drivers/mtd: add support for direct MTD access to raw flash

Documentation/components/drivers/special/mtd.rst

@@ -49,8 +49,29 @@ See include/nuttx/mtd/mtd.h for additional information.
    #. Provide that instance to the initialization method of the
       higher level device driver.
 
+-  **Registering MTD Drivers**. MTD partition can be registered as a
+   standard character device driver. This allows to perform standard
+   ``open/read/write/ioctl`` calls directly on the MTD layer (and thus
+   directly on raw flash). This has some advantages compared to FTL
+   and BCH layers used to register flash device. FTL layer has to buffer
+   the entire erase page, because write operation leads to erase page
+   read, erase and then write the modified buffer. Apart from larger
+   RAM memory consumption, the data may be lost during power cut off,
+   because erase takes a considerable amount of time. On the other hand,
+   an access with FTL/BCH layer takes care of page erase if needed, thus
+   simplifying the application logic.
+
+   Direct access through MTD layer can be faster and lower RAM consumption,
+   but the application has to take care of page erase before write if needed.
+   The erase can be performed by ioctl call ``MTDIOC_ERASESECTORS``.
+   The driver is registered with ``mtd_partition_register`` function call.
+
+   Configuration option ``CONFIG_MTD_PARTITION_REGISTER`` has to be selected
+   to support this feature.
+
 -  **Examples**: ``drivers/mtd/m25px.c`` and ``drivers/mtd/ftl.c``
 
+
 EEPROM
 ======
 

Documentation/implementation/drivers_design.rst

@@ -57,6 +57,12 @@ on top of another MTD driver, it changes the apparent page size of the FLASH to
 ``drivers/mtd/mtd_partitions.c`` can be used to break up a large FLASH into
 separate, independent partitions, each of which looks like another MTD driver.
 
+It is possible to register the partition as a character device driver. This
+provides a direct access to the raw flash, but doesn't ensure any logic
+needed to operate the flash (page erase prior to write, wear leveling,
+fault block detection). The driver is implemented in
+``drivers/mtd/mtd_register.c``.
+
 ``drivers/mtd/ftl.c`` is also interesting. FTL stands for FLASH Translation Layer.
 The FTL driver is an MTD driver that when layered on top of another MTD driver,
 converts the MTD driver to a block driver. The permutations are endless.

drivers/mtd/CMakeLists.txt

@@ -33,6 +33,10 @@ if(CONFIG_MTD)
     list(APPEND SRCS mtd_partition.c)
   endif()
 
+  if(CONFIG_MTD_PARTITION_REGISTER)
+    list(APPEND SRCS mtd_register.c)
+  endif()
+
   if(CONFIG_MTD_SECT512)
     list(APPEND SRCS sector512.c)
   endif()

drivers/mtd/Kconfig

@@ -40,6 +40,25 @@ config MTD_PARTITION
 		managing the sub-region of flash beginning at 'offset' (in blocks)
 		and of size 'nblocks' on the device specified by 'mtd'.
 
+config MTD_PARTITION_REGISTER
+	bool "Register MTD partition as a driver"
+	default n
+	depends on MTD_PARTITION
+	---help---
+		This option allows to register created MTD partition as a device
+		driver and perform standard open/close/read/write/ioctl calls
+		directly on the MTD layer. This has some pros and cons compared
+		to the access through FTL or/and BCH layers. FTL layer buffers
+		the erase page, thus increasing RAM memory consumption. It also
+		means the data may be lost during power cutoff as flash write
+		means read, erase, write and erase operation may take time. On the
+		other hand, the access through FTL/BCH takes care of page erase
+		if needed, thus is easier to manage from an application.
+
+		Direct access to MTD layer can be faster and lower RAM memory
+		consumption, but the application has to take care of page erase
+		before writes if needed. This can be done with ioctl calls.
+
 config FTL_WRITEBUFFER
 	bool "Enable write buffering in the FTL layer"
 	default n

drivers/mtd/Make.defs

@@ -41,6 +41,10 @@ ifeq ($(CONFIG_MTD_PARTITION),y)
 CSRCS += mtd_partition.c
 endif
 
+ifeq ($(CONFIG_MTD_PARTITION_REGISTER),y)
+CSRCS += mtd_register.c
+endif
+
 ifeq ($(CONFIG_MTD_SECT512),y)
 CSRCS += sector512.c
 endif