drivers: disk: add fakefat disk driver

Fakefat disk emulates a disk formatted with the FAT file system.
The driver supports FAT16 and FAT32 emulation and multiple instances.
A backend can register at least 15 emulated files on a disk instance.
The file structure contain read and write callbacks that are called
when the filesystem driver accesses the volume. A backend can have
multiple files with read callbacks, but usually only one or zero with
write callbacks.

Fakefat disk can be used for testing or to map services such as
firmware updates to a file on the emulated disk.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>

drivers/disk/CMakeLists.txt

@@ -5,6 +5,7 @@ if(CONFIG_DISK_DRIVERS)
 
 zephyr_library()
 
+zephyr_library_sources_ifdef(CONFIG_DISK_DRIVER_FFAT ffatdisk.c)
 zephyr_library_sources_ifdef(CONFIG_DISK_DRIVER_FLASH flashdisk.c)
 zephyr_library_sources_ifdef(CONFIG_DISK_DRIVER_RAM ramdisk.c)
 zephyr_library_sources_ifdef(CONFIG_DISK_DRIVER_LOOPBACK loopback_disk.c)
@@ -15,4 +16,6 @@ zephyr_library_sources_ifdef(CONFIG_MMC_SUBSYS mmc_subsys.c)
 
 add_subdirectory_ifdef(CONFIG_NVME nvme)
 
+zephyr_linker_sources_ifdef(CONFIG_DISK_DRIVER_FFAT DATA_SECTIONS ffatdisk.ld)
+
 endif()

drivers/disk/Kconfig

@@ -9,6 +9,7 @@ menuconfig DISK_DRIVERS
 if DISK_DRIVERS
 
 source "drivers/disk/Kconfig.ram"
+source "drivers/disk/Kconfig.ffat"
 source "drivers/disk/Kconfig.flash"
 source "drivers/disk/Kconfig.sdmmc"
 source "drivers/disk/Kconfig.mmc"

drivers/disk/Kconfig.ffat

@@ -0,0 +1,19 @@
+# Copyright (c) 2023 Nordic Semiconductor ASA
+# SPDX-License-Identifier: Apache-2.0
+
+config DISK_DRIVER_FFAT
+	bool "Fake FAT Disk"
+	default y if DT_HAS_ZEPHYR_FFAT_DISK_ENABLED
+	help
+	  Fake FAT disk emulates a disk formatted with the FAT file system
+	  and provides very limited functionality. It can be used for testing
+	  or to map services such as firmware updates to a file on
+	  the emulated disk.
+
+if DISK_DRIVER_FFAT
+
+module = FFATDISK
+module-str = ffatdisk
+source "subsys/logging/Kconfig.template.log_config"
+
+endif # DISK_DRIVER_FFAT