[coreboot] MTRR setup strategy

[Carl-Daniel Hailfinger]

Hi,

the problems Jason is seeing with slow boot can be explained with our
incomplete and less than efficient MTRR setup.

We never use subtractive MTRRs, so if a range is not power-of-two sized,
we try to combine it from subranges which are power-of-two sized. For
large ranges which are a little smaller than a power of two, we waste
MTRRs and in some cases even run out of MTRRs. That's not something to
be proud of.

Example:
We want to cache 0MB - (2G-64M-64k).
Current setup:
reg00: base=0x00000000 (   0MB), size=1024MB: write-back, count=1
reg01: base=0x40000000 (1024MB), size= 512MB: write-back, count=1
reg02: base=0x60000000 (1536MB), size= 256MB: write-back, count=1
reg03: base=0x70000000 (1792MB), size= 128MB: write-back, count=1
reg04: base=0x78000000 (1920MB), size=  32MB: write-back, count=1
reg05: base=0x7a000000 (1952MB), size=  16MB: write-back, count=1
reg06: base=0x7b000000 (1968MB), size=   8MB: write-back, count=1
reg07: base=0x7b800000 (1976MB), size=   4MB: write-back, count=1
--Here we run out of MTRRs, additionally needed MTRRs follow--
reg08: base=0x7bc00000 (1980MB), size= 2048kB: write-back, count=1
reg09: base=0x7be00000 (1982MB), size= 1024kB: write-back, count=1
reg10: base=0x7bf00000 (1983MB), size= 512kB: write-back, count=1
reg11: base=0x7bf80000 (1983MB), size= 256kB: write-back, count=1
reg12: base=0x7bfc0000 (1983MB), size= 128kB: write-back, count=1
reg13: base=0x7bfe0000 (1983MB), size=  64kB: write-back, count=1

We could achieve the same effect with a subtractive setup:
reg00: base=0x00000000 (   0MB), size=2048MB: write-back, count=1
reg01: base=0x7c000000 (1984MB), size=64MB: uncached, count=1
reg02: base=0x7bff0000 (1983MB), size=64kB: uncached, count=1

However, a subtractive setup is not always more efficient. That means we
have to select the best setup type. I devised a slightly tricky
algorithm to do that:
1. Check if there are multiple disjoint cached areas in a given
power-of-two sized area.
1a. If no, go to step 2
1b. If yes, stop here. Need advanced setup not described here.
2. additive_count=bitcount(top_cached_addr+1)
3.
subtractive_count=bitcount(rounduptonextpowerof2(top_cached_addr)-(top_cached_addr+1))
4. if (additive_count>subtractive_count) go to subtractive_method else
go to additive_method

Regards,
Carl-Daniel

-- 
http://www.hailfinger.org/