rtems-docs/bsp_howto/shared_memory_support.rst

Shared Memory Support Driver
############################

The Shared Memory Support Driver is responsible for providing glue
routines and configuration information required by the Shared
Memory Multiprocessor Communications Interface (MPCI).  The
Shared Memory Support Driver tailors the portable Shared
Memory Driver to a particular target platform.

This driver is only required in shared memory multiprocessing
systems that use the RTEMS mulitprocessing support.  For more
information on RTEMS multiprocessing capabilities and the
MPCI, refer to the *Multiprocessing Manager* chapter
of the *RTEMS Application C User's Guide*.

Shared Memory Configuration Table
=================================

The Shared Memory Configuration Table is defined in the following
structure:
.. code:: c

    typedef volatile uint32_t vol_u32;
    typedef struct {
    vol_u32 \*address;        /* write here for interrupt    \*/
    vol_u32  value;          /* this value causes interrupt \*/
    vol_u32  length;         /* for this length (0,1,2,4)   \*/
    } Shm_Interrupt_information;
    struct shm_config_info {
    vol_u32           \*base;       /* base address of SHM         \*/
    vol_u32            length;     /* length (in bytes) of SHM    \*/
    vol_u32            format;     /* SHM is big or little endian \*/
    vol_u32          (\*convert)(); /* neutral conversion routine  \*/
    vol_u32            poll_intr;  /* POLLED or INTR driven mode  \*/
    void             (\*cause_intr)( uint32_t );
    Shm_Interrupt_information   Intr; /* cause intr information   \*/
    };
    typedef struct shm_config_info shm_config_table;

where the fields are defined as follows:

*base*
    is the base address of the shared memory buffer used to pass
    messages between the nodes in the system.

*length*
    is the length (in bytes) of the shared memory buffer used to pass
    messages between the nodes in the system.

*format*
    is either SHM_BIG or SHM_LITTLE to indicate that the neutral format
    of the shared memory area is big or little endian.  The format
    of the memory should be chosen to match most of the inter-node traffic.

*convert*
    is the address of a routine which converts from native format to
    neutral format.   Ideally, the neutral format is the same as the
    native format so this routine is quite simple.

*poll_intr*
    is either INTR_MODE or POLLED_MODE to indicate how the node will be
    informed of incoming messages.

*cause_intr*

*Intr*

    is the information required to cause an interrupt on a node.  This
    structure contains the following fields:

    *address*

        is the address to write at to cause an interrupt on that node.
        For a polled node, this should be NULL.

    *value*

        is the value to write to cause an interrupt.

    *length*

        is the length of the entity to write on the node to cause an interrupt.
        This can be 0 to indicate polled operation, 1 to write a byte, 2 to
        write a sixteen-bit entity, and 4 to write a thirty-two bit entity.

Primitives
==========

Convert Address
---------------

The ``Shm_Convert_address`` is responsible for converting an address
of an entity in the shared memory area into the address that should be
used from this node.  Most targets will simply return the address
passed to this routine.  However, some target boards will have a special
window onto the shared memory.  For example, some VMEbus boards have
special address windows to access addresses that are normally reserved
in the CPU's address space.
.. code:: c

    void \*Shm_Convert_address( void \*address )
    {
    return the local address version of this bus address
    }

Get Configuration
-----------------

The ``Shm_Get_configuration`` routine is responsible for filling in the
Shared Memory Configuration Table passed to it.
.. code:: c

    void Shm_Get_configuration(
    uint32_t           localnode,
    shm_config_table \**shmcfg
    )
    {
    fill in the Shared Memory Configuration Table
    }

Locking Primitives
------------------

This is a collection of routines that are invoked by the portable
part of the Shared Memory Driver to manage locks in the shared
memory buffer area.  Accesses to the shared memory must be
atomic.  Two nodes in a multiprocessor system must not be manipulating
the shared data structures simultaneously.  The locking primitives
are used to insure this.

To avoid deadlock, local processor interrupts should be disabled the entire
time the locked queue is locked.

The locking primitives operate on the lock``field`` of the ``Shm_Locked_queue_Control``
data structure.  This structure is defined as follows:
.. code:: c

    typedef struct {
    vol_u32 lock;  /* lock field for this queue    \*/
    vol_u32 front; /* first envelope on queue      \*/
    vol_u32 rear;  /* last envelope on queue       \*/
    vol_u32 owner; /* receiving (i.e. owning) node \*/
    } Shm_Locked_queue_Control;

where each field is defined as follows:

*lock*
    is the lock field.  Every node in the system must agree on how this
    field will be used.  Many processor families provide an atomic
    "test and set" instruction that is used to manage this field.

*front*
    is the index of the first message on this locked queue.

*rear*
    is the index of the last message on this locked queue.

*owner*
    is the node number of the node that currently has this structure locked.

Initializing a Shared Lock
~~~~~~~~~~~~~~~~~~~~~~~~~~

The ``Shm_Initialize_lock`` routine is responsible for
initializing the lock field.  This routines usually is implemented
as follows:
.. code:: c

    void Shm_Initialize_lock(
    Shm_Locked_queue_Control \*lq_cb
    )
    {
    lq_cb->lock = LQ_UNLOCKED;
    }

Acquiring a Shared Lock
~~~~~~~~~~~~~~~~~~~~~~~

The ``Shm_Lock`` routine is responsible for
acquiring the lock field.  Interrupts should be
disabled while that lock is acquired.  If the lock
is currently unavailble, then the locking routine
should delay a few microseconds to allow the other
node to release the lock.  Doing this reduces bus contention
for the lock.  This routines usually is implemented as follows:
.. code:: c

    void Shm_Lock(
    Shm_Locked_queue_Control \*lq_cb
    )
    {
    disable processor interrupts
    set Shm_isrstat to previous interrupt disable level
    while ( TRUE ) {
    atomically attempt to acquire the lock
    if the lock was acquired
    return
    delay some small period of time
    }
    }

Releasing a Shared Lock
~~~~~~~~~~~~~~~~~~~~~~~

The ``Shm_Unlock`` routine is responsible for
releasing the lock field and reenabling processor
interrupts.  This routines usually is implemented as follows:
.. code:: c

    void Shm_Unlock(
    Shm_Locked_queue_Control \*lq_cb
    )
    {
    set the lock to the unlocked value
    reenable processor interrupts to their level prior
    to the lock being acquired.  This value was saved
    in the global variable Shm_isrstat
    }

Installing the MPCI ISR
=======================

The ``Shm_setvec`` is invoked by the portable portion
of the shared memory to install the interrupt service routine
that is invoked when an incoming message is announced.  Some
target boards support an interprocessor interrupt or mailbox
scheme and this is where the ISR for that interrupt would be
installed.

On an interrupt driven node, this routine would be implemented
as follows:
.. code:: c

    void Shm_setvec( void )
    {
    install the interprocessor communications ISR
    }

On a polled node, this routine would be empty.

.. COMMENT: COPYRIGHT (c) 1988-2009.

.. COMMENT: On-Line Applications Research Corporation (OAR).

.. COMMENT: All rights reserved.