section regarding shared memo added.
Signed-off-by: Christophe Milard <christophe.mil...@linaro.org>
---
doc/users-guide/users-guide.adoc | 129 +++++++++++++++++++++++++++++++++++++++
1 file changed, 129 insertions(+)
diff --git a/doc/users-guide/users-guide.adoc b/doc/users-guide/users-guide.adoc
index bbb53a7..07d8949 100644
--- a/doc/users-guide/users-guide.adoc
+++ b/doc/users-guide/users-guide.adoc
@@ -543,6 +543,135 @@ lookup. The lookup function is particularly useful to
allow an ODP application
that is divided into multiple processes to obtain the handle for the common
resource.
+== Shared memory
+=== Allocating shared memory
+Blocks of shared memory can be created using the +odp_shm_reserve()+ API
+call. The call expects a shared memory block name, a block size, an alignment
+requirement, and optional flags as parameters. It returns a +odp_shm_t+
+handle. The size and alignment requirement are given in bytes.
+
+.creating a block of shared memory
+[source,c]
+----
+#define ALIGNMENT 128
+#define BLKNAME +shared_items+
+
+odp_shm_t shm;
+
+typedef struct {
+...
+} shared_data_t;
+
+shm = odp_shm_reserve(BLKNAME, sizeof(shared_data_t), ALIGNMENT, 0);
+----
+
+=== Getting the shared memory block address
+The returned odp_shm_t handle can then be used to retrieve the actual
+address (in the caller's ODP thread virtual address space) of the created
+shared memory block.
+
+.getting the address of a shared memory block
+[source,c]
+----
+shared_data_t *shared_data;
+shared_data = odp_shm_addr(shm);
+----
+
+The address returned by +odp_shm_addr()+ is valid only in the calling ODP
+thread space: odp_shm_t handles can be shared between ODP threads and remain
+valid within any threads, whereas the address returned by +odp_shm_addr(shm)+
+may differ from ODP threads to ODP threads (for the same 'shm' block), and
+should therefore not be shared between ODP threads.
+For instance, it would be correct to send a shm handle using IPC between two
+ODP threads and let each of these thread do their own +odp_shm_addr()+ to
+get the block address. Directly sending the address returned by
++odp_shm_addr()+ from one ODP thread to another would however possibly fail
+(the address may have no sense in the receiver address space).
+
+The address returned by +odp_shm_addr()+ is nevertheless guaranteed to be
+aligned according to the alignment requirements provided at block creation
+time, even if the call to +odp_shm_addr()+ is performed by a different ODP
+thread than the one which originally called +odp_shm_reserve()+.
+
+All shared memory blocks are contiguous in any ODP thread addressing space:
+'address' to 'address'+'size' (where 'size' is the shared memory block size,
+as provided in the +odp_shm_reserve()+ call) is read and writeable and
+mapping the shared memory block. There is no fragmentation.
+
+=== Memory behaviour
+By default ODP threads are assumed to behave as cache coherent systems:
+Any change performed on a shared memory block is guaranteed to eventually
+become visible to other ODP threads sharing this memory block.
+(this behaviour may be altered by flags to +odp_shm_reserve()+ in the future).
+Nevertheless, there is no implicit memory barrier associated with any action
+on shared memories: *When* a change performed by an ODP thread becomes visible
+to another ODP thread is not known: An application using shared memory
+blocks has to use some memory barrier provided by ODP to guarantee shared data
+validity between ODP threads.
+
+=== Lookup by name
+As mentioned, shared memory handles can be sent from ODP threads to ODP
+threads using any IPC mechanism, and then the block address retrieved.
+A simpler approach to get the shared memory block handle of an already created
+block is to use the +odp_shm_lookup()+ API function call.
+This nevertheless requires the calling ODP thread to provide the name of the
+shared memory block:
++odp_shm_lookup()+ will return +ODP_SHM_INVALID+ if no shared memory block
+with the provided name is known by ODP.
+
+.retrieving a block handle and address from another ODP task
+[source,c]
+----
+#define BLKNAME 'shared_items'
+
+odp_shm_t shm;
+shared_data_t *shared_data;
+
+shm = odp_shm_lookup(BLKNAME);
+if (shm != ODP_SHM_INVALID) {
+ shared_data = odp_shm_addr(shm);
+ ...
+}
+----
+
+=== Freeing memory
+Freeing shared memory is performed using the +odp_shm_free()+ API call.
++odp_shm_free()+ takes one single argument, the shared memory block handle.
+Any ODP thread is allowed to perform a +odp_shm_free()+ on a shared memory
+block (i.e. the thread performing the +odp_shm_free()+ may be different
+from the thread which did the +odp_shm_reserve()+). Shared memory blocks should
+be freed only once, and once freed, a shared memory block should no longer
+be referenced by any ODP threads.
+
+.freeing a shared memory block
+[source,c]
+----
+if (odp_shm_free(shm) != 0) {
+ ...//handle error
+}
+----
+
+=== Memory creation flags
+The last argument to odp_shm_reserve() is a set of ORed flags.
+Two flags are supported:
+
+==== ODP_SHM_PROC
+When this flag is given, the allocated shared memory will become visible
+outside ODP. Non ODP threads (e.g. usual linux process or linux threads)
+will be able to access the memory using native (non ODP) OS calls such as
+'shm_open()' and 'mmap' (for linux).
+Each ODP implementation should provide a description on exactly how
+this mapping should be done on that specific platform.
+
+==== ODP_SHM_SW_ONLY
+This flag tells ODP that the shared memory will be used by the ODP application
+software only: no HW (such as DMA, or other accelerator) will ever
+try to access the memory. No other ODP call will be involved on this memory
+(as ODP calls could implicitly involve HW, depending on the ODP
+implementation), except for +odp_shm_lookup()+ and +odp_shm_free()+.
+ODP implementations may use this flag as a hint for performance optimization,
+or may as well ignore this flag.
+
== Queues
Queues are the fundamental event sequencing mechanism provided by ODP and all
ODP applications make use of them either explicitly or implicitly. Queues are
--
2.1.4
_______________________________________________
lng-odp mailing list
lng-odp@lists.linaro.org
https://lists.linaro.org/mailman/listinfo/lng-odp
