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
receives commands to format a flash device
or initialise access to an already formatted device.
is the data file, giving access to the logical blocks on the formatted flash.
For example, it can be given to
for use as a file system.
The length of the
file as returned by
shows the total logical (formatted) space available for
use by the driver's clients.
The target flash device is identified to this driver
in a control message defined below.
The flash device must have the following properties:
- It must have a corresponding control file
which must be writable.
- The flash control file must accept a command of the
which must cause the flash erase unit
starting at the given byte
to be erased.
- 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.)
- 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
- format device [ offset [ n [ erasesize ] ] ]
the given flash
starting at the given byte
and format the erased region
for use by the flash translation layer. Omitting the
optional parameters is equivalent to setting them to
is the number of bytes in the flash device's erase unit; setting
takes the value from the underlying device.
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
then everywhere from
to the end of the
underlying device is erased. Otherwise,
must be multiples of
Make the newly formatted device's contents available on
- init device [ offset [ n [ erasesize ] ] ]
- Make available on
the logical blocks (with existing content) of a previously-formatted
The parameters are as defined for the
- part name start [ limit ]
- Add a partition. This creates a new data file
with similar properties to
but which constrains the range of the formatted data accessed to
and end at
or the last byte of the formatted data if
- delpart name
- Removes a partition.
- Stop flash translation on
the corresponding flash device,
and close it.
An error results instead if
- Force scavenging of reusable blocks (mainly intended to be used when
testing flash or debugging the driver).
- trace n
- Trace the actions of the flash translation driver.
No tracing is done if
Larger values of
increase the level of detail.