On Jul 28, 2004, at 12:59 PM, David Ho wrote:

I am just curious about how the 8xx does a block mapping (e.g. mapping
a
video frame buffer) without BATs.

It doesn't.  Everything is mapped with 4K pages.

- it allocates the smallest possible page size for the block requested.
i.e. it will have to allocate an 8-Myte page for the 2-Mbyte block,
very
wasteful it seems.

Worse than that.  Due to alignment the 8M page will open up lots of
aliases and access problems.

Could you shed some light on how this works.  It will help me assess
how
efficient block mapping is on the 8xx.

On the 860 (or other variants with the large number of TLBs), you can
allocate a few pinned entries for the lower kernel memory and the IMMR
space.

I have some experimental code that would map larger spaces with big
pages, but the complexity of the TLB miss handler badly hurt the
performance
of the typical 4K case.  I haven't found anything that works well, yet.


	-- Dan


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