flash – flash memory

bind –a #F[n] /dev

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 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.

The top level directory contains a single directory named flash for bank 0, and flashn for each other bank n. It contains two files for each partition: a data file part and an associated control file partctl, where 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 flash representing the whole of flash memory, and the corresponding control file flashctl. Other partitions can be created by writing to flashctl as described below.

The data file 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 0xFF (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.

The control file partctl 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 manufacturer ID, the flash device ID, the memory width in bytes, and a string giving the flash type (currently either nor or 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:
add name start end
Create a new partition that ranges from start to end within the current partition. Each value must be numeric (decimal, octal or hexadecimal) and a multiple of the erase unit size. Name must not be the name of an existing partition. On success, new files name and namectl will appear in the parent flash directory.
erase allErase the whole flash partition, setting all bytes to 0xFF, except those that are hardware write–protected.
erase offsetErase the segment that begins at the given offset within the partition, setting all bytes to 0xFF, except those that are hardware write–protected. The offset is given in bytes, but must be a multiple of the segment (erase unit) size.
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 off removes that protection. Writing 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.
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 strtoul (in atof(2)); the default base is 10.


flashfs(4), paqfs(4)

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.