8xx block address mapping...

[Dan Malek]

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


** Sent via the linuxppc-embedded mail list. See http://lists.linuxppc.org/