How to create a FAT image consumable by micropython

[pulkin]

I tried to use pyfatfs with default arguments and it does not work (ENODEV is raised when trying to mount fat.img).

from pyfatfs.PyFat import PyFat

image = PyFat()
with open("fat.img", 'wb') as f:
    image.mkfs(f.name, 12, 32 * 1024)
    image.close()

[peterhinch]

In general system-level drivers written for CPython are unlikely to work in MicroPython. MP does support FAT on a disk-type device, but does not support creating a FAT partition in a file.

[dpgeorge]

You can do this using the unix port of MicroPython. For example:

import uos


TRACE = 1

class RAMBlockDevice:
    def __init__(self, block_size, num_blocks):
        self.block_size = block_size
        self.data = bytearray(num_blocks * self.block_size)

    def readblocks(self, block, buf, off=0):
        if TRACE:
            print(f"readblocks({block=}, {len(buf)=}, {off=}")
        addr = block * self.block_size + off
        for i in range(len(buf)):
            buf[i] = self.data[addr + i]

    def writeblocks(self, block, buf, off=0):
        if TRACE:
            print(f"writeblocks({block=}, {len(buf)=}, {off=}", buf[:100] if any(buf) else "ZERO")
        addr = block * self.block_size + off
        for i in range(len(buf)):
            self.data[addr + i] = buf[i]

    def ioctl(self, op, arg):
        if op == 4:  # block count
            return len(self.data) // self.block_size
        if op == 5:  # block size
            return self.block_size
        if op == 6:  # erase block
            return 0


# Create and mount a user filesystem.
bdev = RAMBlockDevice(512, 128)
print("mkfs")
uos.VfsFat.mkfs(bdev)
print("mkfs done")
print("mount")
uos.mount(uos.VfsFat(bdev), "/userfs")
print(uos.statvfs("/userfs"))
print("mount done")

# Test open for writing.
print("create file")
with open("/userfs/test.txt", "w") as f:
    print(f.write("".join(f"hello{i}" for i in range(100))))
print("create file done")

Then the filesystem's raw data is in bdev.data, which can be saved to a file.

[pulkin]

Increased image size and it did help image.mkfs(f.name, 12, 1024 * 1024).

Full story: I mount this image using the built-in os.mount on ESP32. I use/develop a small library to serve the image as a network block device.