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unified/doc: Minor updates to message queues in Kernel Primer

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Change-Id: I4e691f97818874d594ac1d9225c14f6af3c0101d
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
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Allan Stephens 2016-10-03 10:37:33 -05:00 • committed by Benjamin Walsh
commit ddf7bb5eb3
1 changed files with 8 additions and 6 deletions
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14
doc/kernel_v2/data_passing/message_queues.rst
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@ -39,7 +39,7 @@ If a thread attempts to send a data item when the ring buffer is full,
the sending thread may choose to wait for space to become available.
Any number of sending threads may wait simultaneously when the ring buffer
is full; when space becomes available
it is given to the highest priority sending task that has waited the longest.
it is given to the highest priority sending thread that has waited the longest.
A data item can be **received** from a message queue by a thread.
The data item is copied to the area specified by the receiving thread;
@ -48,8 +48,8 @@ the size of the receiving area *must* equal the message queue's data item size.
If a thread attempts to receive a data item when the ring buffer is empty,
the receiving thread may choose to wait for a data item to be sent.
Any number of receiving threads may wait simultaneously when the ring buffer
is empty; when a data item becomes available
it is given to the highest priority receiving task that has waited the longest.
is empty; when a data item becomes available it is given to
the highest priority receiving thread that has waited the longest.
.. note::
The kernel does allow an ISR to receive an item from a message queue,
@ -149,9 +149,11 @@ in an asynchronous manner.
.. note::
A message queue can be used to transfer large data items, if desired.
However, it is often preferable to send pointers to large data items
to avoid copying the data. The kernel's memory map and memory pool object
types can be helpful for data transfers of this sort.
However, this can increase interrupt latency as interrupts are locked
while a data item is written or read. It is usally preferable to transfer
large data items by exchanging a pointer to the data item, rather than the
data item itself. The kernel's memory map and memory pool object types
can be helpful for data transfers of this sort.
A synchronous transfer can be achieved by using the kernel's mailbox
object type.