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    SubjectRe: MTRR initialization
    On Fri, Sep 14, 2007 at 09:33:30AM -0700, Howard Chu wrote:
    > Hi, was wondering if anyone else has been tripped up by this... I've got
    > 4GB of
    > RAM in my Asus A8V Deluxe and memory hole mapping enabled in the BIOS. By
    > default, my system boots up with these MTRR settings:
    >
    > reg00: base=0x00000000 ( 0MB), size=4096MB: write-back, count=1
    > reg01: base=0x100000000 (4096MB), size=1024MB: write-back, count=1
    > reg02: base=0xc0000000 (3072MB), size=1024MB: uncachable, count=1
    > reg03: base=0xc0000000 (3072MB), size= 256MB: write-combining, count=1
    >
    > The X server and various other programs try to add a mapping for my video
    > card's buffer, at 0xd0000000, size=256MB, type=write-combining, and this
    > always
    > fails with a type mismatch error (old type is write-back). Apparently it's
    > conflicting with mapping register 0. I can't just disable the existing
    > settings
    > and re-add them; the system hangs soon after disabling reg01.
    >
    > I guess the kernel must be getting the initial setup from the BIOS. I've
    > hacked
    > around this in mtrr/generic.c by explicitly changing the MTRR state in
    > get_mtrr_state to split the first mapping into two; one at base 0 size
    > 2048M
    > and one at base 2048M size 1024M. So now I have this, which is pretty much
    > what
    > I wanted:
    >
    > reg00: base=0x00000000 ( 0MB), size=2048MB: write-back, count=1
    > reg01: base=0x80000000 (2048MB), size=1024MB: write-back, count=1
    > reg02: base=0x100000000 (4096MB), size=1024MB: write-back, count=1
    > reg03: base=0xc0000000 (3072MB), size=1024MB: uncachable, count=1
    > reg04: base=0xc0000000 (3072MB), size= 256MB: write-combining, count=1
    > reg05: base=0xd0000000 (3328MB), size= 256MB: write-combining, count=1

    BTW, having overlapping WC, UC regions make the end result UC. So in this
    case, you may not be getting the desired performance.

    >
    > So the question is - was there an easier/correct way to do this?
    >
    > It might have been nice if the MTRR ioctls allowed the register number to
    > be
    > specified on the Set commands, though I'm not sure that would have helped
    > in
    > this case.
    -
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