samples/fs/fat_fs: Readme update

The sample now support's setting up FAT FS on devices without
SD card access and this new ability requires update in documentation.

Signed-off-by: Dominik Ermel <[email protected]>

samples/subsys/fs/fat_fs/README.rst

@@ -7,27 +7,66 @@ Overview
 ********
 
 This sample app demonstrates use of the filesystem API and uses the FAT file
-system driver to mount an SDHC card connected over a SPI bus or to an on-chip
-SDHC controller.
+system driver with SDHC card, SoC flash or external flash chip.
 
-Requirements
-************
+To access device the sample uses :ref:`disk_access_api`.
+
+Requirements for SD card support
+********************************
 
 This project requires SD card support and microSD card formatted with FAT filesystem.
 See the :ref:`disk_access_api` documentation for Zephyr implementation details.
+Boards that by default use SD card for storage:
+``arduino_mkrzero``, ``esp_wrover_kit``, ``mimxrt1050_evk``, ``nrf52840_blip``
+and  ``olimexino_stm32``.
+The sample should be able to run with any other board that has "zephyr,sdmmc-disk"
+DT node enabled.
+
+Requirements for setting up FAT FS on SoC flash
+***********************************************
+
+For the FAT FS to work with internal flash, the device needs to support erase
+pages of size <= 4096 bytes and have at least 64kiB of flash available for
+FAT FS partition alone.
+Currently the following boards are supported:
+``nrf52840dk_nrf52840``
+
+Requirements for setting up FAT FS on external flash
+****************************************************
+
+This type of configuration requires external flash device to be available
+on DK board. Currently following boards support the configuration:
+``nrf52840dk_nrf52840`` by ``nrf52840dk_nrf52840_qspi`` configuration.
 
 Building and Running
 ********************
 
-This sample can be built for a variety of boards, including ``nrf52840_blip``,
-``mimxrt1050_evk``, ``olimexino_stm32``, ``esp_wrover_kit`` and ``arduino_mkrzero``.
+Boards with default configurations, for example ``arduino_mkrzero`` or
+``nrf52840dk_nrf52840`` using internal flash can be build using command:
 
 .. zephyr-app-commands::
    :zephyr-app: samples/subsys/fs/fat_fs
    :board: nrf52840_blip
    :goals: build
    :compact:
 
-To run this sample, a FAT formatted microSD card should be present in the
+Where used example board ``nrf52840_blip`` should be replaced with desired board.
+
+In case when some more specific configuration is to be used for a given board,
+for example ``nrf52840dk_nrf52840`` with MX25 device over QSPI, configuration
+and DTS overlays need to be also selected. The command would look like this:
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/subsys/fs/fat_fs
+   :board: nrf52840dk_nrf52840
+   :gen-args: -DOVERLAY_CONFIG=nrf52840dk_nrf52840_qspi.conf -DDTC_OVERLAY_FILE=nrf52840dk_nrf52840_qspi.overlay
+   :goals: build
+   :compact:
+
+In case when board with SD card is used FAT microSD card should be present in the
 microSD slot. If there are any files or directories present in the card, the
 sample lists them out on the debug serial output.
+
+.. warning::
+   In case when mount fails the device may get re-formatted to FAT FS.
+   To disable this behaviour disable :kconfig:option:`CONFIG_FS_FATFS_MOUNT_MKFS` .