14. Non-Volatile Memory Driver¶

The Non-Volatile driver is responsible for providing an interface to various types of non-volatile memory. These types of memory include, but are not limited to, Flash, EEPROM, and battery backed RAM. The capabilities provided by this class of device driver are:

Initialize the Non-Volatile Memory Driver
Optional Disable Read and Write Handlers
Open a Particular Memory Partition
Close a Particular Memory Partition
Read from a Particular Memory Partition
Write to a Particular Memory Partition
Erase the Non-Volatile Memory Area

There is currently only one non-volatile device driver included in the RTEMS source tree. The information provided in this chapter is based on drivers developed for applications using RTEMS. It is hoped that this driver model information can form the basis for a standard non-volatile memory driver model that can be supported in future RTEMS distribution.

14.1. Major and Minor Numbers¶

The major number of a device driver is its index in the RTEMS Device Address Table.

A minor number is associated with each device instance managed by a particular device driver. An RTEMS minor number is an unsigned32 entity. Convention calls dividing the bits in the minor number down into categories that specify an area of non-volatile memory and a partition with that area. This results in categories like the following:

area - indicates a block of non-volatile memory
partition - indicates a particular address range with an area

From the above, it should be clear that a single device driver can support multiple types of non-volatile memory in a single system. The minor number is used to distinguish the types of memory and blocks of memory used for different purposes.

14.2. Non-Volatile Memory Driver Configuration¶

There is not a standard non-volatile driver configuration table but some fields are common across different drivers. The non-volatile memory driver configuration table is typically an array of structures with each structure containing the information for a particular area of non-volatile memory. The following is a list of the type of information normally required to configure each area of non-volatile memory.

memory_type

is the type of memory device in this area. Choices are battery backed RAM, EEPROM, Flash, or an optional user-supplied type. If the user-supplied type is configured, then the user is responsible for providing a set of routines to program the memory.

memory

is the base address of this memory area.

attributes

is a pointer to a memory type specific attribute block. Some of the fields commonly contained in this memory type specific attribute structure area:

use_protection_algorithm

is set to TRUE to indicate that the protection (i.e. locking) algorithm should be used for this area of non-volatile memory. A particular type of non-volatile memory may not have a protection algorithm.

access

is an enumerated type to indicate the organization of the memory devices in this memory area. The following is a list of the access types supported by the current driver implementation:

simple unsigned8

simple unsigned16

simple unsigned32

simple unsigned64

single unsigned8 at offset 0 in an unsigned16

single unsigned8 at offset 1 in an unsigned16

single unsigned8 at offset 0 in an unsigned32

single unsigned8 at offset 1 in an unsigned32

single unsigned8 at offset 2 in an unsigned32

single unsigned8 at offset 3 in an unsigned32

depth

is the depth of the progamming FIFO on this particular chip. Some chips, particularly EEPROMs, have the same programming algorithm but vary in the depth of the amount of data that can be programmed in a single block.

number_of_partitions

is the number of logical partitions within this area.

Partitions

is the address of the table that contains an entry to describe each partition in this area. Fields within each element of this table are defined as follows:

offset: is the offset of this partition from the base address of this area.
length: is the length of this partition.

By dividing an area of memory into multiple partitions, it is possible to easily divide the non-volatile memory for different purposes.

14.3. Initialize the Non-Volatile Memory Driver¶

At system initialization, the non-volatile memory driver’s initialization entry point will be invoked. As part of initialization, the driver will perform whatever initializatin is required on each non-volatile memory area.

The discrete I/O driver may register device names for memory partitions of particular interest to the system. Normally this will be restricted to the device “/dev/nv_memory” to indicate the entire device driver.

14.4. Disable Read and Write Handlers¶

Depending on the target’s non-volatile memory configuration, it may be possible to write to a status register and make the memory area completely inaccessible. This is target dependent and beyond the standard capabilities of any memory type. The user has the optional capability to provide handlers to disable and enable access to a partiticular memory area.

14.5. Open a Particular Memory Partition¶

This is the driver open call. Usually this call does nothing other than validate the minor number.

With some drivers, it may be necessary to allocate memory when a particular device is opened. If that is the case, then this is often the place to do this operation.

14.6. Close a Particular Memory Partition¶

This is the driver close call. Usually this call does nothing.

With some drivers, it may be necessary to allocate memory when a particular device is opened. If that is the case, then this is the place where that memory should be deallocated.

14.7. Read from a Particular Memory Partition¶

This corresponds to the driver read call. After validating the minor number and arguments, this call enables reads from the specified memory area by invoking the user supplied “enable reads handler” and then reads the indicated memory area. When invoked the argument_block is actually a pointer to the following structure type:

typedef struct {
  uint32_t  offset;
  void     *buffer;
  uint32_t  length;
  uint32_t  status;
} Non_volatile_memory_Driver_arguments;

The driver reads length bytes starting at offset into the partition and places them at buffer. The result is returned in status.

After the read operation is complete, the user supplied “disable reads handler” is invoked to protect the memory area again.

14.8. Write to a Particular Memory Partition¶

This corresponds to the driver write call. After validating the minor number and arguments, this call enables writes to the specified memory area by invoking the “enable writes handler”, then unprotecting the memory area, and finally actually writing to the indicated memory area. When invoked the argument_block is actually a pointer to the following structure type:

typedef struct {
  uint32_t   offset;
  void      *buffer;
  uint32_t   length;
  uint32_t   status;
} Non_volatile_memory_Driver_arguments;

The driver writes length bytes from buffer and writes them to the non-volatile memory starting at offset into the partition. The result is returned in status.

After the write operation is complete, the “disable writes handler” is invoked to protect the memory area again.

14.9. Erase the Non-Volatile Memory Area¶

This is one of the IOCTL functions supported by the I/O control device driver entry point. When this IOCTL function is invoked, the specified area of non-volatile memory is erased.