Flash map
The <zephyr/storage/flash_map.h> API allows accessing information about device
flash partitions via flash_area structures.
Each flash_area describes a flash partition. The API provides access
to a “flash map”, which contains predefined flash areas accessible via globally
unique ID numbers. The map is created from “fixed-partition” compatible entries
in DTS file. Users may also create flash_area objects at runtime
for application-specific purposes.
This documentation uses “flash area” when referencing single “fixed-partition” entities.
The flash_area contains a pointer to a device,
which can be used to access the flash device an area is placed on directly
with the flash API.
Each flash area is characterized by a device it is placed on, offset
from the beginning of the device and size on the device.
An additional identifier parameter is used by the flash_area_open()
function to find flash area in flash map.
The flash_map.h API provides functions for operating on a flash_area.
The main examples are flash_area_read() and flash_area_write().
These functions are basically wrappers around the flash API with additional
offset and size checks, to limit flash operations to a predefined area.
Most <zephyr/storage/flash_map.h> API functions require a flash_area object pointer
characterizing the flash area they will be working on. There are two possible
methods to obtain such a pointer:
obtain it using flash_area_open;
defining a
flash_areatype object, which requires providing a validdeviceobject pointer with offset and size of the area within the flash device.
flash_area_open() uses numeric identifiers to search flash map for
flash_area objects and returns, if found, a pointer to an object
representing area with given ID.
The ID number for a flash area can be obtained from a fixed-partition
DTS node label using FIXED_PARTITION_ID(); these labels are obtained
from the devicetree as described below.
Relationship with Devicetree
The flash_map.h API uses data generated from the Devicetree API, in particular its Fixed flash partitions. Zephyr additionally has some partitioning conventions used for Device Firmware Upgrade via the MCUboot bootloader, as well as defining partitions usable by file systems or other nonvolatile storage.
Here is an example devicetree fragment which uses fixed flash partitions for both MCUboot and a storage partition. Some details were left out for clarity.
/ {
soc {
flashctrl: flash-controller@deadbeef {
flash0: flash@0 {
compatible = "soc-nv-flash";
reg = <0x0 0x100000>;
partitions {
compatible = "fixed-partitions";
#address-cells = <0x1>;
#size-cells = <0x1>;
boot_partition: partition@0 {
reg = <0x0 0x10000>;
read-only;
};
storage_partition: partition@1e000 {
reg = <0x1e000 0x2000>;
};
slot0_partition: partition@20000 {
reg = <0x20000 0x60000>;
};
slot1_partition: partition@80000 {
reg = <0x80000 0x60000>;
};
scratch_partition: partition@e0000 {
reg = <0xe0000 0x20000>;
};
};
};
};
};
};
Partition offset shall be expressed in relation to the flash memory beginning address, to which the partition belongs to.
The boot_partition, slot0_partition, slot1_partition, and
scratch_partition node labels are defined for MCUboot, though not all MCUboot
configurations require all of them to be defined. See the MCUboot
documentation for more details.
The storage_partition node is defined for use by a file system or other
nonvolatile storage API.
Numeric flash area ID is obtained by passing DTS node label to
FIXED_PARTITION_ID(); for example to obtain ID number
for slot0_partition, user would invoke FIXED_PARTITION_ID(slot0_partition).
All FIXED_PARTITION_ macros take DTS node labels as partition
identifiers.
Users do not have to obtain a flash_area object pointer
using flash_map_open() to get information on flash area size, offset
or device, if such area is defined in DTS file. Knowing the DTS node label
of an area, users may use FIXED_PARTITION_OFFSET(),
FIXED_PARTITION_SIZE() or FIXED_PARTITION_DEVICE()
respectively to obtain such information directly from DTS node definition.
For example to obtain offset of storage_partition it is enough to
invoke FIXED_PARTITION_OFFSET(storage_partition).
Below example shows how to obtain a flash_area object pointer
using flash_area_open() and DTS node label:
const struct flash_area *my_area;
int err = flash_area_open(FIXED_PARTITION_ID(slot0_partition), &my_area);
if (err != 0) {
handle_the_error(err);
} else {
flash_area_read(my_area, ...);
}