Internal Filesystem

There are two layers involved with the internal filesystem. FlashFX Pro does wear leveling and bad block management while Reliance is the filesystem on top of it.

FlashFX Pro

FlashFX Pro maps logical addresses to physical addresses on the NAND. The physical space is divided into regions which are divided into units (erase blocks), each unit divided up into blocks (each equal to page size of the NAND). All integers are read as little endian.

The first block in a unit has a header describing the unit:

	0x0	0x1	0x2	0x3
0x00	Signature
0x04	Signature
0x08	Signature
0x0C	Signature
0x10	clientAddress
0x14	eraseCount
0x18	serialNumber
0x1C	ulSequenceNumber
0x20	lnuTotal
0x24	lnuTag
0x28	numSpareUnits		blockSize
0x2C	lnuPerRegion		partitionStartUnit
0x30	unitTotalBlocks		unitClientBlocks
0x34	unitDataBlocks		checksum

The spare area of the NAND is 1/32th of the page size and holds extra information

1	2	3	4
Alloc info		ones-complement of byte 0 XOR byte 1	error-correcting Hamming code of bytes 0-2
seems to always be FF FF FF 0F for used pages, FF FF FF FF for unused
error-correcting code of second half of page data
error-correcting code of first half of page data

Checksum

The checksum in the unit header is calculated by adding all the bytes in the header mod 2^16.

uint16_t checksum(void *_ptr, size_t size) {
	uint16_t sum = 0;
	uint8_t *ptr = _ptr;

	while (size--) {
		sum += *ptr++;
	}

	return sum;
}

Allocation information

This contains the status and logical address of the block. A unit header will have a magic signature (0x48E2).

Status

Bits 12-15 indicate the status of the block.

Probably irrelevant for this version of FlashFX (always 0x4).

Logical address

Mask with 0x0FFF to get address. 0x8E2 is a magic number for unit headers.

Retrieving a page

The best algorithm for retrieving page data from a FlashFX partition is to first create a lookup table resolving logical page numbers to physical page numbers and use that to retrieve the page.

To build the table, first iterate through every unit. Your table needs to hold two pieces of data: the physical page number and a sequence number. The key should be the logical page number. The table will be unitClientBlocks * lnuTotal entries long.

For each unit, read the header, then iterate through the blocks in the unit. You need to gather two pieces of information for each block in the unit: the logical page address of that block and the sequence number of the unit it belongs to. The logical page number is equal to the unit's (clientAddress / blockSize) + the block's logical address.

If the page number is larger than the one already existing in the lookup table and if the sequence number is greater, the entry for that logical page number is set to the current block. Basically, the latest version of the logical page is the one with the greatest physical page number and sequence number.

Use the lookup table to map logical page addresses to physical page addresses.

If a lookup table entry is empty, it means no physical page has been allocated to it and the data at that logical address is a blank page (a page filled with 0xFF s).

Reliance

TODO

Resources

Goplat's post on Omnimaga

Patent for FlashFX