shithub: 9ferno

ref: b502a62da2ec6058923db94f87ecc2d29db2fa77
dir: /man/3/flash/

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flash \- flash memory
.BI "bind -a #F" \fR[\fPn\fR]\fP " /dev"

.B /dev/flash
.BI /dev/flash/ part
.BI /dev/flash/ part ctl
The flash memory device serves a two-level directory,
giving access to files representing part or all of a bank of flash memory.
A platform might have more than one bank of flash, numbered starting from 0.
The attach specifier
.I n
is a decimal integer that selects a particular bank of flash (default: 0).
Both NOR and NAND flash is supported.
For both types of flash, the driver gives a read/write/erase interface to the raw flash device,
which can impose constraints on operations beyond those imposed by the driver.
Other drivers such as
.IR ftl (3)
.IR logfs (3)
implement any higher-level format required, including ECC for NAND flash, for instance.
The top level directory contains a single directory named
.B flash
for bank 0, and
.BI flash n
for each other bank
.IR n .
It contains two files for each partition:
a data file
.I part
and an associated control file
.IB part ctl ,
.I part
is the name of the partition.
Each partition represents a region of flash memory that starts and ends on a flash segment (erase unit) boundary.
The system initially creates a single standard partition
.B flash
representing the whole of flash memory, and the corresponding control file
.BR flashctl .
Other partitions can be created by writing to
.B flashctl
as described below.
The data file
.I part
provides read and write access to the bytes on the system's flash memory.
Bytes can be read and written on any byte boundary:
the interface hides any alignment restrictions.
A read returns the value of the bytes at the current file offset, where zero is the start of the partition.
A write reprograms the flash to the given byte values, at the current
file offset (relative to the start of the partition), using the physical
device's reprogramming algorithm.
An erased flash byte is logically
.B 16rFF
(regardless of the conventions of the physical flash device).
A write can change a bit with value 1 to a 0,
but cannot change a 0 bit to 1;
that can only be done by erasing one or more flash segments.
NAND flash typically has restrictions on the number of writes allowed to a page before requiring a block erase.
Reads and writes are unbuffered.
The control file
.BI part ctl
can be read and written.
A read returns several lines containing decimal and hexadecimal numbers
(separated by white space)
revealing the characteristics of memory within the partition.
The first line gives the 
the manufacturer ID,  the flash device ID, the memory width in bytes, and a string giving the flash type
(currently either
.B nor
.BR nand ).
Subsequent lines give characteristics of each group of erase units within the partition,
where the erase units within a group have the same properties.
Each line gives the start and end (as byte addresses) of the erase units in the region
that lie within the partition, followed by the size in bytes of each erase unit, which is followed
for NAND flash by the size in bytes of a page.
The sizes for NAND flash include the extra bytes per page typically used to hold an ECC and block status.
A write contains one of the following textual commands:
.TF erasexx
.BI add " name start end"
Create a new partition that ranges from
.I start
.I end
within the current partition.
Each value must be numeric (decimal, octal or hexadecimal) and a multiple of the erase unit size.
.I Name
must not be the name of an existing partition.
On success, new files
.I name
.IB name ctl
will appear in the parent
.B flash
.B erase all
Erase the whole flash partition, setting all bytes to
.BR 16rFF ,
except those that are hardware write-protected.
.BI erase " offset"
Erase the segment that begins at the given
.I offset
within the partition,
setting all bytes to
.BR 16rFF ,
except those that are hardware write-protected.
.I offset
is given in bytes, but must be a multiple
of the segment (erase unit) size.
.BR protectboot [ " off " ]
By default the system prevents erase unit 0 of the flash from being
erased or written, assuming it
contains the primary bootstrap.
Writing this command with parameter
.B off
removes that protection.
.B protectboot
with any other parameter (or none) restores the protection.
Note that a manufacturer might also have locked the flash in hardware,
and that protection must be removed in a device-dependent way.
.B sync
If the underlying device must buffer or cache (current devices do not), flush the buffer(s).
The syntax of all numbers is that of
.BR strtoul (10.2) ;
the default base is 10.
.B /os/*/devflash.c
.B /os/*/flash*.c
.IR ftl (3)
A write will return an error if
an attempt is made to change a 0 bit to 1,
or if the flash memory fails to be programmed correctly.
The flash cannot be written if the kernel is executing directly from flash,
because the physical flash cannot be read during programming,
and the driver does not copy the programming code to DRAM.