With >= 4GB memory installed we get a memory map split in the middle
due to remap that has boundaries that are inconveniently aligned for
MTRRs due to the various UMA regions.
0000MB-2780MB 2780MB RAM (writeback)
2780MB-2782MB 2MB TSEG (uncached/SMRR)
2782MB-2784MB 2MB GFX GTT (uncached)
2784MB-2816MB 32MB GFX UMA (uncached)
2816MB-4096MB 1280MB EMPTY (N/A)
4096MB-5368MB 1272MB RAM (writeback)
5368MB-5376MB 8MB ME UMA (uncached)
The default MTRR allocation method of trying to cover everything
with one MTRR and then carve out a single uncached region does
not work for the GPU aperture which needs write-combining type,
and it also has issues trying to cover the uneven boundaries
in the avaiable variable MTRRs.
My goal was to make a minimal set of changes and avoid modifying
behavior on existing systems with an algorithm that is not always
optimal for a typical memory layout. So the flag 'above4gb=2'
will change these allocation behaviors:
1) Detect the number of available variable MTRRs rather than
limiting to hardcoded value. We need every last MTRR.
2) Don't try to cover all RAM with one MTRR, instead let each
RAM region get covered independently.
3) Don't assume uma_memory_base is part of the last region
and increase the size of that region. In this case the UMA
region is carved out from the lower memory region and it is
already declared as part of the ram region.
4) If a memory region can't be covered with MTRRs >= 16MB then
instead make a larger region and trim it with uncached MTRRs.
Change-Id: I5a60a44ab6d3ae2f46ea6ffa9e3677aaad2485eb
Signed-off-by: Duncan Laurie <dlaurie@google.com>
Reviewed-on: http://review.coreboot.org/761
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
7389fa945f