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:
- format device [ offset [ n [ erasesize ] ] ]
- 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.
- init device [ offset [ n [ erasesize ] ] ]
- 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.
- part name start [ limit ]
- 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.
- delpart name
- Removes a partition.
- detach
- Stop flash translation on
the corresponding flash device,
and close it.
An error results instead if
ftldata
is open.
- scavenge
- 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
n
is zero.
Larger values of
n
increase the level of detail.