source: rtems/doc/bsp_howto/shmsupp.t @ d1fbaa6

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@c  COPYRIGHT (c) 1988-2002.
@c  On-Line Applications Research Corporation (OAR).
@c  All rights reserved.
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@c  $Id$
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@chapter 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 @b{Multiprocessing Manager} chapter
of the @b{RTEMS Application C User's Guide}.

@section Shared Memory Configuration Table 

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

@example
@group
typedef volatile rtems_unsigned32 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)( rtems_unsigned32 );
  Shm_Interrupt_information   Intr; /* cause intr information   */
@};

typedef struct shm_config_info shm_config_table;
@end group
@end example

where the fields are defined as follows:

@table @b
@item base
is the base address of the shared memory buffer used to pass
messages between the nodes in the system.

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

@item 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.

@item 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.

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

@item cause_intr

@item Intr
is the information required to cause an interrupt on a node.  This
structure contains the following fields:
@table @b
@item address
is the address to write at to cause an interrupt on that node.
For a polled node, this should be NULL.

@item value
is the value to write to cause an interrupt.

@item 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.
@end table
@end table

@section Primitives

@subsection Convert Address

The @code{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. 

@example
@group
void *Shm_Convert_address( void *address )
@{
   return the local address version of this bus address
@}
@end group
@end example

@subsection Get Configuration

The @code{Shm_Get_configuration} routine is responsible for filling in the
Shared Memory Configuration Table passed to it.  

@example
@group
void Shm_Get_configuration(
  rtems_unsigned32   localnode,
  shm_config_table **shmcfg
)
@{
   fill in the Shared Memory Configuration Table 
@}
@end group
@end example

@subsection 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
@code{field} of the @code{Shm_Locked_queue_Control}
data structure.  This structure is defined as follows:

@example
@group
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;
@end group
@end example

where each field is defined as follows:

@table @b
@item 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.

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

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

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

@end table

@subsubsection Initializing a Shared Lock

The @code{Shm_Initialize_lock} routine is responsible for 
initializing the lock field.  This routines usually is implemented
as follows:

@example
@group
void Shm_Initialize_lock(
  Shm_Locked_queue_Control *lq_cb
)
@{
  lq_cb->lock = LQ_UNLOCKED;
@} 
@end group
@end example

@subsubsection Acquiring a Shared Lock

The @code{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:

@example
@group
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
    @}
@}
@end group
@end example

@subsubsection Releasing a Shared Lock

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

@example
@group
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
@}
@end group
@end example

@section Installing the MPCI ISR

The @code{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:

@example
@group
void Shm_setvec( void )
@{
  install the interprocessor communications ISR
@}
@end group
@end example

On a polled node, this routine would be empty.