one of my boxes didnt boot the 2.6.20-rc1-rt0 kernel rpm, it hung during
early bootup. After an hour or two of happy debugging i narrowed it down
to the CALGARY_IOMMU_ENABLED_BY_DEFAULT option, which was freshly added
to 2.6.20 via the x86_64 tree and /enabled by default/.

commit bff6547b

 claims:

    [PATCH] Calgary: allow compiling Calgary in but not using it by default

    This patch makes it possible to compile Calgary in but not use it by
    default. In this mode, use 'iommu=calgary' to activate it.

but the change does not actually practice it:

 config CALGARY_IOMMU_ENABLED_BY_DEFAULT
        bool "Should Calgary be enabled by default?"
        default y
        depends on CALGARY_IOMMU
        help
          Should Calgary be enabled by default? if you choose 'y', Calgary
          will be used (if it exists). If you choose 'n', Calgary will not be
          used even if it exists. If you choose 'n' and would like to use
          Calgary anyway, pass 'iommu=calgary' on the kernel command line.
          If unsure, say Y.

it's both 'default y', and says "If unsure, say Y". Clearly not a typo.

disabling this option makes my box boot again. The patch below fixes the
Kconfig entry. Grumble.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>