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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