eieio rule-of-thumb?

Dan Malek dan at embeddededge.com
Fri May 24 03:28:27 EST 2002

Allen Curtis wrote:

> All of these make sense,....

What doesn't make sense is why we use eieio at all......All of the
mapped I/O space is marked uncached 'guarded' in the PTE, which enforces
in-order load/store operations.  This should also prevent store gathering in
bridges since they shouldn't see a burst write from a processor store operation.

If you want higher peformance programmed I/O access, then you should cache
some of the space, and at that time you must use eieio if there are cached
areas subject to out of order access problems.

On the 8xx and 8260 family, all of the I/O (including the internal memory
space) is mapped uncached and guarded.  I've never used eieio nor seen
any reason it was necessary.

Where you will see problems, especially on 4xx and potentially on 8xx,
is the use of "regular" memory for control structures and special registers
for other control.  You can write to memory, which gets stuck in pipelines,
then whack a DCR (which seems to have some magical fast path update) causing
the peripheral to start up before the pipelined writes have made it to
memory.  I'm wondering if we aren't just lucky with the eieio side effects
when a 'sync' would be the logically correct operator.

	-- Dan

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