esp-idf/docs/api-reference/storage/spi_flash.rst

.. include:: ../../../components/spi_flash/README.rst

See also
--------

- :doc:`Partition Table documentation <../../api-guides/partition-tables>`
- :doc:`Over The Air Update (OTA) API <../system/ota>` provides high-level API for updating app firmware stored in flash.
- :doc:`Non-Volatile Storage (NVS) API <nvs_flash>` provides a structured API for storing small items of data in SPI flash.


.. _spi-flash-implementation-details:

Implementation details
----------------------

In order to perform some flash operations, we need to make sure both CPUs
are not running any code from flash for the duration of the flash operation.
In a single-core setup this is easy: we disable interrupts/scheduler and do
the flash operation. In the dual-core setup this is slightly more complicated.
We need to make sure that the other CPU doesn't run any code from flash.


When SPI flash API is called on CPU A (can be PRO or APP), we start
spi_flash_op_block_func function on CPU B using esp_ipc_call API. This API
wakes up high priority task on CPU B and tells it to execute given function,
in this case spi_flash_op_block_func. This function disables cache on CPU B and
signals that cache is disabled by setting s_flash_op_can_start flag.
Then the task on CPU A disables cache as well, and proceeds to execute flash
operation.

While flash operation is running, interrupts can still run on CPUs A and B.
We assume that all interrupt code is placed into RAM. Once interrupt allocation
API is added, we should add a flag to request interrupt to be disabled for
the duration of flash operations.

Once flash operation is complete, function on CPU A sets another flag,
s_flash_op_complete, to let the task on CPU B know that it can re-enable
cache and release the CPU. Then the function on CPU A re-enables the cache on
CPU A as well and returns control to the calling code.

Additionally, all API functions are protected with a mutex (s_flash_op_mutex).

In a single core environment (CONFIG_FREERTOS_UNICORE enabled), we simply
disable both caches, no inter-CPU communication takes place.

API Reference - SPI Flash
-------------------------

.. include:: /_build/inc/esp_spi_flash.inc

API Reference - Partition Table
-------------------------------

.. include:: /_build/inc/esp_partition.inc

API Reference - Flash Encrypt
-----------------------------

.. include:: /_build/inc/esp_flash_encrypt.inc