.