FTL(3) Library Functions Manual FTL(3)

ftl - flash translation layer

bind -a '#X' /dev
/dev/ftlctl
/dev/ftldata

The flash translation layer device provides the interface for management of rewritable blocks on a flash memory device. Flash memory differs from normal disc or memory in that it is organised in large blocks (erase units), typically 64k bytes or more in size, and although writes can reset bits they cannot set them; instead an entire erase unit must be erased at once. These properties make it unsuitable for direct use by a conventional block-oriented file system. The flash translation layer compensates by implementing a logical to physical mapping that allows 512-byte blocks to be read or written in the same way as rewritable disc blocks. The translation layer manages the details of block remapping, copying erase units to reclaim obsolete physical versions of rewritten logical blocks, erase unit load wearing, etc.

The flash translation device serves a one-level directory, giving access to two files. The control file ftlctl receives commands to format a flash device or initialise access to an already formatted device. Ftldata is the data file, giving access to the logical blocks on the formatted flash. For example, it can be given to kfs (3) for use as a file system. The length of the ftldata file as returned by Sys->stat shows the total logical (formatted) space available for use by the driver's clients.

The target flash device is identified to this driver by name (eg, #F/flash) in a control message defined below. The flash device must have the following properties:

1.
It must have a corresponding control file devicectl (eg, #F/flashctl), which must be writable.
2.
The flash control file must accept a command of the form erase offset which must cause the flash erase unit starting at the given byte offset to be erased.
3.
The device must allow reads and writes of any number of bytes on arbitrary byte boundaries (file offset). (In other words, the flash driver must hide alignment restrictions.)
4.
A write request must allow previously-written regions to be updated provided the new data does not change any 0 bit to 1 (ie, writes can clear bits to 0 but will not change any 0 bits to 1).

The following control messages can be written to ftlctl:


Erase n bytes of the given flash device starting at the given byte offset, and format the erased region for use by the flash translation layer. Omitting the optional parameters is equivalent to setting them to 0xffffffff. Erasesize is the number of bytes in the flash device's erase unit; setting to 0xffffffff takes the value from the underlying device. If offset is 0xffffffff, then the underlying device is searched from the start for an existing flash translation layer header, and the remaining parameters are taken from there. If n is 0xffffffff, then everywhere from offset to the end of the underlying device is erased. Otherwise, offset and n must be multiples of erasesize. Make the newly formatted device's contents available on ftldata.

Make available on ftldata the logical blocks (with existing content) of a previously-formatted device. The parameters are as defined for the format command, above.

Add a partition. This creates a new data file ftlname with similar properties to ftldata, but which constrains the range of the formatted data accessed to begin at start, and end at limit-1, or the last byte of the formatted data if limit is omitted.

Removes a partition.
Stop flash translation on the corresponding flash device, and close it. An error results instead if ftldata is open.
Force scavenging of reusable blocks (mainly intended to be used when testing flash or debugging the driver).

Trace the actions of the flash translation driver. No tracing is done if n is zero. Larger values of n increase the level of detail.

/os/*/devftl.c

flash (3)

Reads and writes of ftldata must be multiples of 512 bytes in length and start on a 512-byte block boundary.
Only one flash device can be active.